THE DISINTEGRATION OF NEON BY FAST NEUTRONS
by
G i l b e r t James P h i l l i p s
A t h e s i s submitted i n p a r t i a l f u l f i l m e n t of
the requirements f o r the degree of
. .
MASTER OF ARTS
i n the Department
'
of
PHYSICS
We accept t h i s t h e s i s as conforming to the
standard required from candidates f o r the
degree of MASTER OF ARTS
Members of the Department o
PHYSICS
THE UNIVERSITY OF BRITISH COLUMBIA
A p r i l , 1952
ABSTRACT
A normal mixture of the stable neon isotopes,
Ne
90.51%, Ne
w i t h 2.68 Mev.
0.28$, Ne
9.21%, has been bombarded
neutrons from the r e a c t i o n H (dn)He
.
A gridded ion chamber, w i t h 1 l i t r e s e n s i t i v e volume,
f i l l e d w i t h neon to 6^ atmospheres pressure, was
used
to detect d i s i n t e g r a t i o n s . Pulses from the i o n chamber
were a m p l i f i e d , arid recorded on an 18-channel k i c k s o r t e r .
The
ground-state t r a n s i t i o n of Ne (n«i)0
was
observed, w i t h 1% counting s t a t i s t i c s . The Q,-value was
measured as
-0.77*0.08 Mev.,
i n f a i r agreement w i t h
previously reported r e s u l t s .
Careful search f a i l e d to r e v e a l any
evidence
17
.
of excited states i n 0 . A l e v e l at 0.87
and one at 1.6 Mev.
Mev.
i s known,.
i s suspected, but c a l c u l a t i o n of
b a r r i e r p e n e t r a b i l i t y f o r alpha p a r t i c l e s corresponding
to these l e v e l s i n d i c a t e s that they are not l i k e l y to
be observed at the neutron energy used.
A second r e a c t i o n , w i t h d i s i n t e g r a t i o n energy
corresponding to a Q,-value of +0.48*0.10 Mev.
observed. I t was
was
not c l e a r whether t h i s r e a c t i o n
N ^ n p j C ^ o r N e ^ U ^ O o ' * . This point i s to be
by f u r t h e r i n v e s t i g a t i o n .
also
was
clarified
ACKNOWLED&EMEHTS
The author i s pleased to express h i s thanks
to Dr. J . B. Warren f o r the suggestion and d i r e c t i o n
of t h i s research.
The assistance of 'Mr. F. C. Flack throughout
the course of the experiments,
and i n the preparation
of t h i s thesis i s deeply appreciated.
Thanks i s also due to Dr. S. B. Woods f o r the
c o n s t r u c t i o n of p o r t i o n s of the apparatus, and f o r
h e l p f u l suggestions.
INDEX
I.
II.
INTRODUCTION
A.
Knowledge of N u c l e i
1
33.
(n
2
C.
Choice of Experiment
) Reactions
3
EXPERIMENTAL METHOD
A.
The Experimental Problem
33.
Production of Past Neutrons
4
1.
East Neutron Sources
k
2.
The H* (dn)He* Reaction
5
3.
The 5 0 kev. Ion Generator
7
4.
Heavy Ice Target
8
5.
33ackground R a d i a t i o n from
9
Neutron Source
C.
Measurement of A l p h a - P a r t i c l e Energies
10
1.
Choice of Detectors
10
2.
The Gridded Ion Chamber
11
3.
D e s c r i p t i o n of the Ion Cham"ber 13
4.
Electrode System
14
5.
P i l l i n g Gas
15
6.
Operating Conditions
17
7.
Pulse
Measurement
Amplification
and
18
8.
Energy C a l i b r a t i o n
20
9.
Experimental Procedure
21
III.
RESULTS AND DISCUSSION
23
A.
Experimental Results
23
33.
Discussion
2^
1. Threshold Energies f o r
Fast Eeutron D i s i n t e g r a t i o n of Neon
2.
24i=
I n t e r p r e t a t i o n of the
Main Peak
25
3.
I n t e r p r e t a t i o n of the
Minor Peak
28
l+.
C.
Summary of Results
3Turther Investigations
30
31
IV.
CONCLUSIONS
33
V.
BIBLIOGRAPHY
3k
ILLUSTRATIONS
Plate
4
3
I.
H
II.
H* (dn)He* Thick Target Y i e l d Function
III.
IV.
H
(dn)He
4
Cross Section vs. Deuteron Energy
Facing Page
5
3
(dn)He
Neutron Energy v s . Angle of Emission
Heavy Ice Target
6
7
8
Pulse Formation i n Ungridded Ion Chamber
12
Gridded Ion Chamber
13
Electrode System f o r Gridded Ion Chamber
14
Wide-33and Low Noise Head A m p l i f i e r
IS
Pulse-Height D i s t r i b u t i o n from Polonium Alphas
20
X.
Complete Energy Spectrum
23-:
XT.
Energy Spectrum: D e t a i l s
24
V.
VI.
VII.
VIII.
IX.
1.
THE PI SI MSG-RAT I ON OF NEON BY FAST NEUTBOMS
I.
A.
INTRODUCTION
KNOWLEDGE OF NUCLEI
It i s much easier to speak with optimism regarding the future of
Nuclear Physics than to discuss with s a t i s f a c t i o n i t s present status.
While
the general p r i n c i p l e s and methods of quantum mechanics appear.to he s a t i s factory f o r dealing with nuclear phenomena, yet the most f r u i t f u l approach,
p a r t i c u l a r l y i n dealing with nuclear reactions and energy l e v e l s , i s essent i a l l y a phenomenological
one.
The main d i f f i c u l t i e s i n attempting to formulate a theory of n u c l e i ,
comparable i n scope to that of atomic structure, are twofold.
F i r s t l y , the
nature and proper description of the short-range forces between the protons
and neutrons of which the nuclei are composed are not understood, and secondly,
even with a comprehensive knowledge of the short-range forcesj the mathemati c a l d i f f i c u l t y of dealing with such a many body problem accurately i s s t i l l
formidable. .-'In t h i s case even approximation methods make l i t t l e headway—in
the words.of Gamow., "the theory of complex nuclei i s d i f f i c u l t mathematically
because everything i s of the same order of magnitude, and nothing can be neglected. "
(GAMOW, 1937)
The problem has then been approached experimentally by c o l l e c t i n g
as much data as possible on a l l aspects of nuclear structure:
functions, energy l e v e l s , spin values, and phenomenological
the dispersion formula, deduced to f i t the observed
excitation
theory, such as
results.
Attempts to progress further have resulted i n the proposal of
various nuclear models.
One example i s the "alpha p a r t i c l e model", which
pictures the nucleons as being grouped within the nucleus as alpha p a r t i c l e s ,
2.
the alpha i n turn forming s h e l l s , with extra nucleons i n outer o r b i t s .
Such a picture suggests exceptional s t a b i l i t y f o r closed, shells of alpha
p a r t i c l e s , as i n C
and 0
.
It was hoped that some of this model's pre-
dictions f o r nuclei of the 4N + n type might "be tested i n the present experiment as one such nucleus, namely 0
product i n the reaction T$e*° (xi* )
(4<*+n ) was a known d i s i n t e g r a t i o n ;
0
1 7
The alpha p a r t i c l e model i s "by no means the only model postulated,
and recently the "orbit model" has been extensively and successfully developed
by MAYER,
(1948).
However, with l i g h t nuclei the alpha p a r t i c l e model has
had some success i n correlating experimental data.
It i s hoped that the
present experiment w i l l contribute another small fact to the ever increasing
information about nuclear reactions.
B. (n oc ) REACTIONS
Nuclear energy levels are usually excited by bombardment with nucleons,
or other heavy p a r t i c l e s .
For neutron bombardment, capture adds about 8
to the nucleus, t h i s energy being d i s t r i b u t e d among the nucleons.
Mev.
The compound
nucleus so formed remains i n this excited state u n t i l random fluctuations
concentrate s u f f i c i e n t energy i n one or more nucleons f o r t h e i r emission.
Photon
emission may also occur, but the l e v e l s f o r this process are sharper than f o r
p a r t i c l e emission, and the l a t t e r process i s much more probable, i f energetically
possible.
The most favoured t r a n s i t i o n i s that which leaves the r e s i d u a l nucleus
i n the most stable configuration. Generally, neutron emission requires somewhat
less e x c i t a t i o n energy than proton or alpha emission, because of the Coulomb
b a r r i e r f o r the charged p a r t i c l e s .
Alpha emission, however, commonly occurs f o r
the l i g h t nuclei, i n which the Coulomb b a r r i e r i s r e l a t i v e l y low, while the
3.
average energy per nucleon i n the compound nucleus i s high, due to the sharing
of e x c i t a t i o n energy between r e l a t i v e l y few p a r t i c l e s .
C.
CHOICE OF EXPERIMENT
Much work has been done with f a s t neutrons, and the d i s i n t e g r a t i o n
of c e r t a i n nuclei, i n p a r t i c u l a r those of carbon and nitrogen by such p a r t i c l e s
i s well known.
Neon however, has received l i t t l e attention.
using neutrons of less than 3 Mev.
energy, showed with rather poor s t a t i s t i c s
only one single-energy group of emitted alphas.
the ground state t r a n s i t i o n of Ie*°(n<t) o ' .
r
This group was
No evidence
been reported, but since a l e v e l i n o'at 0 . 8 7 Mev.
1 . 6 Mev.
Previous experiments,
i d e n t i f i e d with
of excited states has
i s known, and another at
suspected, further i n v e s t i g a t i o n seemed worthwhile.
Neon i s readily obtainable i n pure form, and l i k e the other noble
gases i s suited f o r use i n an ion chamber (WILKINSON,
With an 18 channel
1950).
pulse amplitude analyser a v a i l a b l e i n this laboratory, i t was expected to obtain
data; more rapidly•and hence more p r e c i s e l y than heretofore.
It was decided therefore to bombard neon with fast neutrons,
an ion chamber as a detector.
The Devalue of the Ne
(n«)
0
using
reaction was
to be c a r e f u l l y measured, and a thorough search made f o r other groups of
p a r t i c l e s , p a r t i c u l a r l y any associated with excited states of o ' '
.
The
remainder of this thesis discusses the d e t a i l s of this experiment,- and
results obtained.
the
4.
II.
A.
EXPERIMENTAL METHOD
THE EXPERIMENTAL PROBLEM
Since neon l i k e the other noble gases has excellent e l e c t r o n
c o l l e c t i o n properties, i t seemed obvious to detect the reaction d i r e c t l y by
i o n i z a t i o n .produced i n a gas target.
Among the various possible detectors,
cloud chamber, proportional counter, etc., a gridded ion chamber was chosen
as being a fast operating detector with excellent l i n e a r i t y between pulse-size
and energy.
Three sources of fast neutrons were a v a i l a b l e f o r the experiment:
a 5 0 m i l l i c u r i e Radium-Beryllium source, a 50 kev. i o n accelerator, and a Van
de G-raaff generator, at present producing protons with energies up to 2 Mev.
The Ra-Be source provides a reasonable y i e l d of neutrons, but the straggling
of alphas within the source produces a very broad spectrum of neutron energies.
Neutrons can be generated i n both the 5 0 kev. accelerator and the Van de G-raaff
by means of a nuclear reaction, such as H*(dn) He
Since an intense, monoenergetic
by using the H
(dn) He
3
or H
3
(dn ) He * .
source was required, i t was decided to start
reaction with the 5 0 kev. i o n accelerator and a thick
target of heavy i c e .
B.
PRODUCTION 03? EAST NEUTRONS
1.
Fast Neutron Sources
Many nuclear reactions may be used as neutron sources.
In p a r t i c u l a r , bombardment of l i g h t n u c l e i with protons or deuterons produces
neutrons of a wide range of energies.
The choice of a reaction depends on the
y i e l d , on the number of neutron energy groups produced, and on the energies
required.
A few of the better known fast neutron sources are:
HORNYAK et a l , 1 9 5 0 ) .
(HANSON et a l 1 9 4 9 ;
Plate
10
D(dn)f He ;
I
Cros s Section {cy) vs. D euteron Erlergy (Ed;
/o
10
or
z
cms.
JO
•
•It
/o
\
-J9
/O T
•
> '
o.S
AO
E d ; Mev.
JO
5.
Reaction
H* (dn) He
3
H
3
3
(pn) He
quvalue
Threshold
+ 3.256 Mev.
- 0.764
"
l i ' ( p n ) Be"
- 1.647
"
V*'(pn) C r
- 1.50
3
#
(dn) He *
4 7
< 0 . 0 1 5 Mev.
0.98
»
+17.60
E
(dn)
He
"
,< 0 . 0 1 5
"
1 . 8 -> 7 . 2
Mev.
0.001-^4.
12.
->20.
3.
"
»
1.882
«
0.001
1.53
"
0.002 -> 0.020 Mev.
f o r bombarding energies up to about 4
As mentioned, H
Ranee of Neutron
Energies #
"
.
Mev.
neutrons generated "by the 5 0 Kev. ion accelerator
were chosen f o r the i n i t i a l i n v e s t i g a t i o n .
It was hoped to extend the neutron
energy up and down from the 2.68 Mev. value by using t h i s reaction with f a s t e r
deuterons, the H
(dn ) He
reaction, and slower neutrons from the V
(pn) Cr
reaction.
2.
The H (dn) He' Reaction
a
Since i t was f i r s t reported by OLIPHANT,'HARTECK and '
RUTHERFORD, ( 1 9 3 4 ) , t h i s reaction has received the a t t e n t i o n of many workers.
The l i t e r a t u r e contains extensive information on the y i e l d function (ROBERTS,
1937;
AMALDL et a l , 1937) the cross section (LADENBURG et a l 1937) and the
angular d i s t r i b u t i o n (BONIER, 1 9 3 7 ) , f o r deuteron energies from 15 kev. (BESTSCHERT;,
et a l , 1948) to 10 Mev.
(LEITERS et a l , 1950)
The reaction proceeds i n two ways, with about equal p r o b a b i l i t y :
U*
(dn)
He
H*
(dp)
H
3
3
Q*
* 3 . 2 5 6 ± 0.018
+4.036± 0.022
Mev.
Mev.
These 0-values are magnetic spectrometer determinations (TOLLESTRUP et a l , 1 9 4 9 ) .
Exact r e l a t i v e y i e l d s of protons and neutrons have s t i l l not been established, but
are
known to be approximately equal.
Plate
D (dn)He -, Thick T a r g e t
2
Total
3
Yield (Y)
oS
II
Yield
vs. D e u t e r o n
to
E d ; Mev.
Function
Energy
(Ed)
/.s
20
6.
Curves of cross section and t o t a l y i e l d f o r the reaction showsmooth increases with the energy of the incident deuterons.
These are i l l -
ustrated i n Plates I and II respectively, f o r deuteron energies up to 2
Mev.
The former gives the integrated cross section, and the l a t t e r the t o t a l
y i e l d , over the whole s o l i d angle, f o r a thick heavy i c e target.
These curves
are "based on several investigations, as l i s t e d "by HORNYAK et a l ,
The angular d i s t r i b u t i o n of neutrons i s anisotropic.
(1950)
For deuteron
energies of less than 5 0 0 kev., i t i s adequately described by a r e l a t i o n of the
form N (?)
•B (
1*A c o s * ^ ) i n the centre of mass system. (MANNING et a l ,
1942).
At higher deuteron energies, more neutrons are observed i n the forward d i r e c t i o n
than this r e l a t i o n predicts, and terms i n higher powers of cos*^
must be added.
The more extensive investigations of t h i s aspect of the reaction include those
of BENNETT et a l , (1946) f o r deuterons of up to 1.8 Mev., HUNTER and RICHARDS,
(1949), 0.5 to 3.7 Mev. deuterons, and ERICKSON et a l (1949) f o r 10 Mev.
deuterons.
In the present experiment, the angular d i s t r i b u t i o n was calculated
using the value of A = 0.45 f o r 50 kev. deuterons. (HUNT00N, 1940)
The neutron energy varies with the angle of emission measured r e l a t i v e
to the deuteron beam, and i s also a function of the deuteron energy.
This
dependence may be calculated from the masses and energies of the p a r t i c l e s
involved. HANSON et a l (1949) have carried out these calculations f o r several
of the reactions used as neutron sources, f o r a wide range of deuteron energies.
Plate III shows the results of t h e i r calculations f o r deuterons up to 2 Mev.
It i s evident that i f f a i r l y high energy deuterons are available, as with a
Van de Graaff generator, the reaction provides a wide and continuously variable
range of neutron energies.
The extent to which the neutrons obtained are t r u l y homogeneous depends
on several f a c t o r s .
%
Variation i n energy of the incident deuterons, and
note e s p e c i a l l y BRETSCHER et a l ( l 9 4 8 ) f o r low energies.
Plate
En
*
Mev.
D(dn)He
3
.
Neutron
III
J
Energy
E
(En)
vs.
n
Angle
M
e
v
of
frf?
0°
Emission
{^)
straggling of deuterons i n thick targets w i l l produce a spread i n neutron
energies.
Unless the s o l i d angle subtended by the detector i s very small,
a range of neutron energies w i l l be observed, due to the angular dependence
of the reaction.
Variations i n deuteron energy may be controlled by magnetic analysis
of the i o n beam.
Deuteron straggling i n thick targets i s d i f f i c u l t to determine,
as the rate of energy loss, (yj
i s not well known.
In the present experiment,
the effect of t h i s mechanism on the homogeneity of the neutron energy w i l l be
small.
In the region of deuteron energies used (50 kev.), the y i e l d function
of the reaction r i s e s very rapidly.
Therefore, f o r deuterons of even s l i g h t l y
less than average energy, the neutron y i e l d drops very sharply.
Thus r e l a t i v e l y
few low energy neutrons w i l l result from straggled deutrons.
3.
The 50 kev. Ion Generator
The accelerating voltage of the 5 0 kev. generator was
supplied by a commercial half-wave transformer-rectifier set, (FEREANTI X-RAY)
with condenser f i l t e r i n g .
Ion voltage was monitored by observing the current
through a .calibrated o i l immersed resistance stack.
to be much less than 1
(KIRKALDY,
Ripple voltage was stated
Construction of the unit i s described elsewhere
1951).
The i o n beam was produced by.a radio-frequency i o n source, of a type
developed i n this department. (KIRKALDY,
1951;
i n the discharge i s less than 100 v o l t s .
WOODS,
1952).
Energy spread
Provision i s included f o r magnetic
analysis o f the i o n beam.
Deuterium gas was obtained from heavy Water i n t h i s laboratory.
The
heavy water ( 9 9 * $) was electrolysed i n a closed system, which included a
cold trap and phosphorous pentoxide d r i e r .
The gas was passed into"the i o n
i
PLATE
IV
HEAVY
ICE
TARGET
source through a Palladium
leak, thereby eliminating most contamination, with
the exception of hydrogen.
Beam currents were measured by- metering the target current to ground.
biased to + 3 0 0 v o l t s to prevent back electron current.
The target was
of 100 to 20Cyfc-amp. resolved H
h.
Beams
ions were obtained.
Heavy Ice Target
The f i r s t targets used consisted of heavy water adsorbed
on Phosphorous Pentoxide but the neutron y i e l d s obtained were low.
target, as shown i n Plate IV was
A heavy i c e
then constructed.
The target was attached to the ion generator by a two inch'Pyrex pipe
flare fittings.
After/ evacuation,
f i l l e d with l i q u i d a i r .
the inner chamber of the target
Heavy water vapour was
was
then admitted to the body
of the target v i a a needle valve and the copper tube, indicated i n Plate
IV.
The vapour froze to the copper target-leaf, forming a layer of heavy i c e .
The 0-ring seal on the copper tube allowed i t to be moved out of the way
the beam a f t e r the target was
formed.
Formation of the target was
target body through the Pyrex pipe.
and the target was
of
f i r s t observed by looking into the
This view proved to be rather r e s t r i c t e d ,
l a t e r modified by cutting a c i r c u l a r part i n the side
of the body, (position indicated i n Plate IV), and attaching a glass window
by means of an 0-ring seal.
The Pyrex pipe e f f e c t i v e l y insulated the target from the i o n
generator, and allowed the beam current to be metered.
It was
found
that the i n t e r i o r of the glass pipe became charged, causing serious
initially
defoc-
ussing of the beam and so a great reduction i n the u s e f u l beam current.
A
sleeve of wire gauze on the i n t e r i o r of the pipe, connected to the target,
overcame the d i f f i c u l t y .
and
9.
The inner chamber of the target accomodated about 200 cc. of
With a normal beam of 150y(f.amp. on the target, i . e . 7«5
liquid air.
watts beam-power, this was s u f f i c i e n t f o r a maximum of one hour's operation.
Normally i t was topped up every half hour.
A beam of 150
amp.
corresponds to 9 . 3 o 1 0
; f
deuterons per
second, and from the y i e l d curve f o r the reaction, (Plate I I ) , the y i e l d
f o r 50 kev. deuteron energy i s approximately 2 / 1 0
'neutrons per deuteron,
so that the t o t a l y i e l d was about 2 * 10 ' neutrons per second.
\\
v,
5.
Background Radiation from Neutron Source
It was necessary to consider the p o s s i b i l i t y of other
penetrating- radiations o r i g i n a t i n g i n the heavy ice target, i . e . other
(dn) reactions.
E* , o " ,
0
The l i g h t n u c l e i exposed to the deuteron beam were B'
, o'*from the heavy water, and traces of c'^and C
/f
o i l contamination.
o"(dn) F "
M
/ J
ft
Q.--1.6
Mev.
(NEWS0N, 1935;
« +3.5
Mev.
(WELLES, 1946)
(isotopic abundance 0.04$)
(BONNER et a l 1949)
(threshold 0.328 Mev.)
Q» -0.26
Mev. .
»x
C
REYDENBURG-, 1948)
ft
The high thresholds eliminate the C - + 0
/a
f rom pump
The only reactions to be considered were:
o'**(dn) J ? " -
C (dn) N
/ J
,
reactions, while both
17
and 0
have too few atoms present to be s i g n i f i c a n t .
Soft (50 kev.) jtf-rays w i l l be present from the ion generator, but
would not be expected to penetrate the l/4 inch s t e e l walls of the i o n i z a t i o n
chamber.
Additional protection was given by a l/4 "
sheet of lead-wrapped
around the chamber.
There i s expected to be a background of scattered neutrons from
the concrete walls of the laboratory, and from such massive objects i n the
v i c i n i t y as the analysing magnet.
Tests with a cadmium absorber would
indicate whether thermal neutrons were producing any e f f e c t s , such as ( n * )
10.
reactions i n the walls of the ion chamber.
Scattered neutrons with
somewhat less energy than those from the heavy ice target would be
expected to cause some spreading of pulse d i s t r i b u t i o n s .
C.
MEASUREMENT 03? ALPHA PARTICLE ENERGIES
!•
Choice of Detectors
As mentioned previously,' such d i f f i c u l t i e s as
straggling i n windows can be avoided by detecting the i o n i z a t i o n of
the products of the reaction i n the target gas
itself.
The Wilson Cloud Chamber has often been used f o r -such
observations—the f i r s t records of Neon disintegrations were so obtained,
(HARZINS,
1933).
However, there are several drawbacks i n using i t
for accurate measurement of p a r t i c l e energies.
Unless s p e c i a l high-S
pressure chambers are used, there w i l l be r e l a t i v e l y fev; target atoms,
and correspondingly few disintegrations w i l l be observed.
F a i r l y acc-
urate energy determinations from cloud chamber photographs are possi b l e , but both the c o l l e c t i o n and analysis of the data are tedious.
Proportional counters are capable of f a s t and accurate energy
determinations.
the device.
Most d i f f i c u l t i e s a r i s e i n maintaining s t a b i l i t y of
For detecting energetic p a r t i c l e s , the gas pressure must
be high i n order to confine the p a r t i c l e range to the gas volume.
voltages are then required to obtain gas-amplification.
High
For consistant
r e s u l t s , the gas-amplification must remain constant, and this requires
w e l l - s t a b i l i z e d high voltage.
The a m p l i f i c a t i o n i s also very sensitive
to s l i g h t changes i n the constitution of the f i l l i n g gas, and the
release of even small amounts of occluded gases from the walls of the
chamber can be very troublesome.
33ackground i s f a i r l y high i n proportional
11.
counters, because of t h e i r s e n s i t i v i t y to stray radiations, such as
soft x-rays.
Such an increase
i n "noise" w i l l raise the useful lower l i m i t
of the device, making the detection of low energy events d i f f i c u l t .
F i n a l l y , the pure noble gases are unsuited f o r use i n proportional
counters, c h i e f l y because of t h e i r tendency to form metastable states.
Addition of a quenching gas i s necessary to de-excite
so prevent spurious pulses.
these states,
and
(WILKINSON, 1950)
Ionization chambers, and p a r t i c u l a r l y gridded ion chambers,
are well suited to these investigations.' They require very stable
amplifiers, but modern c i r c u i t s meet these demands.
l e s s sensitive to v a r i a t i o n s i n c o l l e c t i n g voltage
Ion chambers are
than proportional
counters, are much less sensitive to background radiations, and
r e l a t i v e l y unaffected
filling
are
by s l i g h t changes i n the composition of the
gas.
The
speed with which data may
recording equipment a v a i l a b l e .
be collected depends on
the
Frequently the voltage pulses are
on an oscilloscope and photographed.
displayed
This mejbhod y i e l d s accurate r e s u l t s ,
but; the rate of c o l l e c t i n g data i s usually l i m i t e d by the photographic
equipment, and analysis of the data i s a b i t tedious.
attained with a pulse-amplitude analyser,
was
Greatest speed i s
or "kicksorter".
Such a device
available f o r the present experiment.
2. . The Gridded Ion Chamber
Measurement of p a r t i c l e energies with an ion
chamber requires that the i o n i z a t i o n produced be proportional to the
particle-energy,
and that the voltage pulses from the chamber be pro-
p o r t i o n a l to t h i s i o n i z a t i o n .
Events of a single energy may
i n practice
PLATE
V
PULSE
FORMATION
IN
UN6RIDDED
PULSE
HEIGHT
PULSE
DURATION
,
SECONDS
ION
CHAMBER
12.
produce a d i s t r i b u t i o n of pulse-heights.
The spread of t h i s d i s -
t r i b u t i o n depends on several f a c t o r s — s o u r c e thickness, i n the case
of s o l i d sources placed i n the chamber, straggling of i o n i z a t i o n , nonionizing, e x c i t a t i o n of the f i l l i n g gas, noise .in the associated
e l e c t r o n i c c i r c u i t s , and the influence of p o s i t i v e ions on pulse-formation.
In a gridded ion chamber, the effect of the positive, ions
i s eliminated.
How
t h i s i s achieved may
pulse-formation
i n an ungridded chamber.
be understood by considering
We assume that electron-
attachment does not occur i n the f i l l i n g gas, and that the c o l l e c t i n g
f i e l d i s s u f f i c i e n t to prevent recombination of the ions.
An i o n i z i n g event occurs between the electrodes, and
electrons are attracted to the p o s i t i v e c o l l e c t o r .
the
Electron collection
-S
requires 10
seconds or l e s s , and r e s u l t s i n a sharply r i s i n g voltage
pulse on the c o l l e c t o r (see Plate V).
i s about 1000
The p o s i t i v e ions, whose mobility
times less than that of the electrons, do not move appreciably
before electron c o l l e c t i o n i s completed.
While the p o s i t i v e ions are being
c o l l e c t e d , the p o t e n t i a l of the c o l l e c t o r continues
to r i s e , and
i t s maximum when a l l p o s i t i v e ions have been c o l l e c t e d (at the
electrode).
reaches-
negative
The pulse then decays as the charge leaks away through the
resistance and capacity of the electrode system.
Hot only i s such pulse-formation
slow, but the height and
duration
of the portion r e s u l t i n g from the c o l l e c t i o n of p o s i t i v e ions depends on the
o r i e n t a t i o n of the ions at the time of formation.
In many experiments t h i s
w i l l be a f a i r l y random matter, and the r e s u l t i n g spread of the pulseheight d i s t r i b u t i o n w i l l be correspondingly widened.
An a m p l i f i e r to
reproduce such slow pulses requires a low low-frequency cut-off, thus
introducing the a d d i t i o n a l problem of microphonics.
LATE
1820-20
VI
GRIDDED
8
H
O
L
E
ION
CHAMBER
8
S
SIDE
SECTION
HOLES
SCALEO
INCHES
I
2
13.
The
d i f f i c u l t y i s completely overcome "by placing a g r i d
between the electrodes, near the c o l l e c t o r .
The g r i d shields the
c o l l e c t o r e l e c t r o s t a t i c a l l y from the f i e l d of the p o s i t i v e
ions.
When an i o n i z i n g event occurs, the c o l l e c t o r potential remains constant u n t i l the electrons have diffused past the g r i d , then r i s e s
to i t s maximum value as electron c o l l e c t i o n occurs.
as the charge leaks from the electrode system.
rapidly
The pulse then decays
Much f a s t e r
amplifiers
may be used f o r such pulses, improving the signal-to-noise r a t i o .
f a s t e r pulses also allow shorter resolving
rates.
The
time and much higher counting
Suitable choice of the g r i d voltage prevents electron
collection
by the g r i d .
Some fra.ction of the i o n i z i n g events w i l l occur i n the p o r t i o n
of the sensitive volume between the g r i d and the c o l l e c t o r .
This region
w i l l behave as an ungridded chamber, with c o l l e c t i o n of p o s i t i v e
at the g r i d .
ions
Such pulses w i l l have random heights, and w i l l cause some
spreading of the d i s t r i b u t i o n .
For some events, only a portion of the i o n i z a t i o n w i l l occur
i n the sensitive volume, the p a r t i c l e s either s t r i k i n g an electrode,
or passing out of the sensitive
region.
This effect w i l l cause an asymmetry
of the low-energy side of the d i s t r i b u t i o n of pulses from single energy
i o n i z i n g events.
The degree of asymmetry w i l l depend upon the range of
the ionizing p a r t i c l e s i n the chamber.
3-
Description of the Ion Chamber
Plate VI shows the gridded ion chamber used i n the
experiment.
flanges.
The body of the chamber was s t e e l tubing, with welded
The end-plates were sealed with lead gaskets, thus avoiding
the exposure of the f i l l i n g gas to rubber, a common source of contamination.
PLATE
ELECTRODE
VII
SYSTEM
FOR
GRIDDED
ION
S C A L E : INCHES
O
I
2
COLLECTOR
AND
GUARD
GUARD
BRASS,EXCEPT
WHERE NOTED
GRID,
1
AND
MATERIAL \
RING
G Rl ^.
COLLECTOR
CHAMBER
RING
'////////////////////;//////\
MYCALEX ^
SIDE
ELEVATION
END
ELEVATION
14.
The volume of the chamber was 4.64 l i t r e s . .
A l l i n t e r i o r surfaces were
coated with "aquadag" c o l l o i d a l graphite, to reduce the background
from natural alpha-activity i n the s t e e l .
One end plate carried the gas p u r i f i e r , constructed of brass
and copper tubing.
An e l e c t r i c heating c o i l of about 3 0 ohms (cold) r e s i s -
tance was wound about the centre of the p u r i f i e r , and insulated with
glass wool and asbestos paper.
75 v o l t s across the heater produced a
O
temperature of about 3 0 0 C. at the i n t e r i o r .
at
A copper cooling c o i l
one end aided i n convection of the gas, and protected the 0 - r i n g
s e a l on the f i l l i n g cap.
and water vapour.
The most common impurities are oxygen, nitrogen,
Calcium metal i s known to be e f f e c t i v e i n removing
a l l of these, and so was used as the p u r i f y i n g agent. (REIMANN, 1 9 3 4 ;
WILKINSON, 1950);
The opposite end-plate carried a needle-valve f o r f i l l i n g
the chamber, and three kovar terminals.
The electrode system was also
mounted on t h i s plate, and the electrodes connected
to the kovars.
The chamber was designed by Mr. 3P. Flack, and the electrode
system constructed by Dr. S. B. Woods.
Its use i n other experiments
has been described elsewhere (WOODS, 1 9 5 2 )
4.
Electrode System
The design of grids f o r such chambers has been
c a r e f u l l y investigated, both i n theory and experiment by BUNEMANN,
CHANSBAW, and HARVEY, 1 9 4 9 .
1
The electrode system of the present
chamber was designed largely according to t h e i r recommendations.
The electrodes were constructed of brass, with Mycalex
and Lucite i n s u l a t i o n .
was wound from
Plate 7;II shows the arrangement.
The g r i d
3 6 Coppel wire, spaced 1 mm. centre-to-centre.
c o l l e c t o r spacing was 1 - 5 2 cms. and g r i d to plate 7 . 6 cms.
Grid-
For such
15.
an arrangement, the "grid i n e f f i c i e n c y " , i . e . the extent to which the
number of l i n e s of force ending on the c o l l e c t o r i s dependent upon the f i e l d
due to p o s i t i v e ions, i s 0 . 0 1 , i n d i c a t i n g very e f f i c i e n t shielding of
the c o l l e c t o r .
The curvature of the high-voltage plate i s a s l i g h t modifi c a t i o n to the design of Bunemann et a l . F i e l d p l o t s have shown that
the curvature increases the number of l i n e s of force ending on the
c o l l e c t o r , and insures that the sensitive volume approximates to the
a c t u a l volume above the c o l l e c t o r .
The sensitive volume was estimated to be about 1 l i t r e .
About 15$
of t h i s volume constitutes the unshielded region between the
g r i d and the c o l l e c t o r .
To a f i r s t approximation, we may
the i o n i z i n g events are d i s t r i b u t e d uniformly throughout
assume that
the s e n s i t i v e
volume, and so about 15$ of the events w i l l contribute to the spreading
of the p u l s e - d i s t r i b u t i o n .
5-
Filling
Gas
Like a l l noble gases, neon i s w e l l suited to use
i n an i o n i z a t i o n chamber .
E l e c t r o n attachment does not occur, and electron
c o l l e c t i o n can be achieved, even at high pressures, with reasonable
c o l l e c t i n g voltages.
Although the purest obtainable gas was used, the p u r i f i e r
on the chamber was s t i l l necessary, because of occluded gases on the
chamber walls.
Outgassing of such a chamber i s very d i f f i c u l t ,
though
some attempt was made by evacuating the chamber while baking i t under
a heat lamp, p r i o r to f i l l i n g .
The manufacturers ^ of the neon used stated the l i m i t s of
# The Matheson Co., East Rutherford New
Jersey; private communication
16.
impurity to be
0.2$
Helium
Nitrogen
0.02$
Other less than
0.02$
The three stable neon isotopes were present i n the normal r a t i o s :
Ne
2
0
Ne '
1
Ne *
a
90.51$
0.28$
9.21$
(MATTAUCH and FLAMMERSFELD, 1 9 4 9 )
It was desirable to have as high a pressure of neon as possible,
for
two reasons.
F i r s t , high pressure increased the number of
target atoms, making disintegrations more probable.
Second, the
higher pressure shortened, the tracks of the i o n i z i n g p a r t i c l e s ,
so increasing the number of events which are completely confined
to
the sensitive volume, r e l a t i v e to those which are only p a r t i a l l y
i n this region.
The pressure, may be limited i f the e l e c t r o s t a t i c
f i e l d i s not s u f f i c i e n t to achieve electron c o l l e c t i o n , but this
consideration d i d not a f f e c t the present experiment.
A f t e r evacuation, the chamber was f i l l e d by equalizing
pressures between the chamber and the neon storage cylinder.
The
gas was passed through a l i q u i d nitrogen cold-trap during f i l l i n g .
The f i n a l pressure was 4 ? 3 - 5 cms. mercury (at 22*C) and hence the
to,
.
n
range of the alpha p a r t i c l e s prodticed i n the Ne (n«;0 reaction
with 2 . 6 8 Mev. neutrons'would be 0 . 2 cms., a r e l a t i v e l y small d i s tance compared to the dimensions of the sensitive volume, (roughly
?.5
*9.*
15.
cms.)
17.
6.
Operating Conditions
In the present chamber, the voltage was
limited
by sparking at the high-voltage kovar seal inside the chamber, which
occurred at about - 9 5 0 v o l t s .
This was more than adequate to achieve
saturation, i . e . complete electron c o l l e c t i o n , without recombination.
In operation, a plate voltage of «800 v o l t s and g r i d voltage of - 2 0 0
v o l t s was used.
The plate voltage was obtained from a Dynatron
Hadio Type 200 regulated supply, and the g r i d voltage was
supplied
from the plate by a r e s i s t o r network.
The chamber was stationed with i t s centre-line about
5 1/2 inches from the heavy i c e target.
Jn_this p o s i t i o n the s e n s i t i v e
volume received about l / 3 5 of the t o t a l y i e l d of neutrons, i . e . about
5X 10 * neutrons per second f o r a 150^-amp. beam.
The range of neutron energies received by the s e n s i t i v e volume
was determined by the half-angle subtended at the target by the sensitive
volume, i n t h i s case about 45 degrees.
A value f o r the average neutron
energy was obtained as follows.
The s o l i d angle was treated i n three regions: 0 - 1 5 ,
and 3 0 - 4 5 degrees.
15-30,
A value of neutron energy f o r each region was
calculated from the general formula f o r the angular energy dependence
i n c o l l i s i o n processes (HANSON et a l , 1 9 4 9 ) .
Substituting appropriate
masses and energies f o r 5 0 kev. deuterons bombarding deuterium, the expression f o r neutron energy i s :
E^r ( 0 . 0 1 2 5 ) | ^ c o s ^ - H 9 6
+
cos ^
/cosV+392
Neutron energies f o r the three regions were obtained from this formula
with*s7.5,
2 2 . 5 , and 3 7 . 5 degrees.
The r e l a t i v e number of neutrons i n each region was
then
P L A T E
VIII
W I D E - B A N D
L O W
N O I S E
H E A D
A M P L I F I E R
18.
calculated fa?om the angular d i s t r i b u t i o n function N(^) = B(l>A cos"*/),
which i s v a l i d f o r 5 0 kev. deuterons, using A = 0 . 4 5 (HUHT00N, 1 9 4 0 ) .
In this r e l a t i o n , f& i s measured i n centre-of-mass co-ordinates.
The
angle •& i n the laboratory system i s related t o ^ b y the expression
sin
$
tan •tf.cos 0 4- ST
(SCRTFF, / 9¥
9)
For the present reaction, V = 0 . 0 5 0 4 , and at 45 degrees the correction
i s 2 degrees, which was considered n e g l i g i b l e .
F i n a l l y , adjustment was made f o r the number of target atoms
exposed to each of the three neutron groups, i . e . f o r the f r a c t i o n of
the sensitive volume included i n each of the angular regions.
The f i n a l value obtained f o r the average neutron energy
was 2.68 t 0 . 0 7 Mev.
The error on t h i s figure makes allowance f o r
s l i g h t variations i n deuteron energy, c h i e f l y due to straggling i n
the heavy i c e target.
7.
Pulse Amplification and Measurement
A wide band, low noise head amplifier with an
approximate gain of 100 was mounted d i r e c t l y on the chamber.
VIII gives the c i r c u i t diagram.
Plate
Gain of the f i r s t tube was about 1 0 .
The tube i t s e l f was selected as having the lowest noise of those
available.
The input capacity of the c o l l e c t o r and f i r s t g r i d was
measured by observing the reduction i n height of standard pulses when
ten
a known capacity was connected i n p a r a l l e l and was found to b e y y t . f a r a d s .
The plate voltage was obtained from a Lambda Model 28 regulated supply,
19.
and s t a b i l i z e d DC was used f o r the filaments of the f i r s t
tube and
the ring-of-three.
The output pulses were fed from the cathode follower
through 120 feet of co-axial cable to a Northern E l e c t r i c Type 1444
Linear Amplifier.
This a m p l i f i e r has a gain of about 10 , and gain
s t a b i l i t y better than 1$ over long operating periods.
Provision i s
included f o r up to 33 db. attenuation, variable top and bottom cuts,
and variable duration of output pulses.
Eor optimum signal-to-noise
r a t i o , the amplifier was operated with both top and bottom cuts set
at 5/**-seconds.
The amplified pulses were fed to an 18 channel Marconi
Type
155-935
design).
pulse-amplitude analyser, or "kicksorter", (CHALK EIVEE
Amplitude s t a b i l i t y of the kicksorter discriminators i s stated
as approaching 0 . 0 2 v o l t s under the e x i s t i n g operating conditions.
The
k i c k s o r t e r has a maximum input voltage of 40 v o l t s , and incorporates
a s t a b i l i z e d a m p l i f i e r with a gain of 5 , so that maximum input to the
discriminator i s 200 v o l t s .
The k i c k s o r t e r channels were set up using pulses from a
Standard Pulse Generator.
The amplitude of these pulses was stable to
about 0 . 0 1 v o l t s over long periods.
These pulses were fed through the
k i c k s o r t e r amplifier when adjusting the channels.
The minimum p r a c t i c a l channel-width was found to be 0 . 2 V i n
terms of the Standard Pulse Generator.amplitude.
This channel width
corresponded to about 0 . 0 4 5 Mev. d i s i n t e g r a t i o n energy, which provided
adequate
resolution f o r the present experiment.
P L A T E
P U L S E -
H E I G H T
I X
D I S T R I B U T I O N
lOOOl
F R O M
P O L O N I U M
A L P H A S
NO. O F
COUNTS
SOO
5.25
DISINTEGRATION
ENERGY ,
MEV.
20.
8.
Energy Calibration
A series of i o n i z i n g events of the same energy i n
the chamber should, within s t a t i s t i c a l l i m i t s , produce voltage pulses
of
the same amplitude.
Moreover, the pulse amplitude
depend l i n e a r l y on the d i s i n t e g r a t i o n energy.
i s expected to
The energy scale was
JL/o
calibrated by a Po
to
source i n the chamber.
This source was attached
the inner surface of the high voltage plate, and covered with a
t h i n aluminum collimator. The alphas from Po
and an air-range of 3.842 cms.
energy,
This corresponds to a range of about
1 cm. i n 6 l / 4 atmospheres of neon.
ion
have 5«3 Mev.
Since i t requires 29*3 ev. per
p a i r i n neon (STETTER, 1943), such p a r t i c l e s should produce about
290 *volt pulses on the l O O ^ i f a r a d s capacity of the c o l l e c t o r and
first
/
grid.. The signal-to-noise ratio f o r those polonium alphas was about
20:1.
A l l polonium alphas are stopped i n the sensitive volume, and
none reach the unshielded region between the grid and the c o l l e c t o r .
Plate IX shows a t y p i c a l d i s t r i b u t i o n of pulses from the
polonium alphas.
1.3$
The half-width of this d i s t r i b u t i o n i s 68 kev., or
of the t o t a l energy.
Ionization produced by disintegrations i n the f i l l i n g
gas
d i f f e r s somewhat from that produced by alpha p a r t i c l e s from radioactive sources i n the chamber.
In the case of a neon d i s i n t e g r a t i o n ,
the reacion energy i s shared between an alpha p a r t i c l e and a r e c o i l i n g
oxygen ion, both of which w i l l produce i o n i z a t i o n i n the f i l l i n g
gas.
For the alpha p a r t i c l e s , the r e l a t i o n between i o n i z a t i o n and
energy i s l i n e a r down to low energies (region of 100 kev.) a f t e r which
the proportionality may break down, due to electron attachment and
i n e l a s t i c non-ionizing c o l l i s i o n s .
The variations i n t o t a l i o n i z a t i o n
produced w i l l however be rather small, so that the determination of
alpha p a r t i c l e energies i s quite accurate.
21.
In the case of the larger fragments, the mechanism
of i o n i z a t i o n i s not c l e a r l y understood, though some investigations
have "been made on r e c o i l p a r t i c l e s i n cloud chambers; (see WBENSHALL,
1940)
and discussion and references given by WILKINSON
(1950)).
Two
points must be considered with regard to energy determination f o r such
particles.
F i r s t , the energy loss may be less uniform than f o r alpha
p a r t i c l e s , so that the amount of i o n i z a t i o n released by single energy
events i s somewhat more random than f o r alphas.
In this case, the r e s u l t
would be a spreading of the pulse-height d i s t r i b u t i o n .
Secondly, the energy c a l i b r a t i o n i s based on the energy loss
of Po
alphas i n neon, i . e . 2 9 . 3 ev. per i o n p a i r .
If the energy loss
per ion p a i r f o r oxygen ions has some other value, the apparent d i s i n t egration energy w i l l be i n error.
The l i n e a r i t y of the detecting system was tested by the
following method.
Pulses from the Standard Pulse Generator were
attenuated and fed into the head amplifier through the g r i d - c o l l e c t o r
capacity.
A f t e r amplification, they were displayed on the kicksorter,
where they appeared i n not more than two channels.
Pulses of the same
amplitude, without attenuation, were then fed d i r e c t l y into the kicksorter amplifier, with the same discriminator settings.
was carried out f o r a range of pulse amplitudes.
This procedure
Comparison of the
two sets of kicksorter readings showed the system to be l i n e a r to
within 1 $ .
9.
Experimental Procedure
Except when shut-downs of several days duration
were anticipated, a l l e l e c t r o n i c equipment, with the exception of the
22.
c o l l e c t i n g voltage on the chamber, ran continually, thus maintaining,
the greatest s t a b i l i t y .
The polonium alphas were observed f o r at least an hour at
the beginning of each run.
A fresh target of heavy ice was then
prepared, and the kicksorters set to cover the desired part of the
energy spectrum.
peak was
At the end of the run a second polonium alpha-
taken to ensure that the gain of the amplifiers, and conditions
within the chamber were unchanged.
The calcium p u r i f i e r was heated f o r several hours at intervals
of a few weeks.
The f i l l i n g gas was thus kept free of contamination, such
as occluded gases from the walls of the chamber.
P L A T E
X
2000-,
C O M P L E T E
E N E R G Y
S P E C T R U M
NO. O F
COUNTS
IOOO
2.0
2.5
DISINTEGRATION
3.0
ENERGY,
3.5
MEV.
4.0
4.5
23.
>
III.
A.
RESULTS AMD DISCUSSION
EXPERIMENTAL RESULTS
Plate X shows the complete energy spectrum.
The spectrum
was examined i n sections, and i n moving the kicksorter from one
section to another, an overlap region of s i x channels was allowed.
The data shown i n Plate X was taken from a number of runs, normalized
to approximately the same-average-number of counts per-channel i n
the overlap regions.
It would have been preferable to normalize
on the basis of integrated neutron f l u x , but a neutron monitor was
not available at the time this data was
collected.
The p o s i t i o n of the large peak corresponds to a d i s i n t e g ration energy of 1.91 * 0.01 Mev.
Examination of the low energy
side of this peak showed no evidence of further structure, down to
the onset of e l e c t r o n i c noise at about 450 kev.
The background over t h i s region i s due mainly to scattered
neutrons from the walls of the laboratory and massive objects i n the
v i c i n i t y of the ion chamber.
Neon r e c o i l s from neutrons scattered
i n the chamber have a maximum energy of about 490 kev.
Some c o n t r i -
bution may also come from disintegrations i n the unshielded region
between the g r i d and c o l l e c t o r of the chamber.
Investigation of the high energy side of the main peak
disclosed a second peak of about 100 times less i n t e n s i t y .
This regio
of the spectrum i s shown in greater d e t a i l i n Plate XI.
P o s i t i o n of
this peak corresponds to a d i s i n t e g r a t i o n energy of 3.16
± 0.03
Mev.
24.
No other structure was observed at energies below
those of the polonium alphas, and no pulses of energies greater
than these were observed.
33.
DISCUSSION
1.
Threshold Energies f o r 3Jast Neutron D i s i n t e g r a t i o n
of Neon
The gridded ion chamber might be expected to detect any
(noc) and (n p) reactions a r i s i n g from the d i s i n t e g r a t i o n of neon
nuclei.
The following disintegrations must then be considered:
Isotope
Reactions
Calculated Qr-Yalue
(n ot )
- 0.54
Mev.
(n p )
- 6.14
»
(n «. )
+ 0.71
Ne
Ne*'
(up)
(n * )
- 5.06
Ne
(n p )
These Q-values have been calculated from the mass tables
of MATTAUCH and 3TMMMERS3TELD (1949).
Such values are only approx-
imate, due to uncertainties i n the masses, but are u s e f u l i n estimating the thresholds of reactions.
25-
2.
Interpretation of the Main Peak
There i s no question "but that the main peak i s
due to the reaction Ne'^Cn*) 0 " \
Q ; - 0 . 7 ? ± 0 . 0 8 Mev.
From the measured energy release,
This reaction has previously been reported as'
follows:
(a)
GRAVES and COON ( 1 9 4 6 ) , using an ion chamber, and ,
photographing pulses displayed on an oscilloscope, obtained a value
of Q,= - 0 . 6 Mev., and estimated the cross section to be about 5
barns.
No information v?as given on the number of events
railli-
recorded.
Their Q-value was l a t e r revised to - 0 . 8 to - 0 . 8 5 Mev. i n a p r i v a t e
communication to JOHNSON et a l (see below).
(b)
SIKKEMA ( 1 9 5 0 ) used a large proportional counter
filled
with 2 atmospheres of neon, and recorded pulses on a photographic
strip.
The reaction energy was observed as - 0 . 6 Mev.
The published
curves show peaks 200 counts or less high, with a background as high
as 75 counts.
(c)
JOHNSON, B0CKELMAN and BARSCHALL ( 1 9 5 1 ) used a pro-
p o r t i o n a l counter containing 3 0 atmospheres of neon, and operating
at a gas amplification of about 5 -
They obtained the values Q=-0.75
t 0 . 0 5 Mev. and cross section about 20 m i l l i b a r n s .
The published
curves show peak about 100 counts high.
It i s of interest to r e c a l l the considerations of WILKINSON ( 1 9 5 0 ) mentioned previously, regarding the use of pure noble
gases i n proportional counters.
The formation of metastable states
i n these gases, and the emission of photo-electrons from the cathode
of the counter may produce an objectionably high background.
Such an
26.
effect was observed by SIKKEMA, though he attributed i t to hydrogen
or helium contamination i n the ion chamber.
In the present experiment, the 0„-value' was obtained with
1% counting s t a t i s t i c s , much better than previously reported.
The mass spectrograph data of EWALD (1951)
gives the
isotopic masses:
0 = 17.004,507*0.000,015
/r
Ne' = 19.998,771*0.000,012
17
The 0 mass i s f a i r l y w e l l agreed upon, and from the
above value, and the observed Q,-value i n the present experiment,
the.mass of Ne " i s calculated-to be 19.998,628 ± 0.000V845.
-
BUECBHER et a l (1949) have observed the protons from
0 (dp)0
i n a magneticspectrometer and measured an excited state
of o' at 0.876 * 0.009 Mev.
7
The gamma ray from t h i s l e v e l ; to ground
has been reported by ALB0EGER (1949).
POLLARD and ^DAVIDSON (1947)
have observed the same l e v e l through the reaction N («p)0
, and
i n addition a second particle-groug, which may indicate a second
l e v e l at 1.6 Mev.
,;
However, i t i s possible that the p a r t i c l e s observed
were deuterons from N (<*d)0
.
It i s generally assumed that the grounds-state of Ne has
zero spin and even p a r i t y , (HORNYAK , 1950).
BUTLER, (1950) i n an
analysis of the work of BURROWS et a l (1950) on the angular d i s t r i b u t i o n
H .
.
17
of protons from 0 (dp}0
5/2
/7
concludes that the ground-state of 0 i s
or 3/2., even, and that the f i r s t excited l e v e l i s 1/2 , even.
The spin value of 5/2 f o r the ground state has been confirmed by the
nuclear resonance experiments of ALDER and YJJ, (1951) , and by
GESCHWTND et a l , (1952) from microwave spectroscopy.
Assigning these values of spin and parity, the ground-state
t r a n s i t i o n of Ne (n<* }0
may be viewed as follows:
27.
JO
Spin:
Parity:
i
0
+
Ne
n
Ne
5/2
0
g
even
even
Angular
momentum:
0
1 =-0
1= 1
0
^ (even)
1
' | (odd)
.3/2 (odd)
1 =2
1
3/2 (even).
= 2.
5/2 (even).
1 = 3>
f5/2 (odd)
[7/2 (odd)
The arrows indicate the possible t r a n s i t i o n s f o r neutrons
up to 1 = 3
and alpha p a r t i c l e s up to 1 = 4 . In view of the cross
section of the reaction, i t seems u n l i k e l y that l a r g e r 1-values need
be considered.
17
The t r a n s i t i o n to the excited l e v e l i n 0
, which was not
observed with the 2.68 Mev. neutrons, has the p o s s i b i l i t i e s :
17*
JO
Ne ii*
Ne
n
Spin:
Parity:
i
0
0
z
3
even
even
Angular
momentum:
1-0
1=1
* -a (even)
i - (odd)
(.3/2 (odd)
1=2
(3/2 (even)
1
0
* 1
1
^
1 -2
[5/2 (even)
1 =3
'5/2 (odd)
-^1 = 3
(7/2 (odd)Thus, assuring up to F-wave neutrons,
(1 = 3) , leads to
SI*
seven possible states f o r the compound nucleus, Ne
the ground state t r a n s i t i o n i s forbidden f o r the
states.
. O f these the
even and -J- odd
28.
•It was
hoped that the lack of excited l e v e l s of 0
in
the present experiment would give some i n d i c a t i o n as to which
state of Ne
appeared.
However, the p e n e t r a b i l i t y of the
p o t e n t i a l b a r r i e r f o r the alpha p a r t i c l e s must also be taken into
account. Calculation of the p e n e t r a b i l i t y §
indicates that
the
i n t e n s i t y of the excited-state t r a n s i t i o n would be approximately
10
times that of the ground-state t r a n s i t i o n . A reaction of
such low i n t e n s i t y would not have been detected. Therefore, the
non-observance of excited l e v e l s of 0
regarding
y i e l d s no
information
the state of the compound nucleus, Ne
3. . Interpretation of the Minor Peak
No
workers.
such subsidiary peak has been reported by other
This i s not surprizing i n view of i t s low i n t e n s i t y ,
and the s t a t i s t i c s which they obtained on the
Ne
ln« )0
reaction.
There are three possible causes f o r t h i s peak:
(a) A group of neutrons of higher energy than the main group
from
,
H (dn)He
.
a/,
,
'
lb) A charged-particle
reaction i n Ne
or Ne
(c) A charged-particle
reaction i n some impurity i n the
filling
gas.
The p o s s i b i l i t y of higher energy neutrons has been previously
# see f o r example BETHE, H. A.
R. M. P.
9.-164--1937
£9-
discussed, and
shown to "be most u n l i k e l y .
The only energetically possible reaction i n We
*/.
for
the neutrons used i s Ne
,
(n * ) 0
or Ne
/%
.
For this reaction, the
observed energy release gives Q, - *0.48 * 0.10.
EWALD ( 1 9 5 1 )
gives
the mass values:
o r
18.004,875 * 0.000,013
He's 21.000,393* 0.000,022 '
The above Q,-value corresponds to a mass of
Ne
- 21.000,339 * 0.000,068
Using the cross section of JOHNS Oil et a l ( 1 9 5 1 ) of 2 0 m i l l i b a r n s
for
Ne
(n«)
0
, the i n t e n s i t y of t h i s peak would indicate a cross
section of about 65 millibarns f o r Ne
The
(n«0
0
l i m i t s of inrpurity f o r the neon were stated as 0.2$
and 0.02$ nitrogen, while nitrogen, oxygen and water vapour may
been present in' the chamber p r i o r to f i l l i n g .
helium
have
The e n e r g e t i c a l l y
possible reactions f o r these elements are
/'(np
l/"(n
0"
«
(n «
) 0 "*
QrfO.63
Mev.
) B "
q - - 0.28
"
) C
Qr - 2.31
/S
o " (n « ) c "
The f i r s t
has
1950;
•
+1.73
11
"
of these reactions i s well known, and
been c a r e f u l l y established as + 0 . 6 3 0 * 0 . 0 0 6 Mev.
H0RNYAK et a l 1950) .
T
i t s Q,-value
(FRANZEN et a l ,
h i s value l i e s close to that of the
minor peak, although outside the
probable
error, which i s believed to
be generous.
Por 2 . 8 Mev. neutrons, the cross section f o r N
i s 4 0 millibarns (BALDINGER and RUBER, 1 9 3 9 ) .
(n p )C
h i s would indicate
T
more than 0 . 4 $ nitrogen i n the chamber, though such an estimate i s
very approximate, as this cross section may not hold f o r 2 . 6 8 Mev.
neutrons,
0.1$
^his amount of nitrogen should be accompanied
oxygen.
WILSON et a l ( 1 9 5 0 ) have stated that 5 parts per m i l l i o n
of oxygen i n 6 atmospheres
entirely.
by about
of deuterium w i l l prevent electron c o l l e c t i o n
It would be expected therefore that 0 . 1 $ oxygen would have
noticeable effects i n 6-£ atmospheres
of neon.
The calcium p u r i f i e r
i s expected to remove both nitrogen and oxygen.
If nitrogen were present i n the chamber, the reaction N
should also be observed.
(n «)B
The cross section i s 160 millibarns f o r 2 . 8
Mev. neutron, and so a second small peak should be observed at a d i s integration energy corresponding to Q, -
- 0 . 2 8 Mev.
Some i r r e g u l a r i t i e s
have been noted i n this region of the spectrum, but at the time of
writing, s u f f i c i e n t data was not available to determine whether such
a peak e x i s t s .
4.
Summary of Results
This table shows the Q-values f o r Ne ( n * ) 0
from various experimental investigations, and from calculations based
on mass spectrograph data.
Q,-values
GRAVES and COON ( 1 9 4 6 )
- 0 . 8 0 to.-0.85.
SIKKEMA ( 1 9 5 0 )
-0.6
Mev.
"
JOHNSON et a l ( 1 9 5 1 )
-0.75
Present experiment
-0.77
0.08 "
MATTAUCH and ELAMMERSPELD ( 1 9 4 9 )
-0.54 ±
0.12 "
EWALD ( 1 9 5 D
-0.64±
0.05 "
i
0.Q5 "
Mass Spectrograph
31.
C. FURTHER INVESTIGATIONS
The extension of t h i s i n v e s t i g a t i o n i s now being a c t i v e l y
pursued i n the f o l l o w i n g ways:
(a) Reaction energies are t o be determined more e x a c t l y .
by reducing the error on the neutron energies.
This w i l l be done
by moving the i o n chamber away from, the t a r g e t , thus reducing the
s o l i d angle subtended by the s e n s i t i v e volume, and so decreasing
the
angular v a r i a t i o n of neutron energy.
(b) The region of the spectrum corresponding t o a r e a c t i o n
energy of -0.28 Mev. i s to be c a r e f u l l y re-examined. I f the presence
of a second small peak i s e s t a b l i s h e d , i t would seem to i n d i c a t e that
both small peaks are due to nitrogen i n the i o n chamber.
T h i s would
however r a i s e two i n t e r e s t i n g p o i n t s . F i r s t , how could t h i s amount of
nitrogen appear i n the i o n chamber, and remain i n s p i t e of the calcium
purifier?
Second, i f the minor peak i s due to N (np)C , why i s the
r e a c t i o n energy not c l o s e r to the accepted value?
'
(c) I n v e s t i g a t i o n s are to be c a r r i e d out w i t h thermal neutrons,
•t
3
by moderating the H (dn)He
neutrons wifch p a r a f f i n .
T h i s should
eliminate the main peak, and move the minor peak to the low-energy
end of the spectrum, just above the noise.
Some increase i n the
i n t e n s i t y of t h i s peak would be expected, i f the r e a c t i o n cross
s e c t i o n f o l l o w s the l / v law.
The s h i f t i n the p o s i t i o n of t h i s peak
w i l l give an a d d i t i o n a l check on the l i n e a r i t y of the a m p l i f i e r system.
(dj Some i n i t i a l attempts to thermalize the neutron f l u x
1
were not completely successful i n that the main peak s t i l l appeared,
though w i t h much lower i n t e n s i t y . I f the moderation cannot be improved,
si
//
i t i s proposed to use neutrons from the V (pn)Cr r e a c t i o n , which are
3 0 / 7
below the threshold f o r Ne (n<*)0 •
(e) The r e a c t i o n s w i l l be examined at higher neutron energies,
u s i n g neutrons from H (dn)He
and H (dn)He
, w i t h deutrons accelerated
by the UBC Van de G-raaff Generator, which at present i s operating at
energies up to 2 Mev.
32..
(f) I t would be of i n t e r e s t to introduce some 2% of
nitrogen i n the chamber and repeat the observations.
For a l l these experiments, a neutron monitor i s
d e s i r a b l e . A neutron counter, of the type designed by HANSON and
McKIBBEN (1947) i s at present under construction i n t h i s laboratory.
33.
IV.
CONCLUSIONS
The Q-value of the reaction Ne (n°<-)0
as -0^77*0.08 Mev.
has been measured
The 1% counting s t a t i s t i c s obtained are
considerably better than those reported i n previous investigations
of t h i s reaction.
Careful search of the energy spectrum gave no evidence
n
'Jf o r the appearance of excited states of 0 , using 2.68 Mev. neutrons.
At least one such s t a t e , i s known, (0.87 Mev.) .' Calculation of the
p e n e t r a b i l i t y of the p o t e n t i a l b a r r i e r f o r the alpha p a r t i c l e s
corresponding to t h i s l e v e l indicates an i n t e n s i t y approximately 10
times that of the ground-state t r a n s i t i o n , at t h i s neutron energy.
A reaction of such low i n t e n s i t y would not have been detected i n
the present experiment.
A subsidiary peak was observed at a reaction energy
corresponding to a Q,-value of +0.48*0.10 Mev.
-2/.
.
There i s some
IS
evidence that the reaction is' Ne ( n ^ j O , but the reaction
N (np)C may also be responsible, i f nitrogen is" present i n the
ion chamber. Further i n v e s t i g a t i o n i s necessary to c l a r i f y t h i s point.
34.
V.
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