Rietjens, L. H. Th.
Van den Bold, H . J .
Endt, P. M.
1954
Physica XX
107-114
C O N T I N U O U S AND D I S C R E T E GAMMA-RADIATION
IN T H E DECAY OF l°3Pd
b y L. H. TH. R I E T J E N S , H. J. VAN D E N B O L D and P. M. E N D T
Physiseh Laboratorium der Rijks Universiteit, Utrecht, Nederland
Synopsis
The continuous ~-ray spectrum (internal bremsstrahlung) and the discrete 9,-rays
resulting from the decay of 103pd by electron capture have been investigated with a
scintillation spectrometer.
Weak discrete y-rays have been found with the following energies and intensities:
503 4- 8 keV (0.11 4- 0.02%o), 367 + 6 keV (0.60 + 0.07%o), 305 4- 8 keV (0.11-44- 0.03%0) and (262 4- 15) keV (0.04 + 0.02%0).
The continuous spectrum has been observed, but accurate determinations of its
endpoint, intensity and shape were made difficult by the presence of the discrete
y-rays.
From an analysis of observed ft-values the ~°3Pd spin and parity can be determined
a s J ~ 5/2+ and the 1 0 3 p d - - - 1 0 3 R h m a s s difference as (557+12~) keV.
§ 1. Introduction. The nucleus l°3Pd (half-life 17 days) decays b y electron
capture to l°3Rh m (56 min), which subsequently decays by the emission of a
strongly converted 40 keV y-ray to the l°3Rh ground-state 1) 2) 3). More information about the l°3Rh level scheme can be obtained from the l°3Ru
fl--decay. This decay proceeds t h r o u g h levels at 40 keV (t°3Rh'), 53 keV,
538 keV and perhaps at 611 keV 4). G a m m a - r a y s are observed of 39.6, 52.9,
498.5, 610.6 and perhaps of 295.2 keV 5).
It has been the original object of the present investigation to measure the
shape, i n t e n s i t y and endpoint of the continuous y-ray spectrum (internal
bremsstrahlung) of l°3Pd. This nucleus seemed well suited for such an investigation since no discrete ~,-rays were reported in the decay, except the
strongly converted and easily absorbable 40 keV y-ray from l°3Rh m. The
continuous y-rays accompanying electron capture have only been reported
so far for the nuclides SSFe e), 37A 7) and 7tGe s). The t h e o r y of this process for
allowed transitions has been developed b y M o r r i s o n and S c h i f f 9).
The t o t a l n u m b e r of photons e m i t t e d per K-capture is given by: (a/12z~)
(W/mc2) 2, where a = 1/13~ and W is the total energy available for K-capture
i.e. the mass difference of initial and final nucleus minus the binding energy
of K-electrons. For W = 500 keV the total i n t e n s i t y of the continuous
spectrum would thus be about 0.2%0. The shape of the spectrum is given by
N(E~) = CE~ (W - - E~) 2, where Ey is the ~,-ray energy.
--
107
---
108
L . H . TH. R I E T J E N S , H. J. VAN D E N BOLD AND P. M. E N D T
A l t h o u g h the presence of I°3Pd continuous y-radiation has been clearly
d e m o n s t r a t e d in the present work, its investigation has been severely
h a m p e r e d b y the a p p e a r a n c e of a n u m b e r of weak, discrete y-rays, which
h a d also to be assigned to 1°3Pd. It will be shown, however, t h a t the presence
of the most energetic of these y-rays provides a d e t e r m i n a t i o n of the ~°3Pd
spin and p a r i t y and of the ~°3Pd---l°3Rh mass difference.
§ 2. Experimental equipment. All y - r a y m e a s u r e m e n t s h a v e been perform e d with a scintillation s p e c t r o m e t e r , m a k i n g use of NaI(T1) crystals, an
E.M.I. 6260 p h o t o - m u l t i p l i e r tube, linear amplifier and differential discriminator. Crystals were used of several sizes, the largest being a b o u t one cubic
inch.
-soo
P//min
262 key
100
o
ooo
200
~
367 kev
103pd
300 key
305kevj
--~ . . . . .
100
-~.
200
C ~B
f",
300
tt
S03
400
500
key
600 k e y
Fig. 1. Scintillation spectrum of l°3pd ~-rays.
Curves A, B and C represent the contribution (photo-peaks plus Compton distributions) of 503, 367 and 305 keV v-rays. Curve D is the contribution of the continuous
spectrum computed for an endpoint of 494 keV (see fig. 3). The spectrum remaining
after subtraction of A, B, C and D from the total spectrum is shown in the insert.
It contains a 262 keV v-ray and contributions from scattered v-rays.
F o r e n e r g y calibrations the following sources h a v e been used: 22Na, S4Mn,
6SZn, 137Cs and 2°3Hg. Pulse height was p r o p o r t i o n a l to e n e r g y within 2 % for
pulses at the o u t p u t of the amplifier up to 80 V.
F o r all N a I crystals used the p h o t o - p e a k halfwidth has been between 11
and 13% at E 7 = 511 keV (annihilation radiation).
I n t e n s i t y calibrations were carried out using sources (22Na, 88y, 137Cs'
CONTINUOUS AND DISCRETE GAMMA-RADIATION IN THE DECAY OF
~°3pd 109
2°3Hg) emitting several y-rays and/or X-rays of known intensity ratio.
For each crystal used the photo-efficiency (number of pulses in photo-peak
over number of y-rays incident on the crystal) could thus be determined as a
function of y-ray energy. When necessary, corrections have been applied for
absorption in the Al-cover and MgO-reflector of the crystal. In addition
shapes and intensities of the Compton distributions relative to the photopeaks were determined for m a n y sources with single y-rays.
§ 3. Measurements. Sources for the investigation of the l°3Pd y-spectrum
were
a)
b)
c)
prepared by three different reactions:
]°3Rh (d, 2n) l°3Pd (providing sources of up to 1.5 mC) ;
l°2pd (d, p) l°3Pd (about 100 #C);
1°TAg(d, a2n) l°3Pd (about 10/~C).
1~ ~
103pd
T~=I"Z5_+0.5days
~.,,
8 ~.......
7 t~
--~..~..~
+~.~
A Total spectrum
~'~ ~
(E~,>60kev)
o,O
5 ~ ~ + ~ f ~ C
4 ~"~....~ ~
3
~
ev
367 kev
" f/D
Continuouspart
(E,~=60to 250kev)
~
rays
I
2 0
Fig.
2. D e c a y
I
I
I
I
l
5
l
I
I
I
l
10
l
I
I
I
l
15days
of s e v e r a l c o n s t i t u e n t s of t h e y - r a y s p e c t r u m s h o w n in fig. 1.
All bombardments were performed with 26 MeV deuterons from the Amsterdam cyclotron. After chemical purification the y-spectrum of these sources
was studied with the scintillation spectrometer.
A typical y-spectrum is given in fig. 1. A brass absorber (1.7 g/cm 2) has
been used to prevent overloading of tt{e amplifier by X-ray pulses. Inspection
of fig. I shows the photo-peaks of at least three discrete y-rays with energies
of 503, 367 and 305 keV, and a continuous distribution of pulses between 7 V
and 30 V which is for a good part due to inner bremsstrahlung.
The total intensity of discrete plus continuous spectrum (E Y > 60 keV) is
1.2 YTray per thousand 20.5 keV K-quanta. This intensity ratio was measured
1 10
L. H. TH. R I E T J E N S , H. J. VAN DEN BOLD A N D P. M. E N D T
with the brass asborber removed and with the source at distances of up to 1 m
from the crystal to prevent overloading of the amplifier. In these measurements, where the 7-ray pulse-rate was very small, some background had to
be subtracted.
It is certain that the discrete 7-rays have to be assigned to l°3Pd and not
to some radioactive contaminant of the source. There are at least four arguments to prove this assertion. First, the intensities of the lines, both relative
to the continuous part of the spectrum and relative to the X-rays, were independent of the nuclear reaction b y which the source was prepared. In
these measurements the X-ray intensity from the thick source prepared b y
palladium bombardment had to be corrected for self-absorption in the source.
Second, it has not been possible to eliminate the lines from the spectrum b y
further chemical purification of the sources. Third, half-lives were measured
(see fig. 2) for:
A) the total 7-spectrum (E 7 > ~0 keV);
B) the 503 keV 7-ray photo-peak;
C) the 367 keV 7-ray photo-peak;
D) the continuous part of the scintillation spectrum ( E 7 = 6 0 to 250 keV) ;
E) the X-ray peak.
The half-lives found in these cases agreed within the experimental error. The
average half-life is 17.5 + 0.5 days.
A fourth argument for the assignment of at least the 503 keV 7-ray to
l°~Pd is the fact that its energy agrees within the experimental error with the
energy of the 498 keV y-ray found in the decay of l°~Ru 5).
The same applies to the 7-ray with E 7 = 305 keV which m a y well be
identical with the 295 keV 7-ray observed in the t03Ru decay 5).
As the energy of the 503 keV 7-ray is nearly equal to mc 2 (511 keV) it had
to be shown that this 7-ray is of nuclear origin and does not originate from
the annihilation of positrons. This was done b y using the fact that the two
annihilation quanta from one positron are emitted in opposite directions,
which would cause a peak at 0 -- 180 ° in a ~---7 angular correlation measurement. Two single-channel scintillation spectrometers were used followed b y
a coincidence circuit with a resolving time of 0.25 t* sec. Both channels were
set on E 7 = 510 keV with a channel-width of 100 keV. In this, admittedly
crude, angular correlation experiment the solid angle subtended b y each of the
counters amounted to about 1 steradian.The number of coincidences observed
at ~ = 180 ° was equal to 9. 4- 2, while the theoretical estimate of the number
of true coincidences at 180 °, assumiI{g that the 503 keV 7-ray is due to
positron annihilation, is equal to 125. The reliability of this theoretical estimate was tested with the annihilation radiation of a Na 22 source.
The negative result obtained shows clearly that the 503 keV 7-ray does
not originate from positron annihilation. Moreover it could be shown that the
energy of the y-ray in question is decidedly smaller than 511 keV b y repeated-
C O N T I N U O U S A N D D I S C R E T E G A M M A - R A D I A T I O N IN T H E D E C A Y OF l ° 3 p d
111
ly comparing its photo-peak with the photo-peak of N a 22 annihilation radiation. The positions of these peaks do clearly not coincide.
§ 4. Analysis o~ the scintillation spectrum. The l°3Pd y-ray scintillation
spectrum given in fig. 1 can be analyzed into the discrete and continuous
components given in Table I. The contributions of the various components
have been indicated in fig. I.
TABLE I
Energies and intensities of discrete 7-rays, and
endpoint and total intensity of the continuous
}'-spectrum in the *°°Pd decay.
Energy (keV)
discrete
7-ray s
continuous
},-spectrum
}'t
},~
73
7,
503
367
305
(262
± 8
4- 6
-:-- 8
4- 15)
494+12~
(endpoint)
intensity *)
0.11
0.60
0.11
(0.04
i
4±
~
0.020/,0
0.07°•.o
0.03% o
0.02°/,0)
0.18°/,0 t)
(total intensity)
*) Per unit disintegration.
$) Computed from endpoint euergy.
The existence of 74 is still doubtful. Its photo-peak is not immediately
evident in fig. 1, but if the photo-peaks of Yl, 72 and Ya, their Compton distributions, and the continuous spectrum are subtracted from the measured
spectrum, there remains a restspectrum (shown in the insert of fig. 1) consistently containing a peak at about 260 keV, and a broad distribution below
230 keV which can be accounted for by scattered y-rays as to energy and
intensity.
The contribution of the continuous spectrum could only be estimated by
assuming the theoretical expressions for total intensity and shape to be valid
(implying the assumption that the main E.C. transition is allowed). The
adopted value ~/494+27_t2keV) for the endpoint of the spectrum then fixes
uniquely its intensity and shape. The endpoint energy could only be determined indirectly, because the high-energy part of the continuous spectrum
is hidden under the photo-peaks of the discrete y-rays. The decay energy
( = endpoint + K binding energy of 23 keV) has to be larger than 503 keV
as a discrete y-ray of this energy is present (assuming that this y-ray also
leads to the 40 keV level in l°aRh as the main E.C. transitions do). It has to
be smaller than 650 keV because otheiwise the continuous spectrum would
have filled the valley between the 503 and 367 keV photo-peaks to a larger
extent than was observed actually. Finally the observed intensity of the
continuous spectrum below 250 keV agrees (within the fairly large experimental error) with the intensity computed for an endpoint of 494 keV. An
112
L. H. TH. RIETJENS, H. J. VAN DEN BOLD AND P. M. ENDT
argument narrowing the possibilities for the endpoint still further down is
presented in § 5.
The numerical computation of the pulse distribution to be expected from
a continuous spectrum with an endpoint of 494 keV was performed by correcting the theoretical V-spectrum for absorption in 1.7 g/cm2 brass, dividing
it into a sufficient number of energy intervals and summing their photopeaks (making use of the known photo-efficiency of the crystal) and Compton-distributions (see fig. 3).
103
continuous
Pd
-spectrum
SOOP~n
t
100
1~
200
2~
300
~
400
~
5~
500 key
6~ v
Fig. 3. Pulse distribution (curve C) expected for a continuous ~-ray spectrum (curve
A) with endpoint at 494 keV. Curve 13 represents the spectrum after correction for
absorption in 1.7 g/cm 2 brass. Curve C is found from B b y dividing B into narrow
energy intervals, and s u m m i n g their individual pulse distributions (taking into
account photo-peaks and Compton distributions).
The intensities of discrete v-rays and continuous spectrum per unit disintegration were computed from the measurement (see § 3) of the total
number of v-quanta per 20.5 keV K-quantum. The number of K-quanta per
unit disintegration was assumed to be (83 + 4)% which figure was arrived
at by the following arguments. K-quanta are emitted:
a) after l°3Pd K-capture occurring in 89% of the total number of disintegrations, the other 11% corresponding to L-captures 10) ;
b) after the (almost complete) conversion of the 40 keV v-quantum in the
l°3Rh~ decay, K-conversion occurring in 17 % of the disintegrations 11).
Finally K-quanta are Auger-converted to.an amount of 22% 12), which
brings the total number of K-quanta per unit disintegration down to
(1 - - 0 . 2 2 ) (89 + 17) = 83%.
The relative intensities of discrete v-rays were obtained from fig. 1 after
correction for photo-efficiency and for absorption in the 1.7 g/cm 2 brass.
CONTINUOUS AND DISCRETE GAMMA-RADIATION IN THE DECAY OF l°3pd
113
§ 5. Discussion. The decay scheme proposed for the l°3Ru and ;°3Pd ~tecay
is given in fig. 4. The measurements of C o r k et al. s) and of M e i e t al. 3)
provided most of the material for drawing the ]°3Ru decay scheme. The l°3Rh
ground-state has been measured as J = ½ z3). The shell-model predicts p½
or g%, which establishes odd parity for the l°3Rh ground-state. The spins of
the 40, 53 and 538 keV levels are then uniquely determined as J = 7/2 +,
y~+ 755
( o r ~ ~
/~
\
• /~--
217
\ c ,70)
698
\
3~ +
\ sa8
L
557
5/+
\
_// tog ft =57
~
53.
- 40--~
.
+5/2_~
103 Lm
103Rh
Fig. 4. Decay schemes proposed for t°3Ru and l°3pd.
5/2- and 3/2 + from the measured K/L conversion ratio's of the 7-rays by
which t h e y are deexcited, and from the upper limit of 2 X 10 -9 sec given for
the life-time of the 538 keV level 14). The l°3Ru spin was proposed by C o r k
as J = 5 / 2 +, but this is in contradiction with the fact that no fl--transition
was observed to the l°3Rh 40 keV level (J = 7/2+). A spin of ½+ (or 3/2 +)
would make this transition second forbidden, while the fl--transitions to the
~°3Rh ground-state and 53 keV level are then first forbidden. It is not quite
clear why only the transition to the 53 keV level is observed (log ft = 8.6)
and not to the m°3Rh ground-state. The shell model predicts sl/,, d% or g%
for the l°3Ru ground-state, making spin ½+ preferable.
The main ~°3Pd decay to l°3Rh" is allowed, which limits the ~°3Pd spin to
(5/2, 7/2, 9/2) +. This would render the l°3Pd decay to the 538 keV level
( f = 3/2 +) either allowed or second forbidden. If a log ft-value of 13.0 zs)
is assumed for a second forbidden transition the decay energy computed
from the known life-time and branching ratio (0.0! 1%) would be m a n y MeV.
One m a y thus conclude that the l°3Pd decay to the 538 keV level is allowed,
which determines the X°3Pdspin uniquely as J = 5 / 2 +, in accordance with the
shell-model prediction of sl/~, d% or g%. This same decay provides also the
Physica XX
8*
114
C O N T I N U O U S A N D D I S C R E T E G A M M A - R A D I A T I O N IN T H E D E C A Y OF
l°3pd
most accurate estimate of the l°3pd mass. An assumed log ft-value of 5.7
corresponds to a decay energy of 19 keV, in which case the decay would have
to proceed by L-capture only. Stretching the limits for the log if-value of
allowed transitions as far as 4.5 or 7.0 is) would result in decay energies of 7
keV (L-capture) and 46 keV (mainly K-capture). The decay energy is thus
+27 keV and the corresponding l°3pd--1°3Rh mass difference is (557+1227)
(19_12)
keV.
No attempt has been made to fit the y-rays Y2, Y3 and Y4 observed in the
l°3pd decay into the 1°3Rh level scheme, as it is uncertain at present between
which levels t h e y proceed. One has to assume the existence of a level or levels
at excitation energies between about 300 and 420 keV. These levels should
have negative parity as they are fed from l°3pd by first-forbidden electron
capture transitions (log ft between 7.7 and 9.6).
Acknowledgements. This work is part of the research program of the
Stichting voor Fundamenteel Onderzoek der Materie, which was made possible by a subvention from the Stichting voor Zuiver Wetenschappelijk
Onderzoek.
The authors are indebted to Prof. J. M. W. M i 1 a t z for his interest in
this investigation, to Prof. A. H. W. A t e n Jr. and Miss G. D. d e F e y f e r
for the m a n y chemical separations and to the staff of the Philips cyclotron
at Amsterdam for the bombardments. The calibration of the scintillation
spectrometer was largely performed by J. F. R e i s i g e r and G. J. A rkenbout.
Received 27-1-54.
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