DELAYED PROTON
PROTONS
FOLLOWING
THE
DECAY
OF
ARGON-33
S FOLLOWIN
G TH
E DECA
YO
F ARGON-33
Y AND
L
J. CC.. HARD
HARDY
AND R.
R. LI. VERRAL
VERRALL
Reprinted
from
Reprinted Jrom
CANADIAN JOURNAL OF PHYSICS
43, 418 (1965)
DELAYED
DELAYED PROTONS
PROTONS FOLLOWING
FOLLOWING THE
THE DECAY
DECAY OF
OF ARGON-33
ARGON-33
J. C.
C.
J.
R . I.
I. VERRALL
VERRALL
HARDY AND R.
DELA
YED PROTON
PROTONS
FOLLOWING
THE
DECAY
y OOF
ARGON -333
DELAYED
S FOLLOWIN
G TH
E DECA
F ARGON-3
C.
J. C.
R . 1.
I. VERRALL
HARDY AND R.
RadiationLaboratory,
Laboratory, McGill
McGill University,
University, Montreal,
Montreal, P.Q.
P.Q.
Foster Radiation
Received
N ovem ber 27, 1964
Received November
ABSTRACT
ABSTRACT
The delayed-proton
precursor 33Ar has been produced
produced by proton
proton bombardment
delayed-proton precursor
bombardment
of a lithium
beam of the McGill
lithium chloride
chloride target
target in the internaI
internal beam
McGill synchrocyclotron.
synchrocyclotron.
The half-life
half-life of 33Ar
^sAr was measured
measured to be (178 ±
d= 10) msec. To explain
explain the delayed
delayed
proton spectrum,
proton
spectrum, two new levels, at
at 5.55 and 7.55 MeV, in 33CI
^^Cl are proposed. It is
that the former
former is the expected
expected first TT == 3/2
3/2 state
state in that
that nucleus.
proposed that
also proposed
Delayed
Delayed protons have been observed following the decay of 33Ar.
^^Ar. The only
brief mentions by Reeder et
et al. (1964) and
previous reports of this nuclide are brief
by us (Hardy
by
(Hardy and Verrall 1964a)
1964a) in simultaneous publications that
that ,vere
were
devoted mainly to another
another delayed proton precursor, 37Ca.
^^Ca. We now report our
results in detail; they differ
differ some\vhat
somewhat from those of Reeder etet al.
The measuremen
ts were carried out as in previous experimen
ts in the present
measurements
experiments
series (e.g. McPherson
~IcPherson etet al.
al. 1964), using thin targets bombarded
bombarded in the
circulating
proton
beam
of
the
l\TcGill
synchrocyclotron
circulating
McGill synchrocyclotron and observed by
surface-barrier
surface-barrier silicon detectors. Figure 1 shows the delayed proton spectrum
spectrum
2
from
a
chlorine
target
(2.3
mg/cm
vacuum
evaporated
on
a
200
J.Lg/cm 2
LiCI
from
target
mg/cm^ LiCl
evaporated
/xg/cm^
gold backing). Sho\vn
Shown as an inset in Fig. 1 is the spectrum
spectrum of delayed protons
obtained
\vork of Hardy and
obtained fronl
from a sulphur
sulphur target, which is taken from the work
Verrall (1964b).
(19646). We now argue that
that in the main spectrum
spectrum only the t\VO
two peaks
located at channels 47 and 98 can be said definitely
33Ar.
definitely to follo\v
follow the decay of ^^Ar.
Two other
other peaks, at channels 67 and 114, are too small to be identified
identified positively,
25Si.
while all other peaks are attributed
attributed to 29S
^^S and ^^Si.
The peak in channel 105 of the main spectrum
spectrum "Tas
was sho\vn
shown to follo\v
follow the decay
of 29S
production threshold was
\yas found
^^S by means of the following arguments. Its production
found
to be (50 ±
d= 5) l\Ie\T,
MeV, which agrees with the calculated
calculated threshold for the reaction
35CI(p,
3^Cl(p, a3n)29S.
Q:3n)29S. AIso,
Also, the measured energy of this peak is the sanle
same as that
previously
following the decay
previously found (cf. inset) for the principal proton group follo'Ning
of 29S.
^^S. By comparison
comparison with the inserted spectrum, several other peaks in the
main spectrunl
spectrum can be identified, and these are seen to appear
appear ,vith
with the relative
expected.
intensities expected.
lItt can be seen that
predominant peak in channel 47 ,,-as
that the predominant
was not observed
bombardtnent of sulphur, and consequently
following proton bombardment
consequently the nuclide
responsible for the activity must be an isotope of argon. A measurenlent
measurement of the
threshold for its production
production was difficult
difftcult to obtain because of the relatively
high 13
{3 background;
background; ho,vever,
however, an
an approximate
approximate value
value ofof (39
(39±± ,j)5) ~Ie\7
MeV \vas
was
determined. The argon isotope ln
ust
then
be
33
Ar
since
the
observed
threshold
is
must
^^Ar
consistent
3n)33..~r, whose calculated
consistent only with the reaction 35CI(p,
^^CKp, 3nyK\r,
calculated threshold is
37.8 MeV. A decay curve of the net area under this peak is show'n
shown in Fig. 2, and
half-life as (178 ±
± 10) msec.
gives the haH-life
Canadian
Canadian Journal of Physics. Volume 43 (March.
(March, 1965)
418
418
HARDY A~D
A.YD VERRALL: PROTONS FOLLOWING ARGON-33 DECAY
DECAY
419
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FIG.
FIG. 1.
1. Spectrum of delayed protons from a chlorine target. The energy of each proton peak
MeV, corrected to center of mass. The energies of the two peaks of uncertain origin
is shown in :\IeY,
33. The inset shows the spectrum
spectrum from a sulphur
sulphur target and has
have been calculated for mass 33.
been normalized so that the peak height at 5.59 l\leV
MeV is the same in both spectra.
4r---------,---------~,----------~,--------~,--------~
~
~
<
<t
w
LU
Q.
CL
Z
2f-
DECAY
"33Ar
A r DECA
Y
I~I
(3.26 MeV ) T T,, = ; 178±
I 7 8± 110
0 mse
~MeV)
msecc
~~
l~l~
1
0.1
1
0.2 0.
0.3
0.2
3
TIME —
- ssec
TIME
ec
1
0.4
0.4
0.5
0
.5
Fi(i.
lime decay
half-life
FI<i. 2.
'2. Time
decay of
of the
the net
net area
area under
under the
the main
main peak
peak of
of Fig. 1, showing
showing that
that the
the half-life
of
;1:I.\r is (ITS
(liS d=
± 10) msec.
of ^'Ar
420
420
CANADIA~
CANADL\N JOUR:\AL
JOURNAL OF PHYSICS. VOL.
VOL. -13,
43. 19G5
19G5
From lifetime and threshold measurements, the peak in channel 98 (Fig. 1)
appears also to be associated ,vith
with 33Ar;
^sAr; its intensity is about 4%
4 % of that of the
the
main peak (3.26 MeV) of the 33Ar
^sAr spectrum.
spectrum.
The energies, corrected to center of mass, of the two peaks assigned to ^K\r
33~:\r
are (5.26 ±
=b 0.1) and (3.26 ±
± 0.05) MeV, as is indicated in Fig. 1. Figure :33
shows the proposed decay scheme of 33Ar
^sAr and includes two new levels at (7.55 ±±
two observed
observed
0.1) and (5.55 ± 0.05) MeV which are introduced to explain the t\VO
and
proton peaks. The first level is in an energy region not previously explored, and
proton-scattering experiments.
the second has apparently not been observed in proton-scattering
11.8
1.8
V/zf
33,
'Ar
T
TiY2 == 178
17 8 msec
msec
/2
T=
3/2
T=3/2
7.55 &zSf
-5.75 V
- 5 . 5 5 ('/2)'"T = V2
^ ^ - 5 . 4 6 '/2*
13
Levelss
3 Level
2.29
32
5 +p
^S
+p
33,
CI
0 . 0 0 ^/z''T
= %
33.'\r. Previously
Previously known levels, taken from the compilation
FIG. 3. Proposed decay scheme of ^K\r.
Endt and van der Leun (1962), are shown as solid lines.
of Endt
weIl as observing the peak at
at 3.26 MeV,
l'deV, Reeder et
et al. observed
observed a peak at
As well
(3.90 ±
± 0.1) MeV which they attributed
attributed to ^^Ar.
33Ar. This could be the unidentified
unidentified
peak
peak which we observed
observed at
at about
about 4.0 MeV. Their
Their measured
measured spectrum
spectrum does not
extend to sufftciently
sufficiently high energies to include the ^^Ar
33Ar peak
peak at
at 5.26 l\Ie\7.
extend
MeV.
assigned spin parity
parity of ^"'
!+,",
AIl known nuclei with 15 odd nucleons have an assigned
All
and for this reason, we have assigned
assigned !+
33Ar. On the basis of this assignlnent
and
^^ to ^^Ar.
assignment
33CI which should
should be fed by
by allowed
aIlo\ved ^{3 transitions
there are two known levels in ^^Cl
and which should
should decay by
by emitting
emitting protons within
within the energy
energy range of
of
and
Such proton
proton groups (expected
(expected at
at 3.17 and
and 3.46 AleV)
~Ie\') are certainly
observation. Such
certainh'
not present
present with
with an intensity
intensity comparable
comparable to that
that of
of the observed
observed 3.26-MeV
3.26-l\1e\' peak.
not
This suggests that
that the level at
at 5.55 MeV is fed by
by a superallowed
superaIlo\ved iS+
{3+ transition
This
from the
the TT = = 3/2
3/2 ground
ground state
state of
of ^^Ar,
33Ar, and
and is therefore
therefore the expected
expected first
from
first
T = = 3/2 state in 33C!.
^^Cl. This isotopic spin assignment would also account for
for
the level's nonappearance
nonappearance in resonance data
data for proton
proton scattering
scattering from
from ^^S.
32S.
the
Using
Using the
the semiempirical
semiempirical formula
formula of
of Jaenecke
Jaenecke (1960) for
for Coulomb
Coulomb energy
differences, and
and assuming
assuming isobaric
isobaric invariance, the
the expected
expected energy
energy of
of the
the first
differences,
first
T == 3/2
3/2 level was calculated to be 5.54 Me\!,
Me\', which is in good agreeInent
agreement \vith
with
our
our experimental
experimental value.