124Sb spectrum shape

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Nuclear Physics 73 (1965) 379--384; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

l ~ S b SPECTRUM SHAPE S. T. HSUE, L. M. L A N G E R , S. M. T A N G and D. A. Z O L L M A N

Department of Physics, Indiana University, Bloomington, Indiana t Received 6 May 1965 Abstract: The beta l ~ S b spectrum above 1.0 MeV was measured with a magnetic spectrometer employing a solid state device as detector. The emphasis was on the detailed shape of the highest energy group. The use of the solid state detector resulted in much lower background and helped in the precise determination of the end point and the shape. Attempts to check on the possible existence of a 1.658 MeV beta group and on the shape of the 1.59 MeV group were also made. The experimental shape of the highest energy group was found to be best fitted with an empirical shape factor S~tq2+21p2+D, with D = 164-2. The shape is also fitted very well by S = k(I+aW4-b/W+CW 2) with a = --0.250, b = --0.060 and c = 0.041. The end point of this group was found to be 2.305 4-0.005 MeV, which is somewhat lower than previous determinations. The data suggest the existence of the 1.658 MeV group, or, that if there is no such group, the shape o f the 1.59 MeV group differs from the statistical shape.

E I i

RADIOACTIVITY, 124Sb [from 12aSb(n, ~,)]; measured E# (spectrum shape), Q#.

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1. Introduction The theoretical shape factor of a once forbidden non-unique beta transition can be written in the general form: S = k(1 + a W + b~ W + c W 2), where W is the total energy of the electron and k, a, b and c are functions of the nuclear matrix elements. If one assumes that the B~j matrix elements is dominant 1, 2), then according to Kotani, the shape factor can be expressed as S = l"iL~(q2+AtpZ)+kn(1 + a n W + b / W + c n W 2 ) ,

where p and q are the momenta of the electron and the neutrino, respectively and 21 is a known tabulated function of p and the atomic number Z of the daughter nucleus 3). In addition to the shape factor measurement, information about then uclear matrix elements can be obtained from the measurements of the comparative half-life o f the beta transition ( f t value), the beta-gamma angular correlation and the beta circularly polarized gamma correlation. Thus, in principle, these parameters kn, an, b and Cn can be determined with the information obtained from these independent experiments. Antimony 124 has been one of the most measured nucleides of those having once forbidden non-unique transitions: The part of the decay scheme of interest in this investit W o r k supported by the Office of N a v a l Research under Contract Nonr-1705(02). 379

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gation is shown in fig. 1. The non- statistical shape of its 3- --* 2 + transition has been observed 4-s) and several attempts have been made to determine the matrix elements 9-13). To evaluate the matrix elements, a precise measurement of the shape is vital. Previously, the experimental shape factor was reported to have been fitted with So:q2.-I-~.tp2WD, where D was reported to be 15+5 and 74-4 for two independent measurements 8). A remeasurement, in order to determine the shape with greater precision, was considered to be worthwhile.

3 . 51sbl24~ 1.900 MeV 1.350 Beta Transilion

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Fig. 1. Partial decay scheme of 121Sb.

It has been suspected that there might be a beta group, referred to as f17 by nuclear data sheets 14), with an end point of 1.658 MeV. Also it has been reported that the beta group, called f16, of end point 1.59 MeV has a statistical shape 7). It was our intention to try to clarify these points. For this, a precise measurement of the shape of the outer group is necessary in order that the residual, after the subtraction process, may be definitive.

2. Experimental Procedure The 124Sb beta spectrum was studied in a high resolution, 40 cm radius of curvature, 180 ° focussing magnetic spectrometer 15). Source and detector of 4 mm width were used, resulting in a resolution of 0.65 %. The detector was a surface barrier solid state detector operated at 77°K. At this temperature, the background counting rate was lower than that of a proportional counter by a factor of 10. It has been shown 16) that the solid state detector requires no correction for the variation of back scattering with energy if the electronic detection is integrally biased sufficiently low. When so operated, this detector has the same energy response as the proportional counter over

124Sb S P E C T R U M

381

SHAPE

the energy region o f interest. T h e s p e c t r o m e t e r was c a l i b r a t e d between a n t i m o n y runs with the i n t e r n a l c o n v e r s i o n line o f 137Cs. F o r this experiment, two sources were p r e p a r e d f r o m high specific activity SbCla o b t a i n e d f r o m O a k R i d g e N a t i o n a l L a b o r a t o r y . A b o u t 2 m C u r was used to m a k e a relatively thin source o f a p p r o x i m a t e l y 0.06 m g / c m 2, a n d a b o u t 7 m C u r was used to m a k e an intense source o f thickness a b o u t 0.22 m g / c m 2. T h e thin source was used to ascertain t h a t the t h i c k source d i d n o t d i s t o r t the s p e c t r u m in the region o f interest. T h e thick, m o r e intense source was used to p r o v i d e better statistics. Since SbC1 a is deliquescent, a n t i m o n y sulfide is p r e f e r r e d as a source. The chloride s o l u t i o n was d i l u t e d a n d H2S was p a s s e d t h r o u g h the s o l u t i o n to f o r m Sb2Sa precipitate. T h e p r e c i p i t a t e was w a s h e d several times with water. T h e n a suspension o f Sb2S 3 in w a t e r was d e p o s i t e d o n t o a 20 # g / c m 2 thin Z a p o n film s u p p o r t e d b y a 0.58 m g / c m 2 M y l a r b a c k i n g . Insulin was used to define the source area, a n d a thin Z a p o n c o v e r film was p l a c e d over the source. T h e source m a t e r i a l a p p e a r e d to be very unif o r m l y distributed.

3. Data and Discussion T w o runs were m a d e , one with each source. E a c h r u n involved r e p e a t e d passes over t h e s p e c t r u m f r o m 1.0 M e V to 2.4 MeV, well b e y o n d the e n d p o i n t o f the highest

l

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6.0

Fig. 2. F K plot for the beta spectrum above 1.0 MeV in the decay of z24Sb. The middle curve is with S = 1. The upper curve shows the linearization of the data after application of the best fitting shape factor for this run. The lower curve is the FK plot after the outer group has been stripped off. A is the end point of the fie beta group formerly reported. B is the expected end point of the suspected f17 beta group.

382

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HSUE e t al.

energy group. A l l p o i n t s have a 1 % statistical accuracy. The m i d d l e curve in fig. 2 shows the F K p l o t with S = 1 o f the d a t a o f run 2. T h e b u m p s between W -- 4.2 a n d W = 4.35 are the K , L a n d M internal conversion lines associated with the 1.695 M e V g a m m a r a y 7). The shape f a c t o r p l o t o f the same r u n is shown in fig. 3. T h e d a t a were fitted with the empirical shape factor S~q2+21p2+D. W i t h the least-

~ b 124 D= 15.0\ D = 17.3. \

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5.5

W Fig. 3. Shape factor plot of the 1~4Sbspectrum above 1.0 MeV.

Shape factor fit t o

1248b 0

Run 1 Wo = 5.516 D = 15.6 a b c

--0.252 --0.061 0.042

TABLE 1 spectrum, S = k(1 +aW+b/W+cW2) Run 2 14/o = 5.506 D = 17.3 --0.243 --0.056 0.040

Average 14/o = 5.511 D = 16 --0.250 --0.060 0.041

squares m e t h o d D was f o u n d to be 17.3 for run 2 a n d 15.7 for run 1. The solid line in fig. 3 is the e m p i r i c a l shape with D = 17.3, a n d the d a s h e d lines indicate values o f D o f 15 a n d 19, showing the sensitivity o f the fit. The u p p e r curve in fig. 2, with the internal conversion lines omitted, shows the linearization after the a p p l i c a t i o n o f the best fit shape f a c t o r for r u n 2. Hence the value o f D for a best fit is, using the average value o f the results o f the two runs, 16___2. A least-squares fit o f the d a t a with a shape factor, S = k(l+aW+b/W+eW2), was m a d e using the C D C 3600 c o m p u t e r . The best fit for the average o f the two runs is S = k(1 - 0 . 2 5 0 W - 0 . 0 6 0 / W + 0.041 W2). The consistency o f the fits for the i n d i v i d u a l runs is illustrated in table 1.

lg4Sb SPECTRUM SHAPE

383

The end point of the highest energy group obtained from the first run is W o = 5.516 mo c2 and from the second is W o = 5.506 mo c2 leading to an average value of E o = 2.305_+0.005 MeV and a value of Q~- = 2.906 MeV. The particular end point obtained for each run was used in the shape factor analysis of that run. The end point measured here is somewhat lower than some previously reported results, but appears to be more consistent with the known gamma-ray energies. The F K plot after the outer group has been stripped is shown in the lower left hand of fig. 2; A (1.59 MeV) is the end point of f16, reported by others 5,6, 17), B (1.658 MeV) is the end point of fiT, suggested by the nuclear decay scheme 14, is). If the two well-established gamma rays with energies 0.645 and 0.722 MeV are subtracted from the end point that we measure for the outer group, then one gets A' = 1.583 MeV and B' = 1.660 MeV. Notice that B and B' are about the same and A' is slightly lower than A. After the outer group has been stripped off, the spectrum above 1.0 MeV can be interpreted either as a simple spectrum of one group or as complex consisting of two groups. It is observed that there are data points beyond A, and that these do not shift significantly when different values of D (from 14 to 18) are used in the shape factor for stripping. This suggests that the end point of the beta group next to the highest energy one is greater than A. If there were only one beta group with an end point in the region 1.5-1.7 MeV, the F K plot would differ from a straight line and, hence, would be different from what was reported by Zolotavin et al. 7). Moreover, the end point of such a single group would be about 1.64___0.02 MeV as obtained by extrapolating the data. This is higher than the end point of f16 reported by others s, 6,17) and is close to that of f17 (1.658 MeV) suggested from the decay scheme 14). On the other hand, the data can also be interpreted in terms of two groups. Unfortunately, the energy range between A and B is very short and internal conversion lines lie in this interval. This, plus the cuncertainty introduced by the stripping process, prevent one from saying definitely that the f17 groups exists. However, the data do suggest such a group. If this group does exist, one would be unable to draw any conclusion concerning the detailed shape factors of either the f16 or f17 groups. If, indeed, there are two groups, then the data previously interpreted as f16 should have been regarded as being composed of both f16 and fiT. Consequently, the actual end point of f16 should be a bit lower than A, the previously reported deduction. This suggests why A ' is lower than A. References 1) 2) 3) 4) 5) 6) 7)

T. Kotani, Phys. Rev. 114 (1959) 795 M. Morita and R. S. Morita, Phys. Rev. 109 (1958) 2048 T. Kotani and M. Ross, Phys. Rev. 113 (1959) 622 Lazar, Langer and Moffat, Phys. Rev. 91 (1953) 338 J. Moreau, Compt. Rend. 239 (1954) 800 T. Azuma, J. Phys. Soc. Japan I0 (1955) 167 Zolotavin, Grigorev and Abrovian, Izv. Akad. Nauk SSSR (ser. fiz.) 20 (1956) 289; Columbia Tech. Transl. (1956) p. 271

384 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18)

s.T. HSUE et aL L. M. Langer and D. R. Smith, Phys. Rev. 119 (1960) 1308 J. W. Sunier, Helv. Phys. Acta 36 (1962) 429 P. Lipnik and J. W. Sunier, Nuclear Physics 53 (1964) 305 P. Alexander and R. M. Steffen, Phys. Rev. 124 (1961) 150 G. Hartwig, Z. Phys. 161 (1961) 221 Fischbeck, Greenberg and Newsorac, Bull. Am. Phys. Soc. 9 (1964) 114 Nuclear Data Sheets, National Academy of Sciences, National Research Council (U.S. Government Printing Office, Washington, D.C.) L. M. Langer and C. S. Cook, Rev. Sci. Instr. 19(1948) 257 D. E. Wortman and L. M. Langer, Phys. Rev. 131 (1963) 325 Tomlinson, Ridgway and Gopalakxishman, Phys. Rev. 91 (1953) 484A R. K. Girgis and R. van Lieshout, Physica 25 (1959) 113