“Self-scintillation” study of the beta decay of Rb87

4.E

I

Nuclear Physics 28 (1901) 570----577; ( ~ North-Holland Publishing Co., Amsterdam

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Not to be reproduced by photoprint or microfilm without written permission from the publisher

"SELF-SCINTILLATION"

S T U D Y O F T H E B E T A D E C A Y O F R b s~ G. B. B E A R D

Department o/ Physics, Wayne State University, Detroit, Michigan and W. H . K E L L Y

Department o] Physics and Astronomy, Michigan State University, East Lansing, Michigan t R e c e i v e d 5 J u n e 1961 T h e b e t a d e c a y of R b 87 h a s b e e n s t u d i e d u s i n g t h e self-scintillations p r o d u c e d in t h r e e NaI(T1) c r y s t a l s d o p e d w i t h r u b i d i u m . T h e b e t a s p e c t r u m w a s m e a s u r e d d o w n to 5---8 keY. A n e n d p o i n t e n e r g y of 2744-3 k e V a n d a specific a c t i v i t y e q u a l to 7 8 0 4 - 1 4 s -1 p e r g r a m of n a t u r a l r u b i d i u m were f o u n d . A s s u m i n g a n isotopic a b u n d a n c e of 27.85% for R b sT, a halflife of (5.634-0.10) × 10 l° y is o b t a i n e d .

Abstract:

1. I n t r o d u c t i o n

The long-lived beta activity of Rb s7 has been of interest for some time 1--16). Since the transition is from the { - - state of Rb s~ to the { + ground state of Sr By,it is classified as non-unique third-forbidden. The half-life is of particular interest because of its potential usefulness in determining geological ages from the ratio of radiogenic Sr 8v to Rb By. However, owing to the low specific activity and low energy of the beta particles, it has been difficult to make precise measurements of this half-life and there has been considerable disagreement in the values quoted, ranging from about 4 × 1010 to 6 × 1010 y. Table 1 contains a summary of the published results of half-life determinations. Also listed are the authors and the methods they used. It is of interest to note that these half-life values fall into two groups, one in the range from 4.3 × 101° to 5.1 × 1010 y and the other in range from 5.8 × 1010 to 6.5 × 101° y. Apparently there is no correlation among the results of any particular method, with the possible exception that the two half-life determinations made using self-scintillations produced in RbI(T1) lie within the upper range. However, one should also note that the value obtained using the self-scintillations in a Rb loaded liquid scintillator lies in the lower range. Preston et al. 17) have made an analysis of the general beta spectrum shape using the results of Goodman is). While there is not complete agreement in the experimental shapes obtained in previous investigations the F-K plots lie close enough together to give agreement with the analysis b y Preston et al. above an energy of 40 keV. t W o r k s u p p o r t e d in p a r t b y t h e U n i t e d S t a t e s Air Force u n d e r C o n t r a c t No. A F 49(638)-10 m o n i t o r e d b y t h e Air Force Office of Scientific R e s e a r c h of t h e Air R e s e a r c h a n d D e v e l o p m e n t Command. ' 570

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TABLE 1 Half-life of Rb 9~ beta decay Rb s' half-life (× 10,0 y)

Method

Author (year)

Cloud chamber SrS'/Rb s' in minerals of ages determined from U/Pb Gas counter Gas counter Gas counter High pressure gas counter RbI(TI) scint, crystal Gas counter Gas counter

4.5 6.3

Gas counter(enriched Rb s')

6.2 ± 0 . 3

SreT/RbS~ in minerals of ages determined from U / P b SrS'/Rb 8~ in minerals of ages determined from A4°/K*° Gas counter Rb doped liquid scint.

5.O ~0.2

G. Orban ' (1931) F. Strassman and E. Walling s) (1938) S. Eklund s) (1946) O. Haxel et al. 4) (1948) M. Kemmerich 6) (1949) S. C. Curran et aL e ) (1951) G. M. L e w i s ' ) (1952) j . Flinta and S. Eldund s) (1954) I. Geese-Bahnisch and E. Huster 9) (1954) M. H. MacGregor and M. L. Wiedenbeck x0) (1954) L. T. Aldrich et al. n ) (1956)

4.6 ± 0 . 5

K. Fritze and F. Strassman 1,)

5.074-0.20 4.704-0.05

(1956) V¢. F. Libby ,s) (1957) K. F. Flynn and L. E. Glendenin")

4.72±0.08

(1959) W. l ~ u s c h and W. Schmidt

5.824-0.1 5.534-0.10

(1960) K. Egelkrautand H. Leutz ,e) (1961) Present work (1961)

Gas counter RbI (T1) scint, crystal NaI(TI, Rb) scint, crystal

5.8 6.5 6.0 6.2 5.9 6.2 4.3

4-1.0 4-0.6 4-O.6 ±0.3 ±O.3 4-0.2 4-O.4

~6)

The investigation described here was initially undertaken in an effort to obtain a beta spectrum reliable to lower energies than had hitherto been possible and to make a new measurement of the specific activity t. 2. T h e Self-Scintillating Crystals NaI (T1, Rb) crystals (i.e., rubidium doped NaI (T1) crystals) were grown with 1% b y weight of R b I added to the sodium iodide melt it. Since the end product was somewhat uncertain as to the rubidium content, sections of the ingot were saved from both immediately above (a 1.9 cm diam. b y 0.63 cm high disk) and below (1.9 cm diam. b y 0.63 cm high cone) the centre section. This centre section consisted of a 2.5 cm high b y 1.9 cm diam. cylinder. The top and bottom sections were studied unmounted, whereas the central section was mounted in a thin aluminium can in the usual manner. An undoped NaI(T1) crystal, 2.5 cm high and 1.9 cm diam., was grown under the same conditions and mounted in the same way. This undoped crystal was used for comparison measurements and for all background determinations. t A preliminary account of this work has been given earlier ~). tt These crystals were grown b y the Levinthal Electronic Products, Inc. Palo Alto, California.

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The Rb s~ counting rate per gram of crystal was determined for each of the three sections. The resulting values differed considerably and showed that the rubidium did not distribute uniformly throughout the ingot. The conical tip where the ingot started contained about 0.1 ~ RbI while the mounted crystal had 0.2 ~/o and the disk contained 0.4 %. Considering that Rb + has an ionic radius of 1.49 A while that of Na + is only 0.98 A, it is not surprising that the rubidium segregated out in large measure. Recent work on the distribution of T1 in NaI ('171) crystals 19) shows that it is also not distributed uniformly. To see whether the non-uniform distribution of the Rb had any undesirable effects on the scintillation properties of the doped crystals, their scintillation properties were compared in some detail with those of the comparison crystal. The scintillation efficiencies of the three Rb doped crystals were found to be about 80 O/o of the result obtained using the undoped crystal. The values for the cone and disk were slightly higher and lower, respectively, than that of the mounted section, indicating that the scintillation quenching is dependent on the relative rubidium content. For the 662 keV Cs 13~gamma rays, the resolution of the mounted doped crystal was 13.6 ~o as compared to 12.4 ~o for the undoped comparison crystal. Measurements of the pulse-height spectra of collimated Cs 18~ gamma rays incident on the side of the mounted crystals, normal to the cylinder axis, were made as a function of position above the photomultiplier. A slight effect on the position of the photopeak was noted which was, however, the same for both the doped and undoped crystals and hence was ascribed to geometry effects rather than crystal composition. The pulse-height, as a function of gamma ray energy, was measured for energies in the range from 0.030 to 1.33 MeV with the mounted crystal. The gamma and X-ray sources were placed approximately 10 cm vertically above the crystal in each case. Similar tests with the two unmounted sections were made in the range from 0.008 to 0.662 MeV. In all cases the linearity was found to be better than 2 ~ for energies above 80 keV. Below this energy X-rays were used and linearity was obtained to the accuracy that the average X-ray energies were known (~ 5 %). Since the mean free paths of these different gammas in sodium iodide v a r y from 10 -8 m m to 5 cm, this also implies that the non-uniformity of Rb concentration along the length of the crystals does not adversely affect the scintillation response to a significant degree. We therefore concluded that the position in which a beta particle of given energy is emitted b y a Rb s7 nucleus would have relatively little effect on the amplitude of the resultant photomultiplier pulse and, consequently, little effect in distorting the beta spectrum. Optical transmission measurements were made on the doped pieces and compared with those made on undoped NaI(T1) crystals. No differences were found in the wavelength region in which the photomultipliers are sensitive ~o).

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3. Results

The beta counting was performed in a shielding arrangement that has been previously described 21). No anticoincidence arrangement was used. The integral counting was performed with as many as four completely different counting systems: two single-channel analyzers and two multi-channel analyzer systems. The counting rates obtained with the different systems agreed to within TABLE 2

Results of the chemical analyses and integral counting on the NaI(TI, Rb) crystals

Crystal

Disk (1.9 c m diam. × 0.63 c m high) Cone (1•9 cm diam. × 0.63 c m high) M o u n t e d section (1.9 c m diam. × 2 . 5 4 c m high)

Mass (g)

Total R b c o n t e n t (rag)

Average R b 8~ integral counting rate (counts/min)

R b 87 specific activity R b a~ half-life (counts/min ( x 10 l° y) p e r m g Rb)

6.763

10.034-0.10

4724-9 a)

46.84-1.0

5.534-0.12

2.874 26.76

0.9444-0.02 14.294-0.14

4 3 8 + 18 a ) 671 4- lO b )

46.44-1.9 47.04-0.8

5.584-0.23 5.514-0.10

Weighted average

5.53 4- 0.10

a) D a t a t a k e n with one single-channel s y s t e m and one multi-channel s y s t e m . b) D a t a t a k e n with two separate single-channel s y s t e m s and two separate multi-channel systems• 80C

70C

60C

~ 50C

20(:]

I00

0

I 5

I I0 CHANNEL

I 15 NO.

t 20

2:5

Fig. I. R b s~ integral s p e c t r u m . These d a t a were t a k e n w i t h t h e m o u n t e d NaI (T1, Rb) crystal and one of t h e multi-channel pulse-height analyzer systems. The e x t r a p o l a t i o n to zero energy is linear from a p p r o x i m a t e l y 5 to 6 keV.

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G. B. B E A R D AND W . H. K E L L Y

4- 2 ~/o. Table 2 contains the average values of the integral counting rates obtained for each crystal. The background counting rate was determined using the mounted undoped comparison NaI (T1) crystal. A factor to normalize the background measured with this crystal to that detected b y the disk and cone was found b y comparing the counting rates above the beta end point. No such factor was needed for the mounted crystal. The data presented in table 2 have been corrected for background. Fig. 1 shows a typical integral spectrum. This particular spectrum was taken with an EM19578 S photomultiplier, the mounted crystal and a Nuclear Data model 102 transistorized 256-channel pulse-height analyzer. Quantitative chemical analyses were performed on each of the three doped crystals at the Argonne National Laboratory after the counting was completed. A detailed description of these analyses will be published elsewhere 22). The rubidium content was determined b y precipitating rubidium as the R b # 4 B compound and weighing as such. The crystal was first dissolved in H 2 0 , treated with H N O 8 to remove I~ and then converted to NaC1 b y evaporation to dryness with HC1. Thallium which is also precipitated b y the precipitant, sodium tetraphenylboron, was removed using anion exchange prior to the precipitation of the rubidium. In the analyses determinations were made on duplicate aliquots of the unknown solution as well as on standard solutions. It was necessary to correct the rubidium value first obtained for small amounts of potassium and cesium which are also precipitated in the method of analysis used. The amount of potassium contamination was determined using a flame photometer and the potassium content was found to be not more than a few per cent b y weight relative to the R b content in each case. It is most likely that this potassium was introduced as an impurity in both the NaI and RbI. The contribution to the measured beta activity b y the K 4° betas from this contamination was completely negligible. An estimate of the cesium contamination present was made spectrographically and found to be such as to contribute an error of only about 0.25 °/o in the Rb determination. Considering the various corrections that had to be made, it is felt that the R b analyses are accurate to within 1 °/o. The results of the analyses are given in table 2. The R b s~ weighted average half-life obtained with the three crystal sections is 5 . 5 3 ± 0 . 1 0 × 101° y. This assumes an isotopic abundance of 27.85 °/o for R b s~. The error includes the uncertainty in the R b analysis as well as that of the counting rate determination.

4. Spectrum Shape Fig. 2 shows an uncorrected beta spectrum obtained using the 2.5cm × 1.9 cm diam NaI (1"1, Rb) crystal. Also shown in fig. 2 is the background obtained

' 'SELF-SCINTILLATION"

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with the comparison crystal. Data were taken at normal amplifier gain and also at twice normal gain in order to observe the lower energy region in more detail. Fig. 3 is the Fermi-Kurie plot obtained. Corrections for finite energy resolution ~3) have been made. These corrections were minor except near the end point. The estimated correction for the escape of Rb 87 betas ~4) through the surface before losing all their energy was smaller than the statistical error over most of the spectrum. 3.5

i

x

2C

~x

3.0

-\ &

16

2.5 *. 14

•

:

• Rb a'' NORMAL GAIN t, RbS7 2 x NORMAL GAIN

"

• Rb s7 NORMAL GAIN = Rb s7 2 x NORMAL GAIN

~.. .. :

x EGELKRAUT 6 LEUTZ o FLYNN 8= GLENDENIN

2.0

12

v -7--g

1.5

1.0

-,

s

% •.o

0.5

4ii L BACKGROUNDxlO ' 2 I; "' .% •""@• :"-%

,

0

i

20

%

,%

,

,

40

L

,

I

60 80 CHANNEL NO.

,

I00

120

Fig. 2. B e t a s p e c t r u m of R b a7 a n d b a c k g r o u n d . T h e s e d a t a a r e u n c o r r e c t e d . T h e solid circles s h o w t h e d a t a t a k e n a t n o r m a l a m p l i f i e r gain and the triangles show those taken at 2 times n o r m a l gain.

0

510

,~o ,~o 26o ""'C-250 300 ENERGY (keV)

Fig. 3. R b a7 F e r m i - K u r i e Plot. T h e t r i a n g l e s are t h e p o i n t s t a k e n w i t h 2 t i m e s n o r m a l a m plifier gain. S h o w n for c o m p a r i s o n a r e repres e n t a t i v e p o i n t s f r o m t h e F - K p l o t s of E g e l k r a u t a n d L e u t z ( × ) a n d of F l y n n a n d G l e n d e n i n (o) n o r m a l i z e d a t 80 keV.

Fig. 3 also shows representative points from the F-K plots published by Flynn and Glendenin 14) obtained using a liquid scintillator loaded with rubidium and b y Egelkraut and Leutz 18) determined with a NaI(T1, Rb) crystal. The plots are all normalized at the 80 keV point. It canbe seen that the F-K plot obtained in this work is in good agreement with that reported by Egelkraut and Leutz down to around 40 keV. It is somewhat surprising that the deviation at the lower energies is so great as presumably neither electronic noise nor background should be important for energies above 5 to 10 keV. Egelkraut and Leutz present a figure containing F-K plots using RbI(T1), CsI(T1, Rb) and NaI(T1, Rb) crystals together with the results of other

576

G. B. BEARD AND W. H. KELLY

authors for comparison. Most of the published F-K plots, as well as the unpublished results of Goodman is), agree reasonably well at energies above 40 keV. We estimate that our F-K plot is valid down to about 5-8 keV where electronic noise becomes a significant factor. The actual point where the noise becomes significant is uncertain because of the different scintillation efficiencies of the doped crystal and the comparison one, 'i.e., the signal-to-noise ratios of the two crystals are different. 5. C o n c l u s i o n s

Table 1 also contains the result of the half-life measurement described above. It can be seen that the value obtained in this investigation lies in between the two groupings of previously published results, although it favours the higher set. In general, we are unable to offer any suggestions to explain the differences between the various results. However, a possible explanation of the difference between our half-life value and that of Egelkraut and Leutz could be as follows: Highly purified RbI is hard to prepare and a total contamination of other alkali metals, Na, K and Cs, of as little as 1{ ~ in the RbI used b y them could account for the difference in the half-life values to within the limits of error. The general shape of the beta spectrum agrees with the shapes obtained in previous investigations and so should also be in agreement with the analysis carried out by Preston et al. 17). We wish to thank Mr. K. J. Jensen who performed the analyses for the rubidium content and to express our appreciation to the Argonne National Laboratory for making it possible for Mr. Jensen to do this work for us. The assistance of Mr. W. N. Schreiner and Mr, D. A. Gollnick in the recording of the data and the computations is acknowledged. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13)

G. Orban, Sitzungsber. Akad. Wien l I a 140 (1931) 121 F. Strassman and E. Walling, Ber. Deut. Chem. Ges. 7 1 B (1938) 1 S. Eklund, Ark. Met. Astronom. Fys. A 33 (1946) Nr. 14 O. Haxel, F. G. Houtermanns, and M. Kemmerich, Z. Phys. 124 (1948) 705, Phys. Rev. 7 4 (1948) 1886 M. Kemmerich, Z. Phys. 126 (1949) 399 S. C. Curran, D. Dixon and H. W. Wilson, Phys. Rev. 84 (1951) 151 G. M. Lewis, Phil. Mag. 43 (1952) 1070 J. Flinta and S. Eklund, Ark. Fis. 7 (1954) 401 I. Geese-Bahnisch and E. Huster, l~aturwiss. 41 (1954) 495 M. H. MacGregor and M. L. Wiedenbeck, Phys. Rev. 94 (1954) 138 L. T. Aldrich, G. W. Wetherill, G. R. Tilton and G. L. Davis, Phys. Rcv. 103 (1956) 1045 K. Fritze and F. Strassman, Z. Naturfors. l l a (1956) 277 W. F. Libby, Anal. Chem. 29 (1957) 1566, Chem. Abstr. 52 (1958) no. 5997i

'*SELF-SCINTILLATION" STUDY 14) 15) 16) 17) 18) 19) 20) 21) 22) 23) 24) 25)

577

K. F. Flynn and L. E. Glendenin, Phys. Rev. 116 (1959) 744 W. l ~ u s c h and W.:iSchmidt, Fachausschuss Kernphysik Tagung in Heidelberg, (April, 1960) K. Egelkraut and H. Leutz, Z. Phys. 161 (1961) 13 M. A. Preston, G. H. Keech and J. M. Pearson, Phys. Rev. l l 9 (1960) 305 C, D. Goodman, P h . D . thesis, University of Rochester (1955), unpublished L. M. Shamovskii, L. M. Rodionova, and A. S. Glushkova, Bull. Acad. Sci. USSR, Phys. Series 22 (1958) 1; Colombia technical translations W. J. Van Sciver, private communication G. B. Beard and W. H. Kelly, Phys. Rev. 122 (1961) 1576 K. J. Jensen, to be published J. P. Palmer and L. J. Laslett, AECU Report 1220 (1951) E. der Matoesian and A. Smith, Phys. Rev. 88 (1952) 1186 G, B. Beard and W, H. Kelly, Bull. Amer, Phys. Soc. 4 (1959) 324