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The decay of Se75
Nuclear Physics 2 0 (1961) 6 4 9 - - 6 5 7 ; @ North-Holland Publishing Co., A m s t e r d a m Not to be reproduced by photoprint or microfilm without written permission from the publisher
T H E D E C A Y OF Se ~5 W. F. E D W A R D S ¢ a n d C. J. G A L L A G H E R ,
Jr. *¢
Cali/ornia Institute o/Technology, Pasadena, California *** Received 24 M a r c h 1961 U s i n g t h e D u M o n d b e n t - c r y s t a l g a m m a - r a y s p e c t r o m e t e r , t h e D u M o n d iron-free r i n g - f o c u s i n g b e t a - r a y s p e c t r o m e t e r a n d a n iron free low-field s e m i - c i r c u l a r s p e c t r o m e t e r , t h e energies a n d relative i n t e n s i t i e s of t h e g a m m a r a d i a t i o n s a n d of t h e c o n v e r s i o n e l e c t r o n s following t h e d e c a y of Se 75 i n t o As ~6 h a v e been m e a s u r e d . T h e c o n v e r s i o n coefficients a n d t r a n s i t i o n m u l t i p o l a r i t i e s d e d u c e d f r o m t h i s i n f o r m a t i o n a r e g i v e n a n d discussed. C o m p a r i s o n s are m a d e w i t h m e a s u r e m e n t s b y o t h e r i n v e s t i g a t o r s . I n c o n s i s t e n c i e s b e t w e e n r e p o r t e d v a l u e s of t h e m i x i n g p a r a m e t e r of t h e 279.57 k e V M1 + E2 t r a n s i t i o n are reconciled a n d t h e b e s t v a l u e d e t e r m i n e d is 0 . 5 0 t 0 . 0 6 .
Abstract:
1. I n t r o d u c t i o n
Within the last few years the Se 75 decay scheme, which was previously in considerable doubt, has been well established. Metzger ~), in a study of the resonance fluorescence of several of the transitions, pointed out some inconsistencies in the decay scheme proposed b y Schardt 3). In addition to our preliminary report 3), the published work of numerous groups has served to confirm the decay scheme proposed b y Schardt, and the inconsistencies in the gamma-ray multipolarities reported b y Schardt and Welker 4) have generally been cleared up. Gamma-ray energies, gamma-ray intensities, conversion electron intensities and conversion coefficients reported b y many investigators are discussed in this paper; included are the results of Metzger and Todd 6), Grigorev et al. e), Bashilov and II'in 7) and De Croes and B~ickstrSm s). The last mentioned also report precise transition energies. Pertinent data from angular correlation measurements have been reported b y Schardt and Welker 4), Kelly and Wiedenbeck 9) and Van den Bold et al. 10). 2. I n s t r u m e n t s a n d S o u r c e s
Radioactive sources for the gamma-ray spectrometer were made b y neutron irradiation of SeO 2 with the selenium enriched in the isotope Se 74 (isotopic abundance 12.3 %) at the Materials Testing Reactor, Arco, Idaho. The DuMond * Present address: Utah State University, Logan, Utah. t* P r e s e n t a d d r e s s : I n s t i t u t e of T h e o r e t i c a l P h y s i c s , C o p e n h a g e n , D e n m a r k . tit Work supported by the United States Atomic Energy Commission. 649
650
W.
F.
EDWARDS
AND
C. J .
GALLAGItER, J R .
bent-crystal gamma-ray spectrometer was used to measure gamma-ray energies and intensities. The source for the gamma-ray spectrometer consisted of approximately 3 mg of the SeOz enclosed in a quartz capillary of internal diameter 0.02 cm. With the present source conditions the energy resolution of the gamma-ray spectrometer was A E / E ---- 2 . 3 × 1 0 - 5 E, where E is the gamma-ray energy in keV a n d / 1 E is the full width of the line profile at halfmaximum. For strong lines the errors in the gamma-ray energy measurements were assigned either as ~o of the full width of the line at half maximum or 10 eV, whichever was greater. For weak lines the assigned error was sometimes as great as { the line width. The gamma-ray spectrometer was recently calibrated to permit precise relative intensity measurements 11). The intensities were taken to be proportional to the total number of counts in the photo peak of the NaI(T1) detector with the crystal spectrometer set at an energy corresponding to the centre of the diffracted line. The relative intensities were then corrected for absorption due to the presence of quartz, air, and aluminium in the gamma-ray beam, for the detector efficiency and for the energy dependent reflectivity of the curved crystal, which has been shown 11) to have E -1.9s7±°-°~2 dependence. The statistical error was 1 °/o or less for most lines and was never greater than 60/0. The conversion electrons were measured with the DuMond iron-free ringfocusing spectrometer operating at 0.7 ~/o optimum-match momentum resolution and an iron-free semicircular low-field spectrometer at 1 °/o resolution. The sources for the beta spectrometer were prepared from activities produced by irradiating Se metal and had specific activities of ~ 10#Cur/#g. Sources of thickness between 5 and 30#g/cm ~ were prepared by vacuum evaporation of this material into aluminized mica backings. The Geiger-Mtiller counter in the ring-focusing spectrometer had a 0.96 mg/cm ~ mica window, which caused considerable absorption at electron energies below approximately 80 keV. As a consequence, the intensities of conversion electrons whose energies were below 120 keV were also measured with the semicircular spectrometer in which the G. M. counter had only a 30/~g/cm 2 formvar window, which did not absorb electrons above approximately 10 keV. 3. R e s u l t s
The gamma-ray energies are reported in table 1. They were obtained by converting the measured wavelength in x-units to keV by using the factor E(keV) ---- 12372.44/~ x units as reported by DuMond and Cohen 18). Some disagreement exists between our transition energies and those reported by de Cro~s and B~ckstrSm s), whose values are generally approximately 0.08 lower. A systematic error is indicated. Transition energies establish levels as
<0.3 1.63t0.06 5.57t0.18 28.0 ~ 0 . 6 95.5 4-1.8 2.4 t 0 . 1 100 42.2 -4-O.6 2.29±0.14 19.5 4-0.6
Gamma-ray energy (keV)
24.3 66.05!0.01 96.74=}=0.01 121.12t0.01 135.99=]=0.02 198.604-0.06 264.62±0.07 279.5710.08 304.0 +0.3 400.7 =[:0.2 t0.05 t0.07
0.018 t 0 . 0 0 2 0.00604- 0.0005 0.00761 0.0009 0.045 t 0 . 0 0 6 0.0011t0.0001
(0.0406) a) (0.0240) a)
0.36 0.81
XK a)
9.5 7.9 6.1 8.8
6.5 10.5 7.6
0) e)
K](L+M)
0.42 0.95 0.0445 0.0272 0.018 0.0067 0.0086 0.052 0.0012
CXtotal
Internal conversion coefficients
0.99+0.13 0.94t0.09
0.9710.08
(1.10) a) (0,91) a)
1.10=[= 0.10
(XK theoretical
~ K measured b)
I
MI+E2 E2 E1 E1 MI+E2 M1 MI+E2 E3 E1
(M2)
Multipolarity assignment
1.4±0.1 6.5±0.3 17.4±0.4 58.5±1.1 1.5!0.1 60.1 25.4t0.4 1.4±0.1 11.6i0.4
(%)
Decay fraction e)
a) The conversion coefficients were normalized to the best fit of the coefficients of the 121.12 keV and 135.99 keV transitions with theoretical E1 values. b) The theoretical coefficients are those given by Rose la) based upon the multipolarity assignments in column 7. e) The decay fraction is proportional to (1 + ~total) and is normalized to 100 % feeding of the ground state neglecting any beta branching to that level. o) K : L : M = 3.7 : 1:0.24. e) K : L : M = 8 . 3 : 1 : 0 . 2 .
Gamma-ray relative intensity
TABLE 1 Data for transitions in As 7s
~n
%
o
a*
0~
652
W. F. ~DWARDS AND C. J. GALLAGHER, JR.
shown in fig. 1 at the following energies (in keV); 198.60-¢-0.04, 264.66=[=0.04, 2 7 9 .5 3 i0 .0 4 , 303.914-0.04 and 400.65:J=0.04. G amma- r ay intensities are reported in table 1. Conversion electron intensities are reported as normalized conversion coefficients. The conversion coefficients, also given in table 1, were normalized to the best fit of the 121.12 keV and 135.99 keV K conversion coefficients to the theoretical E1 values. The multipolarity assignments, based on the conversion coefficients and the theoretical values of Rose la), are as indicated. The 24.3 keV gamma r a y was not detected. Using a thin window NaI(T1) scintillation spectrometer the upper limit of its gamma-ray intensity was estimated to be 1 the intensity of the 66.05 keV gamma ray. A 24.3 keV M2 transition has a theoretical K-conversion coefficient of 160. From this and the known ratio of K-conversion electrons, the expected intensity was found to be 0.02 relative to a value of 100 for the 264.62 keV transition, and it is therefore not surprising th at the gamma r a y was not observed. The K : L : M ratios are as theoretically expected assuming an M2 assignment. TABLE 2
Log/t v a l u e s for electron c a p t u r e t r a n s i t i o n s f r o m Se ~a to As 75 E n e r g y of level
Electron capture b r a n c h i n g (%)
Transition
log
0 198.61 264.62 279.57 304.0 400.7 575 b)
< 0 . 0 1 a) <0.5 2.9+1.1
{+ ~ ~+~½~+~~+ ~ {+ ~ ~+ ~+ ~ {+ ~+~[-
>9.6 >9.4 7.8
~< 1+1 ~< 14-1 94=[= 1.3 0.04 b)
/t
~>8.3 ~>8.2 6.0 9.0 b)
a) T h e p e r c e n t a g e b r a n c h i n g s were c a l c u l a t e d a s s u m i n g no b r a n c h i n g to t h e g r o u n d s t a t e . b) R e p o r t e d b y H. L a n g e v i n - J o l i o t a n d M. L a n g e v i n (ref. 1,) a n d included for c o m p l e t e n e s s .
The decay fractions of the transitions indicate weak beta branching to some of the excited levels. With the decay fractions normalized to 100 %, and assuming no p r im a r y beta branching to the ground-state, the feeding of the 264.62 keV level appears to be (2.9±1.1)%. There is also some feeding, of the order of 1 or 2 %, to either the 279.53 keV level or the 303.91 keV level or both. With the present spin assignments a 1st forbidden beta transition to the 279.53 level is expected, whereas the transition expected to the 303.91 keV level is 2nd forbidden. Logfl values have been calculated using these branching ratios or limits on branching ratios and the previously reported electron capture decay energy 14,15) with the aid of Moszkowski's nomograph 16). The results are shown in table 2. The sign of the Gamow-Teller to Fermi matrix element ratio
THE D E C A Y
OF
Se75
653
for the transition to the 400.65 keV state has been measured by Boehm and Gallagher and found to be negative 17). 4. Comparison
with
Previous
Results
The decay scheme of Se 75 originally proposed by Schardt and Welker (see fig. 1) is now well established with the original discrepancies removed. EXCITATION ENERGY (kev)
sd s 863
~//t
800
/ / / 5/~+
/
/ I/t E,ECTRO. 0.04 I / / 1
CAPTURE
/,!t7
/4/Y f,oof ,oo,~t,,=,,. ,,.,,,,,___.~.°°° • ~/,;~,,_ ,,x,o-,,., ,/
/ 4 o o n I la.tz I. El
•
Et
"
I
I//
/ ,,,;,,,, v' r.,
l
.9.s_1_1_92t_~~
s/~_
z64.6-1--1~l--l-lr~ // 304.0 • /l -E3 •
//
II
t41.E2 I
I
9
;~166.0_s IMt*EZ
,.s~ to-',~,,o 2.3 ~ to- ,,,
I 1
I |.,.~,
/If llJ
///////////,.//////////
3/Z-
As 73
Fig. l. The decay scheme of Sev6. In the course of t h e m a n y independent studies of this decay scheme several other apparent discrepancies have appeared, however, and we attempt to resolve these in the following discussion.
654
W.F.
E D W A R D S AND C. J . GALLAGHER, J R .
TABLE 3 Half-lives of levels in As 7s E n e r g y of level (keV)
Reported t,~ (sec)
Ref.
5.2 × 10 -19 a) 2.3 X 10 -11 1.65 X 10 -11 2.5X 10 n 1.5 X 10 -1° h) 0.017 1.5 X 10 -9
198.60 264.62
279.57 304.0 400.7
19) 19)
1) ~) 19)
~) is)
a) Calculated f r o m the average value B (E2) = 0.022 reported b y Alder et al. (ref. 19), where we have used the values (l+~total)199 = 1.018 and [6]199 = 0.38 calculated from the conversion coefficient of the 199 keV t r a n s i t i o n reported in the p r e s e n t paper.
b) Calculated f r o m the average value B(E2) = 0.060 reported in ref. 19), where we have used the values (l+~total)~s9 = 1.0086, ]~12s9 = 0.50 and the 95 % b r a n c h i n g f r o m the 280 keV level t h r o u g h the 280 keV t r a n s i t i o n r e p o r t e d in the p r e s e n t paper. I n using the total B(E2) for the calculation of the half-life of t h e 280 keV level we have a s s u m e d no Coulomb excitation of the 264.62-keV level. TABLE 4 Relative g a m m a - r a y intensities following Se ~5 decay Transition energy (keV)
Schardt and Welker
66.05 96.74 121.12
6.64- 1.5 28 4- 6
135.99 198.60 264.62 279.57 304.0 400.7
94 4.12 3 100 45.7± 4 2.0i0.5 2 4 . 8 ± 2.5
1.84- 1
Gfigorev et al.
Metzger and Todd
de Cro~s and B~ckstr6m
1.534,0.15 5.5 4-0.3 27.9 4-1.3 96 2.6 100 41 2.5 22.3
28 86
4-5 +0.2 4-2.5 ±0.3 ±2.3
100 47 ± l 2.14-0.3 19 ± 2
4- 5 4-15
100 42.5 4-2 2.154,0.3 23 ±2
Our results 1.6 4-0.6 5.6 4-0.2 28.0 ± 0 . 6 95.5 4-1.8 2.4 4,0.1 100 42.2 4-O.6 2.294-0.14 19.5 4-0.6
TABLE 5
Relative K conversion electron intensities following Se 75 Decay Transition energy (keV) 66.05 96.74 121.12 135.99 198.60 264.62 279.57 304.0 400.7
Schardt and Welker
715 173 420 6.44-0.7 I00 4-8 53 4-7 15.65=1.3 3.64-0.6
Grigorev et al. 73.7 -4- 4.4 645 4-25 154 4-2.3 384 -4- 5 7.3 4-0.3 100 -4- 3 49.2 4- 3 16.1 i 0.8 3.764-O.26
Metzger and Todd
377
4-20
i00 53.64- 1.6 15.44- 0.9 3 . 6 ± 0.4
de Cro~s and B~ckstr6m
Our Result
80 4.12 940 -4-60 180 4-12 450 4- 30 6.84.1 100 63 ± 5 16.04- 1 3 . 8 ± 0.3
99 4.12 753 4-60 187 ± 1 5 378 4- 30 7 4. 1 100 53 4 - 5 17.24. 1.7 3.74- 0.4
THE DECAY OF Se 75
655
Although no half-life determinations have been made in the present study, the half-lives of all the strongly populated states in As 75 have now been either directly measured i, 2, is), or the levels have been Coulomb excited lg); for the sake of completeness half-lives of the levels are shown in fig. 1. and listed in table 3. The total electron capture energy obtained from the data reported b y Trail and Johnson 14) and Gossett and Butler 15) is also given in fig. 1. We have observed the 577 keV gamma-ray reported b y Langevin-Joliot and Langevin is) but only in the NaI spectrum. The gamma-ray intensities and conversion electron intensities reported b y the various groups are compared in tables 4 and 5. The conversion electron intensities reported b y the various investigators do not vary beyond the experimental errors, with the exception of the values reported for the 96.74 keV transition and the value reported b y Grigorev et al. for the 121.12 keV transition. The relative gamma-ray intensity measurements are generally in very good agreement; the only exception to this is the measurement of the intensity of the 279.57 keV line b y Metzger and Todd, their value of 474-1 differing considerably from the present value of 42.24-0.06 as well as from the values 414-2.5 and 42.54-2 measured b y Grigorev et al. and de Cro~s and B~tckstrdm respectively. Because this line is very close in energy to the 264.62 keV line, relative to which its intensity was measured, absorption corrections and the curved crystal reflectivity correction were very small; therefore the magnitude of our reported error is almost entirely due to counting statistics. Furthermore, the intensity of this line was remeasured four times. The highest value measured was 42.7 and the mean 42.2. For these reasons we feel that the value of Metzger and Todd is high. A point still open to discussion is the M 1+ E 2 mixing ratio of the 279.57 keV transition. Metzger and Todd 5) and later Manning and Rogers 20) have TABLE 6 Conversion coefficient d a t a for the 279.57 keV t r a n s i t i o n in As :5 K conversion electron intensity
Investigators
Gamma-ray intensity
S c h a r d t and Welker
45.74-4
53
Grigorev et al.
41
49.24-3
+2.5
4-8 7.64-0.8 7.6±1.0
Bashilov and II'in 53.64-1.6
7.04-0.4
42.5~2
63
4-5
9.94-1
42.24-0.6
53
4-5
7.64-0.9
Metzger and Todd
47
de Cro@s and BAckstr6m P r e s e n t investigators
~1
Mixing parameter
K conversion coefficient ( × 103)
~) This value differs f r o m t h a t reported in the original p a p e r 21).
i
0.51 +0.12 --0.13 0.51 +0.15 -0.16 +0.06 0.42_0.07 )
0 ~ +0.19 .vv --0.17 O al +0.14 .v. --0.15
656
W. F. EDWARDS AND C. J. GALLAGHER, J R .
commented upon the disagreement between the mixing parameters 81 as determined by conversion coefficient information and those determined by angular correlation. Table 6 gives the mixing parameters calculated from conversion coefficients assuming the theoretical coefficients of Rose la). The agreement is rather good in spite of the apparently high gamma-ray intensity of Metzger and Todd which lowers their value for the mixing parameter. The agreement is better than reported previously 5, s0) because a calculation error in Metzger and Todd's value for the mixing parameter has been corrected ~1). The angular correlation data for this transition are summarized in table 7. TABLE 7
A n g u l a r correlation d a t a for t h e 279.57 k e V t r a n s i t i o n in As ~5 Investigators
a.,
Schardt and Welker
- - 0 . 4 2 -}-0.03
--0.444-0.11
Kelly and Wiedenbeck
--0.41 4-0.03
--0.414-0.08
v a n d e n Bold et al.
--0.4664-0.02
- - 0 7~+0.26a) •
~ --0.21
a) T h i s v a l u e differs f r o m t h a t r e p o r t e d in t h e original paper, t h e error limits h a v i n g been inc o r r e c t l y a s s i g n e d *~).
The agreement among the various values for the mixing parameter is now reasonable, primarily because the error limits on the value --0.744-0.10 previously reported by van den Bold et al. were in error and have now been corrected in such a way as to improve the agreement 23). The mean of the eight values for the mixing parameter is 0.47. The deviation of the individual values from the mean is certainly reasonable as reflected by the z2-test. The expected value of Z2 which is equal to the number of degrees of freedom or amount of over-determination of the system, is 7. The calculated value is 8.0. The probability that Z~ be equal to 8.0 or more by random deviation alone is 0.33, indicating consistency of the data. If Metzger and Todd's value is eliminated from the set, the mean is 0.50 and X* is 7.2 (the expected value in this case is 6); the probability of the value 7.2 or greater by random deviation is 0.30. We conclude that the determinations of the mixing ratios of the 279.57 keV transition are in satisfactory agreement and that the best value for the mixing parameter is 0.504-0.06 which indicates an 80 % M l + 2 0 % E2 mixture for this transition. We are grateful to Professor J. W. M. DuMond and Professor F. Boehm for their interest and to Dr. G. Manning for helpful discussions.
THE
D~CAY
O F Se 76
657
References 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
F. R. Metzger, Phys. Rev. 110 (1958) 123 A. W. Schardt, Phys. Rev. 108 (1957) 398 W. F. Edwards, C. J. Gallagher, Jr. and J. W. M. DuMond, Bull. Am. Phys. Soc. 4 (1959) 279 A. W. Schardt and J. P. Welker, Phys. Rev. 99 (1955) 810 F. R. Metzger and W. B. Todd, Nuclear Physics 10 (1959) 220 Grigorev, Zolotavin, Klementev and Sinitzyn, Izvestia Akad. Nauk SSSR 23 (1959) 159 A. A. Bashilov and V. V. II'in, Izvestia Akad. Nauk SSSR 23 (1959) 159 M. DeCroSs and G. B~ckstr6m, Ark. f6r Fys. 16 (1960) 567 W. H. Kelly and M. L. Wiedenbeck, Phys. Rev. 102 (1956) 1130 I-L C. Van den Bold, J. Van de Geijn and P. M. Endt, Physica 24 (1958) 24 W. Edwards, P h . D . Thesis, California Institute of Technology, 1960 (unpublished) Cohen, DuMond, Layton and Rollett, Rev. Mod. Phys. 27 (1955) 363 M. E. Rose, Internal conversion coefficients (North-Holland Publishing Co., Amsterdam, 1958) C. C. Trail and C. H. Johnson, Phys. Rev. q l (1953) 474 C. R. Gossett and J. w . Butler, Phys. Rev. 113 (1959) 246 S. A. Moszkowski, Phys. Rev. 82 (1951) 25 F. Boehm and C. J. Gallagher, Jr., Phys. Rev. 119 (1960) 258 14. Langevin - Joliot and M. Langevin, J., Phys. Rad. 19 (1958) 765 Alder, Bohr, Huus, Mottelson and Winther, Revs. Mod. Phys. 28 (1956) 432 G. Manning and J. Rogers, Nuclear Physics 15 (1960) 166 F. R. Metzger, private communication 14. C. van den Bold, private communication
T H E D E C A Y OF Se ~5 W. F. E D W A R D S ¢ a n d C. J. G A L L A G H E R ,
Jr. *¢
Cali/ornia Institute o/Technology, Pasadena, California *** Received 24 M a r c h 1961 U s i n g t h e D u M o n d b e n t - c r y s t a l g a m m a - r a y s p e c t r o m e t e r , t h e D u M o n d iron-free r i n g - f o c u s i n g b e t a - r a y s p e c t r o m e t e r a n d a n iron free low-field s e m i - c i r c u l a r s p e c t r o m e t e r , t h e energies a n d relative i n t e n s i t i e s of t h e g a m m a r a d i a t i o n s a n d of t h e c o n v e r s i o n e l e c t r o n s following t h e d e c a y of Se 75 i n t o As ~6 h a v e been m e a s u r e d . T h e c o n v e r s i o n coefficients a n d t r a n s i t i o n m u l t i p o l a r i t i e s d e d u c e d f r o m t h i s i n f o r m a t i o n a r e g i v e n a n d discussed. C o m p a r i s o n s are m a d e w i t h m e a s u r e m e n t s b y o t h e r i n v e s t i g a t o r s . I n c o n s i s t e n c i e s b e t w e e n r e p o r t e d v a l u e s of t h e m i x i n g p a r a m e t e r of t h e 279.57 k e V M1 + E2 t r a n s i t i o n are reconciled a n d t h e b e s t v a l u e d e t e r m i n e d is 0 . 5 0 t 0 . 0 6 .
Abstract:
1. I n t r o d u c t i o n
Within the last few years the Se 75 decay scheme, which was previously in considerable doubt, has been well established. Metzger ~), in a study of the resonance fluorescence of several of the transitions, pointed out some inconsistencies in the decay scheme proposed b y Schardt 3). In addition to our preliminary report 3), the published work of numerous groups has served to confirm the decay scheme proposed b y Schardt, and the inconsistencies in the gamma-ray multipolarities reported b y Schardt and Welker 4) have generally been cleared up. Gamma-ray energies, gamma-ray intensities, conversion electron intensities and conversion coefficients reported b y many investigators are discussed in this paper; included are the results of Metzger and Todd 6), Grigorev et al. e), Bashilov and II'in 7) and De Croes and B~ickstrSm s). The last mentioned also report precise transition energies. Pertinent data from angular correlation measurements have been reported b y Schardt and Welker 4), Kelly and Wiedenbeck 9) and Van den Bold et al. 10). 2. I n s t r u m e n t s a n d S o u r c e s
Radioactive sources for the gamma-ray spectrometer were made b y neutron irradiation of SeO 2 with the selenium enriched in the isotope Se 74 (isotopic abundance 12.3 %) at the Materials Testing Reactor, Arco, Idaho. The DuMond * Present address: Utah State University, Logan, Utah. t* P r e s e n t a d d r e s s : I n s t i t u t e of T h e o r e t i c a l P h y s i c s , C o p e n h a g e n , D e n m a r k . tit Work supported by the United States Atomic Energy Commission. 649
650
W.
F.
EDWARDS
AND
C. J .
GALLAGItER, J R .
bent-crystal gamma-ray spectrometer was used to measure gamma-ray energies and intensities. The source for the gamma-ray spectrometer consisted of approximately 3 mg of the SeOz enclosed in a quartz capillary of internal diameter 0.02 cm. With the present source conditions the energy resolution of the gamma-ray spectrometer was A E / E ---- 2 . 3 × 1 0 - 5 E, where E is the gamma-ray energy in keV a n d / 1 E is the full width of the line profile at halfmaximum. For strong lines the errors in the gamma-ray energy measurements were assigned either as ~o of the full width of the line at half maximum or 10 eV, whichever was greater. For weak lines the assigned error was sometimes as great as { the line width. The gamma-ray spectrometer was recently calibrated to permit precise relative intensity measurements 11). The intensities were taken to be proportional to the total number of counts in the photo peak of the NaI(T1) detector with the crystal spectrometer set at an energy corresponding to the centre of the diffracted line. The relative intensities were then corrected for absorption due to the presence of quartz, air, and aluminium in the gamma-ray beam, for the detector efficiency and for the energy dependent reflectivity of the curved crystal, which has been shown 11) to have E -1.9s7±°-°~2 dependence. The statistical error was 1 °/o or less for most lines and was never greater than 60/0. The conversion electrons were measured with the DuMond iron-free ringfocusing spectrometer operating at 0.7 ~/o optimum-match momentum resolution and an iron-free semicircular low-field spectrometer at 1 °/o resolution. The sources for the beta spectrometer were prepared from activities produced by irradiating Se metal and had specific activities of ~ 10#Cur/#g. Sources of thickness between 5 and 30#g/cm ~ were prepared by vacuum evaporation of this material into aluminized mica backings. The Geiger-Mtiller counter in the ring-focusing spectrometer had a 0.96 mg/cm ~ mica window, which caused considerable absorption at electron energies below approximately 80 keV. As a consequence, the intensities of conversion electrons whose energies were below 120 keV were also measured with the semicircular spectrometer in which the G. M. counter had only a 30/~g/cm 2 formvar window, which did not absorb electrons above approximately 10 keV. 3. R e s u l t s
The gamma-ray energies are reported in table 1. They were obtained by converting the measured wavelength in x-units to keV by using the factor E(keV) ---- 12372.44/~ x units as reported by DuMond and Cohen 18). Some disagreement exists between our transition energies and those reported by de Cro~s and B~ckstrSm s), whose values are generally approximately 0.08 lower. A systematic error is indicated. Transition energies establish levels as
<0.3 1.63t0.06 5.57t0.18 28.0 ~ 0 . 6 95.5 4-1.8 2.4 t 0 . 1 100 42.2 -4-O.6 2.29±0.14 19.5 4-0.6
Gamma-ray energy (keV)
24.3 66.05!0.01 96.74=}=0.01 121.12t0.01 135.99=]=0.02 198.604-0.06 264.62±0.07 279.5710.08 304.0 +0.3 400.7 =[:0.2 t0.05 t0.07
0.018 t 0 . 0 0 2 0.00604- 0.0005 0.00761 0.0009 0.045 t 0 . 0 0 6 0.0011t0.0001
(0.0406) a) (0.0240) a)
0.36 0.81
XK a)
9.5 7.9 6.1 8.8
6.5 10.5 7.6
0) e)
K](L+M)
0.42 0.95 0.0445 0.0272 0.018 0.0067 0.0086 0.052 0.0012
CXtotal
Internal conversion coefficients
0.99+0.13 0.94t0.09
0.9710.08
(1.10) a) (0,91) a)
1.10=[= 0.10
(XK theoretical
~ K measured b)
I
MI+E2 E2 E1 E1 MI+E2 M1 MI+E2 E3 E1
(M2)
Multipolarity assignment
1.4±0.1 6.5±0.3 17.4±0.4 58.5±1.1 1.5!0.1 60.1 25.4t0.4 1.4±0.1 11.6i0.4
(%)
Decay fraction e)
a) The conversion coefficients were normalized to the best fit of the coefficients of the 121.12 keV and 135.99 keV transitions with theoretical E1 values. b) The theoretical coefficients are those given by Rose la) based upon the multipolarity assignments in column 7. e) The decay fraction is proportional to (1 + ~total) and is normalized to 100 % feeding of the ground state neglecting any beta branching to that level. o) K : L : M = 3.7 : 1:0.24. e) K : L : M = 8 . 3 : 1 : 0 . 2 .
Gamma-ray relative intensity
TABLE 1 Data for transitions in As 7s
~n
%
o
a*
0~
652
W. F. ~DWARDS AND C. J. GALLAGHER, JR.
shown in fig. 1 at the following energies (in keV); 198.60-¢-0.04, 264.66=[=0.04, 2 7 9 .5 3 i0 .0 4 , 303.914-0.04 and 400.65:J=0.04. G amma- r ay intensities are reported in table 1. Conversion electron intensities are reported as normalized conversion coefficients. The conversion coefficients, also given in table 1, were normalized to the best fit of the 121.12 keV and 135.99 keV K conversion coefficients to the theoretical E1 values. The multipolarity assignments, based on the conversion coefficients and the theoretical values of Rose la), are as indicated. The 24.3 keV gamma r a y was not detected. Using a thin window NaI(T1) scintillation spectrometer the upper limit of its gamma-ray intensity was estimated to be 1 the intensity of the 66.05 keV gamma ray. A 24.3 keV M2 transition has a theoretical K-conversion coefficient of 160. From this and the known ratio of K-conversion electrons, the expected intensity was found to be 0.02 relative to a value of 100 for the 264.62 keV transition, and it is therefore not surprising th at the gamma r a y was not observed. The K : L : M ratios are as theoretically expected assuming an M2 assignment. TABLE 2
Log/t v a l u e s for electron c a p t u r e t r a n s i t i o n s f r o m Se ~a to As 75 E n e r g y of level
Electron capture b r a n c h i n g (%)
Transition
log
0 198.61 264.62 279.57 304.0 400.7 575 b)
< 0 . 0 1 a) <0.5 2.9+1.1
{+ ~ ~+~½~+~~+ ~ {+ ~ ~+ ~+ ~ {+ ~+~[-
>9.6 >9.4 7.8
~< 1+1 ~< 14-1 94=[= 1.3 0.04 b)
/t
~>8.3 ~>8.2 6.0 9.0 b)
a) T h e p e r c e n t a g e b r a n c h i n g s were c a l c u l a t e d a s s u m i n g no b r a n c h i n g to t h e g r o u n d s t a t e . b) R e p o r t e d b y H. L a n g e v i n - J o l i o t a n d M. L a n g e v i n (ref. 1,) a n d included for c o m p l e t e n e s s .
The decay fractions of the transitions indicate weak beta branching to some of the excited levels. With the decay fractions normalized to 100 %, and assuming no p r im a r y beta branching to the ground-state, the feeding of the 264.62 keV level appears to be (2.9±1.1)%. There is also some feeding, of the order of 1 or 2 %, to either the 279.53 keV level or the 303.91 keV level or both. With the present spin assignments a 1st forbidden beta transition to the 279.53 level is expected, whereas the transition expected to the 303.91 keV level is 2nd forbidden. Logfl values have been calculated using these branching ratios or limits on branching ratios and the previously reported electron capture decay energy 14,15) with the aid of Moszkowski's nomograph 16). The results are shown in table 2. The sign of the Gamow-Teller to Fermi matrix element ratio
THE D E C A Y
OF
Se75
653
for the transition to the 400.65 keV state has been measured by Boehm and Gallagher and found to be negative 17). 4. Comparison
with
Previous
Results
The decay scheme of Se 75 originally proposed by Schardt and Welker (see fig. 1) is now well established with the original discrepancies removed. EXCITATION ENERGY (kev)
sd s 863
~//t
800
/ / / 5/~+
/
/ I/t E,ECTRO. 0.04 I / / 1
CAPTURE
/,!t7
/4/Y f,oof ,oo,~t,,=,,. ,,.,,,,,___.~.°°° • ~/,;~,,_ ,,x,o-,,., ,/
/ 4 o o n I la.tz I. El
•
Et
"
I
I//
/ ,,,;,,,, v' r.,
l
.9.s_1_1_92t_~~
s/~_
z64.6-1--1~l--l-lr~ // 304.0 • /l -E3 •
//
II
t41.E2 I
I
9
;~166.0_s IMt*EZ
,.s~ to-',~,,o 2.3 ~ to- ,,,
I 1
I |.,.~,
/If llJ
///////////,.//////////
3/Z-
As 73
Fig. l. The decay scheme of Sev6. In the course of t h e m a n y independent studies of this decay scheme several other apparent discrepancies have appeared, however, and we attempt to resolve these in the following discussion.
654
W.F.
E D W A R D S AND C. J . GALLAGHER, J R .
TABLE 3 Half-lives of levels in As 7s E n e r g y of level (keV)
Reported t,~ (sec)
Ref.
5.2 × 10 -19 a) 2.3 X 10 -11 1.65 X 10 -11 2.5X 10 n 1.5 X 10 -1° h) 0.017 1.5 X 10 -9
198.60 264.62
279.57 304.0 400.7
19) 19)
1) ~) 19)
~) is)
a) Calculated f r o m the average value B (E2) = 0.022 reported b y Alder et al. (ref. 19), where we have used the values (l+~total)199 = 1.018 and [6]199 = 0.38 calculated from the conversion coefficient of the 199 keV t r a n s i t i o n reported in the p r e s e n t paper.
b) Calculated f r o m the average value B(E2) = 0.060 reported in ref. 19), where we have used the values (l+~total)~s9 = 1.0086, ]~12s9 = 0.50 and the 95 % b r a n c h i n g f r o m the 280 keV level t h r o u g h the 280 keV t r a n s i t i o n r e p o r t e d in the p r e s e n t paper. I n using the total B(E2) for the calculation of the half-life of t h e 280 keV level we have a s s u m e d no Coulomb excitation of the 264.62-keV level. TABLE 4 Relative g a m m a - r a y intensities following Se ~5 decay Transition energy (keV)
Schardt and Welker
66.05 96.74 121.12
6.64- 1.5 28 4- 6
135.99 198.60 264.62 279.57 304.0 400.7
94 4.12 3 100 45.7± 4 2.0i0.5 2 4 . 8 ± 2.5
1.84- 1
Gfigorev et al.
Metzger and Todd
de Cro~s and B~ckstr6m
1.534,0.15 5.5 4-0.3 27.9 4-1.3 96 2.6 100 41 2.5 22.3
28 86
4-5 +0.2 4-2.5 ±0.3 ±2.3
100 47 ± l 2.14-0.3 19 ± 2
4- 5 4-15
100 42.5 4-2 2.154,0.3 23 ±2
Our results 1.6 4-0.6 5.6 4-0.2 28.0 ± 0 . 6 95.5 4-1.8 2.4 4,0.1 100 42.2 4-O.6 2.294-0.14 19.5 4-0.6
TABLE 5
Relative K conversion electron intensities following Se 75 Decay Transition energy (keV) 66.05 96.74 121.12 135.99 198.60 264.62 279.57 304.0 400.7
Schardt and Welker
715 173 420 6.44-0.7 I00 4-8 53 4-7 15.65=1.3 3.64-0.6
Grigorev et al. 73.7 -4- 4.4 645 4-25 154 4-2.3 384 -4- 5 7.3 4-0.3 100 -4- 3 49.2 4- 3 16.1 i 0.8 3.764-O.26
Metzger and Todd
377
4-20
i00 53.64- 1.6 15.44- 0.9 3 . 6 ± 0.4
de Cro~s and B~ckstr6m
Our Result
80 4.12 940 -4-60 180 4-12 450 4- 30 6.84.1 100 63 ± 5 16.04- 1 3 . 8 ± 0.3
99 4.12 753 4-60 187 ± 1 5 378 4- 30 7 4. 1 100 53 4 - 5 17.24. 1.7 3.74- 0.4
THE DECAY OF Se 75
655
Although no half-life determinations have been made in the present study, the half-lives of all the strongly populated states in As 75 have now been either directly measured i, 2, is), or the levels have been Coulomb excited lg); for the sake of completeness half-lives of the levels are shown in fig. 1. and listed in table 3. The total electron capture energy obtained from the data reported b y Trail and Johnson 14) and Gossett and Butler 15) is also given in fig. 1. We have observed the 577 keV gamma-ray reported b y Langevin-Joliot and Langevin is) but only in the NaI spectrum. The gamma-ray intensities and conversion electron intensities reported b y the various groups are compared in tables 4 and 5. The conversion electron intensities reported b y the various investigators do not vary beyond the experimental errors, with the exception of the values reported for the 96.74 keV transition and the value reported b y Grigorev et al. for the 121.12 keV transition. The relative gamma-ray intensity measurements are generally in very good agreement; the only exception to this is the measurement of the intensity of the 279.57 keV line b y Metzger and Todd, their value of 474-1 differing considerably from the present value of 42.24-0.06 as well as from the values 414-2.5 and 42.54-2 measured b y Grigorev et al. and de Cro~s and B~tckstrdm respectively. Because this line is very close in energy to the 264.62 keV line, relative to which its intensity was measured, absorption corrections and the curved crystal reflectivity correction were very small; therefore the magnitude of our reported error is almost entirely due to counting statistics. Furthermore, the intensity of this line was remeasured four times. The highest value measured was 42.7 and the mean 42.2. For these reasons we feel that the value of Metzger and Todd is high. A point still open to discussion is the M 1+ E 2 mixing ratio of the 279.57 keV transition. Metzger and Todd 5) and later Manning and Rogers 20) have TABLE 6 Conversion coefficient d a t a for the 279.57 keV t r a n s i t i o n in As :5 K conversion electron intensity
Investigators
Gamma-ray intensity
S c h a r d t and Welker
45.74-4
53
Grigorev et al.
41
49.24-3
+2.5
4-8 7.64-0.8 7.6±1.0
Bashilov and II'in 53.64-1.6
7.04-0.4
42.5~2
63
4-5
9.94-1
42.24-0.6
53
4-5
7.64-0.9
Metzger and Todd
47
de Cro@s and BAckstr6m P r e s e n t investigators
~1
Mixing parameter
K conversion coefficient ( × 103)
~) This value differs f r o m t h a t reported in the original p a p e r 21).
i
0.51 +0.12 --0.13 0.51 +0.15 -0.16 +0.06 0.42_0.07 )
0 ~ +0.19 .vv --0.17 O al +0.14 .v. --0.15
656
W. F. EDWARDS AND C. J. GALLAGHER, J R .
commented upon the disagreement between the mixing parameters 81 as determined by conversion coefficient information and those determined by angular correlation. Table 6 gives the mixing parameters calculated from conversion coefficients assuming the theoretical coefficients of Rose la). The agreement is rather good in spite of the apparently high gamma-ray intensity of Metzger and Todd which lowers their value for the mixing parameter. The agreement is better than reported previously 5, s0) because a calculation error in Metzger and Todd's value for the mixing parameter has been corrected ~1). The angular correlation data for this transition are summarized in table 7. TABLE 7
A n g u l a r correlation d a t a for t h e 279.57 k e V t r a n s i t i o n in As ~5 Investigators
a.,
Schardt and Welker
- - 0 . 4 2 -}-0.03
--0.444-0.11
Kelly and Wiedenbeck
--0.41 4-0.03
--0.414-0.08
v a n d e n Bold et al.
--0.4664-0.02
- - 0 7~+0.26a) •
~ --0.21
a) T h i s v a l u e differs f r o m t h a t r e p o r t e d in t h e original paper, t h e error limits h a v i n g been inc o r r e c t l y a s s i g n e d *~).
The agreement among the various values for the mixing parameter is now reasonable, primarily because the error limits on the value --0.744-0.10 previously reported by van den Bold et al. were in error and have now been corrected in such a way as to improve the agreement 23). The mean of the eight values for the mixing parameter is 0.47. The deviation of the individual values from the mean is certainly reasonable as reflected by the z2-test. The expected value of Z2 which is equal to the number of degrees of freedom or amount of over-determination of the system, is 7. The calculated value is 8.0. The probability that Z~ be equal to 8.0 or more by random deviation alone is 0.33, indicating consistency of the data. If Metzger and Todd's value is eliminated from the set, the mean is 0.50 and X* is 7.2 (the expected value in this case is 6); the probability of the value 7.2 or greater by random deviation is 0.30. We conclude that the determinations of the mixing ratios of the 279.57 keV transition are in satisfactory agreement and that the best value for the mixing parameter is 0.504-0.06 which indicates an 80 % M l + 2 0 % E2 mixture for this transition. We are grateful to Professor J. W. M. DuMond and Professor F. Boehm for their interest and to Dr. G. Manning for helpful discussions.
THE
D~CAY
O F Se 76
657
References 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
F. R. Metzger, Phys. Rev. 110 (1958) 123 A. W. Schardt, Phys. Rev. 108 (1957) 398 W. F. Edwards, C. J. Gallagher, Jr. and J. W. M. DuMond, Bull. Am. Phys. Soc. 4 (1959) 279 A. W. Schardt and J. P. Welker, Phys. Rev. 99 (1955) 810 F. R. Metzger and W. B. Todd, Nuclear Physics 10 (1959) 220 Grigorev, Zolotavin, Klementev and Sinitzyn, Izvestia Akad. Nauk SSSR 23 (1959) 159 A. A. Bashilov and V. V. II'in, Izvestia Akad. Nauk SSSR 23 (1959) 159 M. DeCroSs and G. B~ckstr6m, Ark. f6r Fys. 16 (1960) 567 W. H. Kelly and M. L. Wiedenbeck, Phys. Rev. 102 (1956) 1130 I-L C. Van den Bold, J. Van de Geijn and P. M. Endt, Physica 24 (1958) 24 W. Edwards, P h . D . Thesis, California Institute of Technology, 1960 (unpublished) Cohen, DuMond, Layton and Rollett, Rev. Mod. Phys. 27 (1955) 363 M. E. Rose, Internal conversion coefficients (North-Holland Publishing Co., Amsterdam, 1958) C. C. Trail and C. H. Johnson, Phys. Rev. q l (1953) 474 C. R. Gossett and J. w . Butler, Phys. Rev. 113 (1959) 246 S. A. Moszkowski, Phys. Rev. 82 (1951) 25 F. Boehm and C. J. Gallagher, Jr., Phys. Rev. 119 (1960) 258 14. Langevin - Joliot and M. Langevin, J., Phys. Rad. 19 (1958) 765 Alder, Bohr, Huus, Mottelson and Winther, Revs. Mod. Phys. 28 (1956) 432 G. Manning and J. Rogers, Nuclear Physics 15 (1960) 166 F. R. Metzger, private communication 14. C. van den Bold, private communication