DELAYED PROTON
PROTONS
FOLLOWING
AND
DECAY
S FOLLOWIN
G 21Mg
''Mg AN
D 25Si
2551 DECA
Y
R.
R.
J. C.
C.
l\IcPHERSON*
MCPHERSON* AND J.
HARDY
RadiationLaboratory,
Laboratory, McGill
McGill University,
University, Montreal
Montrealt
Foster Radiation
Received
Received August
August 10, 1964
ABSTRACT
ABSTRACT
The delayed-proton
precursors 21Mg
produced by proton
delayed-proton precursors
^^Mg and
and 25Si
^^Si have been produced
proton
bombardment
targets of sodium, magnesium,
magnesium, aluminum,
aluminum, and silicon dioxide
dioxide
bombardment of targets
internal beam
synchrocyclotron. Half-lives
Half-lives of 118 ±
zh 4 and
and
in the internaI
beam of the McGill synchrocyclotron.
225 ±
± 6 msec respectively
respectively were measured,
measured, and a spin-parity
spin-parity assignment
assignment of
225
of
(5/2)+ has been made to the ground
ground state
state of 21Mg.
^iMg. Strong
Strong lines in the delayeddelayed(5/2)+
proton
spectra have been attributed
attributed to decay of the expected
expected first TT == :3/2
3/2
proton spectra
states in 21Na
^iNa and
and 25Al,
^SAI, fed by superallowed
superallowed f3)3 transitions
transitions from
from the respective
respective
states
precursors
^^Mg
precursors 21
Mg and ^^Si.
25Si.
INTRODUCTIO:\T
INTRODUCTION
A neutron-deficient
Q value
neutron-deficient nuclide, bound in its ground state and having a Q
for {3
jS decay larger than the proton separation
separation energy of its {3-decay
(S-decay daughter,
daughter,
can lead to delayed proton emission from unbound excited states of the
daughter. The energetics of a large number of ne\\'
new neutron-deficient
neutron-deficient nuclides
which meet these conditions have been predicted
predicted using isobaric invariance
principles and known masses of neighboring or mirror nuclei (e.g. Goldansky
Goldansky
Observation of the proton transitions from daughter
daughter
1960; Jaenecke 1964). Observation
identification for n1any
many of these short-lived
short-lived
states provides an easy method of identification
nuclides.
As neutrons are removed from stable isotopes of the elements between
oxygen and calcium, the first-encountered
first-encountered delayed-proton
delayed-proton precursors are the
T
21~Ig, 25Si,
..-\,
T,z == -3/2
- 3 / 2 nuclides 130,
^^O, 17:\Te,
^^Ne, ^'Mg,
2551, 29S,
293^ 33
33^:
^^ and
^nd 37Ca.
^^Ca. Each of these
proton-induced
can be produced
produced by removal of at most three neutrons in proton-induced
reactions on the lightest corresponding stable isotopes. For the members of
of
this group heavier than 17Ne,
^^Ne, the possibility exists of having a large {3-decay
/S-decay
component corresponding to superallowed
superallowed transitions to the first TT == 3/2
3/2
component
states of the Tz
—1/2 daughters. Narrow
transitions
T z == -1/2
Narro\v proton lines due to transitions
from the T
2" == 1/2
1/2 admixture in these states should then dominate the delayed
proton spectra.
Delayed
e, 21^^Mg,
~\lg, and
Delayed protons attributed
attributed to four of these nuclides (130,
(^^O, 17X
^^Xe,
25Si)
2^Si) have been observed by Barton
Barton etet al. (1963). In each case, poorly resolved
*Now
*Now at
at Chemistry
Chemistry Department,
Department, Brookhaven
Brookhaven )Jational
National Laboratories.
Laboratories.
tThe
fThe laboratory
laboratory was formally
formally named
named The John
John Stuart
Stuart Foster
Foster Radiation
Radiation Laboratory
Laboratory by order
order
Board of Governors
Governors of McGill
McGill University
University in June, 1964.
of the Board
Canadian
Canadian Journal of Physics. Volume -13
43 (January.
(January, 1965)
1
2
CANADIAN JOURNAL
JOURNAL OF
OF PHYSICS.
PHYSICS. VOL.
VOL. 43,
43. 1965
19G5
CANADL\N
ters. A
were fifitted
in the {3-decay
/3-decay daugh
daughters.
A Iater
later
proton groups \vere
tte d to kknown
no\\!.n 1levels
eve lS 'ln
. .
\vork
work, also from this laboratory (McPherson, Hardy, and Bell 1964), provldes
provides
a m;re
more detailed study of the 17Ne
^^Ne decay. The present paper descri~es
describes t~e
the
results of il11proved
21~Ig, produced ?y
improved nleasurelnents
measurements on ^'Mg,
by proton react1o~s
reactions ln
in
targets of sodiunl
sodium and 111agnesium,
magnesium, and on 25Si,
^^Si, produced ln
in targets of alumlnum
aluminum
and silicon. \Vith
With improved knowledge of the proton spectra, some parts of
the decay schemes of 21^iMg
~ rg and 25Si
al. (1963) have
^^Si proposed by Barton etet al.
modified.
been nlodified.
EXPERIMENTAL TECH:\IQUE
TECHNIQUE
EXPERIMENTAL
Proton bombardments of thin target foils \yere
were perfonned
performed in the internaI
internal
beam of the :\IcGill
McGill synchrocyclotron. Figure 1 sho\vs
shows the configuration
configuration of a
thin target foil and solid-state detector mounted on the end of a radial probe.
TARGET
FOll
Experimental configuration
configuration as mounted
mounted on the end of a radial probe and inserted
FIG. 1. Experimental
into the internal
internaI cyclotron
cyclotron beam.
inserted into the circulating
circulating beam
beam at
at radii corresponding
The probe could be inserted
proton energies between
bet\veen 30 and 98
981\IeV,
and depending
depending on the radius
to proton
MeV, and
chosen for bombardment,
bombardment, the beam
beam traversed
traversed the target
target from
froln 10 to 50 times,
effective current
current of some
sonle tens of
of microamperes.
nlicroamperes. The
The target
target foil was
\vas
giving an effective
suspended
suspended 6 cm from
from the detector
detector and
and intercepted
in tercepted the proton
proton beam
beanl in the
2
median
plane.
A
silicon
surface
barrier
detector
of
200-mm2
area
was
enclosed
median
surface barrier detector
200-lnln area ,,"as enciosed
in an aluminum
alul11inum shield suspended
suspended below
belo\v the median
median plane and
and was
\\"as connected,
through
through the probe, to an
an Ortec
Ortec charge-sensitive
charge-sensitive preamplifier
preamplifier outside
outside the region
of
of strongest
strongest magnetic
magnetic field. The
The detector
detector depletion
depletion depth
depth of
of 300 microns
allowed
allowed measurement
measurement of
of full
full energy
energy for protons
protons up to 6 MeV.
:\Ie\Y. Aluminum
absorber
absorber foils over
over the detector
detector were
\vere used to protect
protect it
it from
from unwanted
unwanted ion
33
McPHERSON
M C P H E R S O N A);D
A N D HARDY:
H A R D Y : 21Mg
2iMg A);D
A N D 25Si
2551 DECA
DECAY
bombardment
bombardment and, by varying their number, to measure the specifie
specific ionizaionizaparticles.
tion of the detected particles.
Counting \yas
was performed
performed bet\veen
between repetitive bursts-typically
bursts—typically 40 msec long
long
-of
—of normal cyclotron operation. A clock-controlled counting period was
was
which was long enough to allow dissiinitiated following a 100-msec delay \vhich
shorter
pation of beam storage effects in the cyclotron; it was found that shorter
delayeddelays resulted in a large background continuum appearing under the delayedperiod,
proton spectrum. Usually the spectrum, taken over the total counting period,
was stored in one haH
half of a 256-channel analyzer. For lifetime runs, however,
however,
\"as
was divided accurately into four equal intervals and the
the
the counting period \"as
quarters
spectra corresponding to each were stored sequentially in the four quarters
the
of the analyzer memory. Thus four-point decay curves for the peaks in the
could
depended
spectrum cou
Id be obtained. The duration of the counting period depended
on the lifetime of the delayed protons and on \vhether
whether or not decay curves
curves
were being sought, but it \"as
was usually less than haH
half a second.
second.
\vere
Pulses from a mercury pulse generator were fed onto the input of the preamplifier, and ,vere
were used in conjunction
conjunction with the 210pO
^^^Po aa line to calibrate the
5.3-MeV a's
a's \vas
was 76
energy response of the system. The resolution for the 5.3-:\IeV
ke
\
F\YH:\I.
ke\^ F W H M .
T
EXPERIMENTAL RESULTS
JIg: Identification
Identification
-Ll/g;
delayed-proton spectrum
spectrum obtained
obtained following the bomFigure 2 shows the delayed-proton
2
bardment
2.3-mg/cm2 foil of natural magnesium; the total counting time
bardn1ent of a 2.3-mg/cm
21
1800
21 Mg
1600
DELA YED
PROTONS
DELAYE
D PROTON
S
TARGET
g
TARGE T 2-3mg/cm
2-3mg/cm22 MMg
1400
1200
1"0
(f)
V
~
v
Z
1000
3
:::>
,;.,
0
v
o
0
o
u
800
600
600
40040
200
200
40
150
50
CHANNEL
FIG. 2. Spectrum
Spectrum of
of delayed
delayed protons
protons following
following the
the decay
decay of
of "Mg.
21Mg. The
The energy
energy of
of eac
each
FIG.
h
proton
proton peak
peak is shown
shown in MeV, corrected
corrected to center
center of
of mass.
44
CANADIAN JOURNAL
JOURNAL OF
OF PHYSICS.
PHYSICS. VOL.
VOL. 43,
43. 1965
19G5
CANADIAN
was 25 minutes. A sonlewhat
somewhat thicker sodium target yielded a silnilar
similar spectrum,
spectrum,
were necessarily less weIl
well resolved. Both targets ,,'ere
were
although the peaks ,vere
bombarded at incident proton energies from threshold to 85 ~\Ie\r,
MeV, and at
at
each energy the delayed spectrum remained qualitatively unchanged. Since
all major
major peaks had the same
same
lifetime measurements also indicated that aIl
half-life, ,,'ithin
within experimental error, it may be concluded that the delayed
delayed
all follow the (3
decay ofof the
the same
same nuclide.
nuclide.
protons aU
{3 decay
Yield curves were plotted for both targets by measuring the production
production
of the main peak as a function
function of the target radius in the cyclotron. Radial
oscillations in the cyclotron beam make the actual bombarding energy at a
given target radius less than the nominal cyclotron energy at that radius by
well determined (2 to 5 MeV), but relative threshold
threshold
an amount that is not weIl
measurements are accurate to better than 2 MeV. Since the proton peaks
fluorine target was used (McPherson et
of Fig. 2 were not observed when a fluorine
et al.
1964), the nuclide responsible for the present activity must be an isotope of
delayed-proton
magnesium or sodium. Any isotope of sodium that can be a delayed-proton
precursor must have a production threshold from stable magnesium lower
than that from sodium. In fact, the production threshold from magnesium
was 8 MeV greater than that from sodium, and was approximately
approximately 50 l\IeV.
MeV.
These results are compatible only with the reactions '^Mg{p,
24Mg(p,d2ny'\lg
d2n)21 ~Ig and
3n)2iMg, whose calculated laboratory
laboratory energy thresholds, based on the
23Na(p, 3n)21:\1g,
estimated Q^^
db 0.3) MeV
^iMg made by J
Jaenecke
estimated
Q~+ of (13.0 ±
~Te V for 21l\1g
aenecke (1964), are
zb 0.3) and (38.9 ±
=b 0.3) MeV respectively. This verifies the assignment
(48.6 ±
^iMg by Barton etet al. (1963).
(1963).
of this activity to 21Mg
21
Mg: Half-life
Half-life
^'Mg:
four-point decay curves appear
appear in Fig. 3 and represent
represent the decay
Typical four-point
obtained for
of individual proton lines as indicated. Similar
Similar curves have been obtained
other prominent
prominent energy peaks and, together
together with the three illustrated, allow
other
adopt the value (118 d=
± 4) msec for the half-life
half-life of ^^Mg.
21~\Ig.
us to adopt
21 Mg: Energies
Energies and
and Decay
Decay Scheme
Scheme
^'Mg:
appear also in the first
column of Table 1;
The energies shown in Fig. 2 appear
first column
I;
they
are
corrected
to
center
of
mass
and
have
the
indicated
uncertainties.
they
corrected
center
indicated
TABLE I1
TABLE
proposed new levels of ^iNa
21.Na as
~s indicated
indicated by the experimental
experimental data; three
Known and proposed
four proposed
proposed levels appear
appear twice
tWlce since
smce they are indicated
indicated by two proton
proton groups
of the four
Er, (c. of
of m.),
£p
MeV
MeV
E p + (20Ne
(2°Ne +
+p),
Ep-f
p),
MeV
MeV
£p
-f-p),
E p + (20Ne*
(2°Ne* +
p),
MeV
33.44±O.O6
.44±0.06
55.89±O.O6
. 8 9 ± 0 . 0 6 7.S2±0 7.52±O.O6
06
6.J,.8±O.06
44.03±O.O6
. 0 3 ± 0 . 0 6 6.48zL0.06
88.11±O.O6
. 1 1 ± 0 06
44.27±O.O8
.27±0.08
66.72±O.O8
. 7 2 ± 0 . 0 8 8.S5±0 8.35±O.O8
08
44.81±O.O4
.81±0.04
77.26±O.O4
. 2 6 ± 0 . 0 4 8.89±0 8.89±O.OJ,.
OA
7.75±.O.OJ,.
55.30±O.O4
. 3 0 ± 0 . 0 4 7.75±.0.04
9.38±0.'o4
9.38±O.O4
(9.78)±O.1
((5.70)±O.1
5.70)±0.1
((8.15)±O.1
8 . 1 5 ) ± 0 . 1 {9.78)±0.1
(8.35)±O.1
(9.98)±O.1
((5.90)±O.1
5 . 9 0 ) ± 0 . 1 {8.S6)±0.1
(9.98)±0.1
8.90±O.OJ,.
lO.53±O.O4
66.45±O.O4
. 4 5 ± 0 . 0 4 8.90±0.04
10.53=b0.04
9.73±O.O6
7.28±O.O6 9.73±0.06
11.36±O.O6
7.28zfc0.06
11.36^.0.06
tSee
tSee Endt
Endt and
and van
van der
der Leun
Leun (1962).
21Na, MeV
Levels in ^^Na,
Knownf
Knownt
Proposed
77.45
45
6*^2
6.52
88.35±O.O8
.35±0.08
88.90±O.O4
.90±0.04
7"7^^nn4
7.75±O.O4
q'7^innfi
9.73±O.O6
R^^toos
8.35±O.O8
S
QOZOOA
8.90±O.O4
973±006
9.73±O.O6
McPHERSON
A~D
Y
MCPHERSON A
N D HARDY:
H A R D Y : 21Mg
^iMg AND
A N D 25Si
2551 DECA
DECAY
~
2
~
<t
<4 4
w 10
1-
a.
z
(J')
to
5f-
~~
1
I I I
~
~,
5
1
21 Mg DECA
DECAY
Y
-
Mg torget:
MeV)=
116'2
msec
r g e t : TT,, (E(E=4.81
= 4.8 I MeV)
= 116
* 2 mse
c
~~
-
~~ .. _~.
~~3MeVl=123±7msec
=4.03 Me\/) = 123* 7 msec
-
~
I~I
z
~I~
T~2(E=4.81) "7'7msecI~;~
:::>
3
oo
u
<
I-
o
10'
No target
Na
target:: T , ( E = 4.8l) == Il7i7mse c
'2
5~o ______~1
________
0.1
~1
______
0.2 0.3
~1
______
0.3
~1
_
~
L-1~
_______ _
0.4
0.5
TIME-sec
T I M E — se c
FIG.
FIG. 3. Typical four-point
four-point decay curves for peaks in the spectrum of protons following
following
2iMg. The target used, the energy of the peak considered, and the measured half-life
half-life are
21lVIg.
indica
ted for each curve.
indicated
Column 22 in that
predicted energy of the corresponding level
that table gives the predicted
in 21Xa
2°Ne,
2^Xa assuming that
that proton decay proceeds to the ground state of ^^Ne,
and column 3 gives the same quantity
quantity assuming that
that the decay proceeds to
the first excited state of 2°Ne.
^^Ne. The italicized energy values are those which
appear
a. Two of the proton peaks
appear to correspond
correspond to actual level energies in 21N
^iNa.
can be seen to fit approximately
a levels at
approximately the proton decay of known 21N
^^Na
at
6.52 and 7.4,)
~IeV. Three proposed new levels at 8.35,8.90,
~IeV can
7.45 MeV.
8.35, 8.90, and 9.73 MeV
th sorne
ty since their deca
ted
be assigned wi
with
some certain
certainty
decayy to the ground and first exci
excited
states of 2°Xe
20~ e would give rise to proton energies agreeing with observed peaks.
The remaining peak at 5.30 MeV has been tentatively
tentatively assigned to an additional
additional
predominantly to the
proposed level at
at 7.7
7.755 MeV which is assumed to decay predominantly
2°N
e ground state. The proposed decay scheme is shown in Fig. 4 and includes
20Ne
aIl
all kno\vn
known levels for 21Na
^^Na that
that appear in the compilation
compilation of Endt
Endt and van der
Leun (1962).
Kienle and Wien (1963) have assigned a spin-parity
spin-parity of (5/2+)
( 5 / 2 + ) to the ground
state of 219F,
the
mirror
nucleus
of
21
Mg.
If,
as
seems
likely, the ground
^^gF,
'^uMg.
If,
12
state of ^^Mg
21~Ig is also 5/2+,
en within the observable energy range (i.e.,
5/2 + , th
then
levels above about
about 5.3 MeV) the only 21Na
^^Na level with known spin-parity
spin-parity which
must
f3 transition
must be excited by an allowed (3
transition isis the
the one
one at
at 6.52
6.52 l\leV;
MeV; the
the other
other
kno\vn
known level in Table 1
I has not been assigned any spin. Within the same range
of energy, there are six remaining known levels which have apparently
apparently not
been excited; of these, three have known spin-parities which, on the basis of
of
5 / 2 + for 2iMg,
5/2+
21~Ig, preclude their being excited by allowed ^f3 transitions, and the
others have no assigned spin-parity. Thus, the proposed decay scheme is quite
consistent with respect to previously
demand
consistent
previously observed levels and seems to demand
6
CANADLAN
CANADIAN JOURNAL
JOURNAL OF
OF PHYSICS.
PHYSICS. VOL.
VOL. 43.
43, 1965
1965
(13.1)
(5/2 ) +
3.1) (5/2)'^
• ----21 Mg
Mg
T.
T=
(3+
Y2
8 msec
=11118
msec
-----9·73
8'90 (5/2 ) +
- - - - 8·35
_
p
-
-
-
T = 3/2
- - - - 7·75
7'45
- - - - - - 6'52 (3/2 ,5/2 )+
-
20
- - - - - - - - - - 2·45
0+
Ne+p
Ne
+p
- - - - - O'OO
0,00 %
~2 ^+
21
21.
Na
No
T
= r
T=!l2
Vz
FIG. 4. Proposed decay scheme of ^^Mg.
MeV;
21Mg. The TT = 3/2
3/2 level is shown dashed at
at 8.90 l'leV;
the compilation of Endt
Endt and van der Leun
(1962).
an
all other levels and assignments in 21Na
^iNa are from
5 / 2 + for the spin-parity
spin-parity of 21Mg.
^iMg. Oda etet al.
( 5 / 2 + ) as the
5/2+
al. (1959) suggest (5/2+)
^iNa but consider (3/2+),
( 3 / 2 + ) , (5/2-),
(5/2 — ), and
and
spin-parity of the 7.45-MeV level in 21Na
(3/2 — ) also
also as
as possibilities;
possibilities; our
our resuIts
results tend
tend to
to confirm
confirm aa (5/2+)
( 5 / 2 + ) or
or (3/2+)
(3/2+)
(3/2-)
assignment to that level.
level.
energy
aenecke (1960) for the Coulomb energy
Using the semiempirical formula of JJaenecke
difference between 21F
^ip and 21Ne,
^^Ne, it is possible to estimate an excitation energy
energy
^iNe. Accepting isobaric
isobaric
of (8.7 ± 0.5) MeV for the first TT == 3/2 level in 21Ne.
^iKa at about this
this
invariance, one then expects the lowest TT == 3/2 level in 211\a
AT = 0 superallowed ^{3 transition should occur from the TT == 3/2
3/2
energy. A llT
ground state of 21Mg
^^Mg to this analogue level.
level.
Relative log
log ft values for {3/? decay to the levels at 8.90
8.90 and 9,73
9.73 l\Ie\!~
MeV n1ay
ma}' be
be
estimated since
since the energies
energies and
and thus relative intensities
intensities of
of aIl
all their possible
possible
estimated
decays are
are within the range of
of observation.
observation. Assun1ing
Assuming that
that the (3
decay
proton decays
{3 decay
to
the
9.73-MeV
level
represents
an
ordinary
allowed
transition,
one
may
to the 9.73-MeV level represents an ordinary allo\ved transition, one may
suppose that
that its
its log
log ftft value
value is
is between
between 4.0
4.0 and
and 5.0.
5.0. On
On the
the basis
basis of
of con1parative
comparative
suppose
total proton
proton intensities,
intensities, the
the log
for {3/3 decay
decay to
to the
the 8.90-l\Iev
8.90-Mev level
level then
then lies
lies
total
log ft for
between 2.8
2.8 and
and 3.8,
3.8, indicative
indicative of
of aa superallowed
superallowed transition.
transition. The
The same
same conconbetween
clusion follows
follows from
from assuming
assuming any
any other
other of
of the
the observed
observed levels
levels to
to be
be excited
excited
clusion
by
log ft range.
by aIlowed
allowed {3/? transitions
transitions of
of the
the same
same log
range.
77
McPHERSON
AND
HARDY: ^iMg
21Mg A
AND
DECA Y
M
CPHERSON A
N D HARDY-.
N D 225Si
551 DECAY
We thus
thus assign
assign the
the 8.90-AleV
8.90-~IeV level
level as
as the
the lowest
lowest TT = = 3/2
3/2 level
level in
in ^iXa,
21;\"a, the
We
analogue to
to the
the ground
ground state
state of
of ^^Mg.
21~\Ig. This
This assignment
assignment is
is quite
quite consistent
consistent with
\vith
analogue
its nonappearance
nonappearance in
in the
the resonance
resonance data
data for
for proton
proton reactions
reactions on
on -^Xe.
20X e. In
In this
its
energy
energy region,
region, the
the TT = = 1/2
1/2 isotopic
isotopie spin
spin impurity
impurity of
of the
the level
level may
may remain
relnain
small
and
so
it
would
be
only
weakly
excited
in
such
a
reaction.
In
fact
small and
it would be only \veakly exeited in such reaetion. In fact such
such
a weak
admixture,
which
would
result
in
particularly
narrow
proton
peaks,
weak admixture, \vhieh \vould result
particularly narro\v proton
seems
indicated
by
our
results:
the
widths
of
seems indieated by our
the widths of the
the peaks
peaks at
at 4.81 and
and 6.45
6.45 MeV
l\leV
can
be
accounted
for
by
the
resolution
of
the
detector
and
the
energy
spread
ean be aeeounted for by the resolution of the detector and the energy
in
in the
the target.
target. Other
Other peaks
peaks in
in the
the spectrum
speetrum appear
appear to
to be
be marginally
marginally wider,
\vider,
perhaps
perhaps indicating
indieating widths
\vidths of
of a few
fe\v tens
tens of
of keV for
for the
the emitting
emitting levels.
Using the
the value
value of
of 8.90
8.90 MeV
l\IeV for
for the
the lowest
lowest TT == 3/2
3/2 state
state of
of ^iXa
2lXa and
and the
Using
ealeulated Coulomb
Coulomb energy
energy difference,
difference, a revised
revised estimate
estilnate of
of the
the Q^+
Q/3+ ofof 21~Ig
calculated
^^Mg
is (13.1 ±± 0.2) l'deY.
MeV.
25Si: Identification
Identification
'^Si:
Figure o;) shows
sho\vs the spectrum
spectrulu obtained
obtained from
from a 2A-mg/cm'^
2.--!-mg/cm 2 target
target of alu111inum;
Figure
aluminum;
2
target produced
produced a siluilar
~-\t b0111barding
a 3.0-111g/cm
3.0-mg/cm2 silicon dioxide target
similar result. At
bombarding
fro111 threshold
threshold to 85 MeV,
~IeV, the spectrum
spectrum remained
remained qualitatively
energies from
CD
J~
1000
1000-
2S Si DELAYED PROTONS
TARGET-2.4mg/cm 2 AI
800
800-
U)
600-
tH
Z
Z
:::>
3
O
°400
o 400-
20
200-
1
OL-~--~~--~~~~~~~~~~~~~~~~~--~II~O
20
50 6
600 7
700
CHANNEL
FIG . .j.
5 Spectrunl
Spectrum of delayed protons following the decay of 25Si.
^^Si. The energy of each proton
proton
peak is
is shown in MeV, corrected to center of massa
mass.
measurements on the four largest-intensity peaks indieated
indicated
unchanged. Lifetime Ineasurements
significant differenee
difference bet\veen
between them, and it \vas
was concluded tha~
that the delayed
delayed
no signifieant
protons in
in these
these four
four peaks aIl
all follo\v
follow the deeay
decay of
of the saIne
same nuchde.
nuclide. The
The t\VO
two
CA~ADIAN
CANADIAN JOURNAL OF PHYSICS.
PHYSICS. VOL.
VOL. 43,
43. 1965
88
lovv-intensity
V were not sufficiently
low-intensity peaks at
at 3.89 and 5.32 l\Ie
MeV
sufficiently above background to allow positive identification
identification of their source. Since the spectrum
from a n1agnesium
magnesium target \vas
was completely
completely different,
different, the nuclide responsible
for the activity in Fig. 5 is restricted to an isotope of either aluminum or
major peak from both targets
silicon. Yield curves for production
production of the major
showed that
that the production
shovved
production threshold from silicon was 8 MeV greater than
approximately 50 MeV. This case is similar to that
from aluminum and was approximately
for 21l\Ig,
\vith the reactions 28Si(p,
2^Mg, and the results are only compatible with
^^Si(p, d2n)25Si
and 27AI(p,
^^Al(p, 3n)25Si,
3n)2^Si, \vhose
whose calculated
calculated laboratory
laboratory energy thresholds, based on
(2/3+ of 25Si
^ssi (12.6 l\IeV),
MeV), are (48.9 ± 0.5) and
the estimate of Jaenecke for the Qf3+
± 0.5) l'deV.
MeV.
(39.2 ±
25Si:
Half-life
255i.- Half-life
Typical decay curves are sho\vn
shown in Fig. 6. From such measurements we
adopt the value (225 ± 6)
25Si.
6) msec for the half-life
half-life of ^^Si.
11
2~
10
10"
~~~
~
25Si
DECAY
2^Si DECA
Y
Aluminum Target
Target
Aluminum
z
TI (E=4.28Mey)=
225t44 mse
msecc
(E = 4.28Mev)= 225t
Y2
TI (E=3.46Mey)
5
<
Y2
'230~5msec
.~
~I
UJ
o.
l
»-
..J
~
~
o
~
~
o
5
TI (E=4.89Mev)= 20St:40msec
:12
~
2
10'
1.0
T'ME-sec
FIG. 6. Typical four-point
.
5^JIG
four-point decay curves for peaks in the spectrum off protons following
25Si.
peak considered
l·fspect:ud~
0
dPrfotons
^Si. The
The energy
energy of
of the
the peak
considered and
and its
its measured
measured half
half-life
are ln
indicated
o r eac
^ c hhfollowmg
curTe.
- 1 e are
Ica te for
curve.
25Si:
"^^Si:Energies
Energiesand
and Decay
DecayScheme
Scheme
energies (corrected to center
f mass)
h·IC h
Table II shows .the proton
which
.
cen t er of
0
mass ) w
were taken from Fig.
FIg. 5. Llke
c 0 1umns 22 an
d 33 th
1evel1
Like Table 1I,' it shows l·n
in columns
and
t he
e leve
McPHE
RSON AXD HARDY
MCPHERSON
HARDY:: 21Mg
«Mg AXD
AND 2sSi
.^si DECAY
9
9
energ ies in 25AI \vhich vvould
l'
decay
to the ou
res~ t ln the obser
ITJ^to^:^^^^^^^
^'^
^ b - ved
- d proto
protonn peaks
peaks,, suppo
supposing
gr nd or first excIted state, respe ctivel y of 24~Ig T,yo of sing
protonn pea
peaks
t th
t h . 7d TT"^ f h
' ' ^ ' ' ' respectively,
of - M g . Two of the
the
proto
k s ffit
. (3/2) level
,~.
. e ecay 0 t e kno,v n spIn
at
7.14
~1e\T.
the
two
t C ssma
L l:IlfTh
r l i '^l^^'^^y
spin (3/2) level at 7.14 MeV; the
peaks
. the
factknown
f rom 25AI,
' If "the y are ^fln
'
could fit an uncer tain level
at
77 .557le·.
1\1
M9T \V ThT e rema
' "^ lnlng "t,vo
" ^"
''''
^^^"^
"^^'
I
d
fit
an
uncertain
levd
at
. nce of a ne,v level
are consI. stent '\1. th the eXIste
at
7
i
^
M
J
V
T
r
^
'
'
'
'
'
'
^
'Z""
^'^
"
^
i
^
t
e
n
t
with
the
existence
of
a
new
at
. 3 ~Ie\
sed decay schem
at /.yd
M e \ .. The
The propo
proposed
schemee is show
shownn in Fig. 7.
7.
1
( 12· 9 )(~,~)+-..,..---25
0^'^)h^hf'
SI
Si
T,~=
= 22
5 mse
c
T
225
msec
(i+
- - - - 7'93(~'I~)+ T= 0/2
= 714(3^
7'14 (~))
3-66
24
Mg+p
Mg + p
2
.-.-- ----- ----2
- 2·29
- 2 9 0 0+
25
25
5 ...
0·00
0-00 %~ T =
'/ ^
T=~
A1
FIG.
sed decay
FIG. 7.
7. Propo
Proposed
decay schem
scheniee of 25Si.
^sSi. The T
T =
= 3/2
3/2 level is shown dashed
dashed at
at 7.93
7.93 MeV;
MeV I
aIl
other
levelss and
ments in ^^A\
25AI are from the compi
all other level
and assign
assignments
compilation
Endt and
and van
van der
der Leun
Leun
lation
of
Endt
(1962)
thee two
(1962).. If
If th
two smaU
small peaks in Fig. 5 are to appea
appearr in this schem
scheme,
they will
will run
run from
from the
the
e,
they
dashed
dashed level
level at
at 7.5 MeV to the ground and first excited states
states of
of (24Mg
(^^Mg 4- p).
p).
+
TABL
TABLE
E II
II
Agree
men
t
betwee
n
predic
ted
Agreement between predicted and known levels in ^sAl
25Al
E p (c. of
Ep
of m.),
MeV
3.46±
3 . 4 6 ±O.O6
0.06
(3.89)
( 3 . 8 9 )±O.1
±0.1
4.28±
4 . 2 8 ±O.O4
0.04
4.89±
4.89d=0.06
O.O6
(5.32) ±O.O8
±0.08
5.62rb0.04
.5.62±
O.O4
Ep +
£p
+ (24Mg
e^Mg +
+ p),
p), Ep
£;p +
+ (24Mg*
(24Mg* +
+ p),
p),
MeV
MeV
MeV
5.7.j±
5 . 7 5 ± 0O.06
.06
(6.18)
( 6 . 1 8 )±O.1
±0.1
6.57±
6 . 5 7 ±O.O4
0.04
7.18±
O.O6
7.18±0.06
(7.61)
±O.O8
(7.61)±0.08
7.91±
O.O4
7.91±0.04
7.12±
7.12±0.06
O.O6
(7.55)
±O.1
(7.SS)zL0.1
7.94±O
.O4
7.94±0.04
8.55±
8 . 5 5 ±O.O6
0.06
(8.98)
(8.98) ±O.
± 0 . 0088
9.28±
9 . 2 8 ±O.O4
0.04
Levels in 25AI,
25A1, MeV
Know
nt
Knownf
Propos
ed
Proposed
7.14
7.14
(7.5)
7.14
7.14
(7
..5)
(7.5)
7.93±
7 . 9 3 ±O.O4
0.04
7.93±
O.O4
7.93±0.04
tSee Endt
Endt and
and van der Leun
Leun (1962).
The
The groun
groundd state
state of 25Si
^^Si could be eithe
eitherr 3,/2+
3 ' 2 + or 5/2+
5 '2 + ,, altho
although
the fact
fact
ugh the
that
the
mirro
nucle us, 25~a,
that the mirrorr nucleus,
^^Na, has aa spinspin-parity
5 / 2 + make
makess the
the latter
latter
parity of 5/2+
value
more
in the obser
vable energ
value more likely
likeh'.. With
Within
observable
energyy range (i.e. levels above
above abou
aboutt
CANADIAN
CA:\~\DL\:\ JOURNAL
JOURNAL OF PHYSICS. VOL. 43, 1965
10
10
5.4 lVle\T)
MeV) there are seven known levels in 25AI,
^^Al, of which only one has definite
definite
spin-parity
spin-parity and to this level allowed ^{3 decay is not possible. Thus, in the
absence of known spin-parity
spin-parity information,
information, the proposed decay scheme is quite
consistent
but does not allow us
consistent \:vith
with respect to previously observed levels, but
3 / 2 + and 5/2+
5 / 2 + for ^^Si.
to decide between 3/2+
25Si.
The proposed level at
~\le\;r in 25AI
at 7.93 MeV
^^Al appears to be the expected first
first
T == 3/2 level.
level. Using the semiempirical Coulomb energy difference between
between
25N
a
and
25~Ig,
it
is
possible
to
predict
the
energy
of
this
level
in
25AI
25Na
'^'Wg,
predict
^sAl as
(7.6 ±
~Ie\fT. Relative log
logft
± 0.5) MeV.
ft values for {3/S decay to the levels at 7.93 and
7.1-1
7.14 l\le\T
MeV may be estimated
estimated by comparing the sum of intensities of the two
proton groups from each level; i.e., the sum of intensities of peaks at 3.46
and -1.89
4.89 l\Ie\:
MeV is cOlnpared
compared \vith
with the sum of intensities of peaks at
at 4.28 and
5.62 MeV.
~\Ie\:. Assuming, as was done for 21:\Ig,
^^Mg, that
that {3/3 decay to the level at 7.14
~Ie\' is allo\ved,
logft value could lie between -l.0
MeV
allowed, its logft
4.0 and 5.0. In this case,
the log ftft value for (3{3 decay to the 7.93-l\Ie\l
7.93-MeV level would be between 3.3
and 4.3, indicative of a superallowed
{3 transition.
superallowed 13
transition.Again,
Again,consistent
consistent\vith
withthe
the
T == 3/2 assignment, the \\'idths
widths of the proton transitions ffOlTI
from this level are
are
the narro\\-est
narrowest in the spectrum and can be accounted
accounted for by instrumental
instrumental
resolution and target thickness.
The calculated
calculated Coulomb energy difference
difference between 25Si
^^Si and 25AI
^^Al can be
applied to give a revised estimate of (12.9 ±
~Ievr for the Q^^
Q{3+ of ^^Si.
25Si.
d= 0.2) MeV
SUMMARY
SUMMARY
The observation
observation of delayed proton spectra following the decay of the
neutron-deficient
neutron-deficient nuclides 21Mg
^^Mg and 25Si
^^Si has led to results which are summarized in Table III. \Vork
Work is being undertaken
undertaken to explore the decays of the
remaining
T
=
-3/2
nuclides
between
remaining Tz z = —3/2 nuclides between oxygen
oxygen and
and calcium.
calcium.
,
" TABLE III
III
Summary
Summary of results
Parent
Parent Qff+
nuclide JT
21Mg
2iMg
26Si
25Si
J7r
+
(5/2)-t(5/2)
Q{3+ (est.),
(est.),
MeV
Tj,
msec
Energyofoflowest
lowestTT = =3/2
3/2
Energy
state of daughter, MeV
13.1±0.2
13.1±0.2
12.9±0.2
12.9±0.2
118±4
118±4
225±6
225±6
8.
(21 N a *)
8 . 9900 ±O
± 0 . .004
4 (2iNa*)
7.93±0.04
(26AI*)
7 . 9 3 ± 0 . 0 4 (^^Al*)
ACKNOWLEDGMENTS
ACKNOWLEDGMENTS
The authors received financial assistance from the National Research Council
of Canada
Canada in the form of Studentships held during the course of this work.
was supported
supported by a grant from the Atomic Energy Control Board.
The research "vas
We should also like to thank
tory, Dr. R. E. Bell,
thank the director of the labora
laboratory.
for a number of helpful
helpful discussions.
McPHERSON
25Si DECAY
MCPHERSON AND HARDY: 21Mg
"Mg AXD
AND ^^si
DECAY
Il
H
REFEREi\CES
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W . T.,
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VANDER
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LEUN, C.
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