The beta-ray spectrum of 71arsenic

Stoker, I?. H. Ong Ping Hok 1953

Physica

SIX

279-286

THE BETA-RAY

SPECTRUM

by P. H. STOKER

OF 7’ARSENIC

“) and ONG PING

HOK

Synopsis The activity of 7’Xs , obtained by bombarding Ge with deutcrons of 26 and 14 NeV has been investigated. From the decay of the electron line of 163,9 keV the half-life was found to be 59.5 $ 2 hours. The 175 kc\’ y-ray yaw the ratio eK/er+Li.,r = 8.3. The Fermi-Iiurie plot of the positon spectrum allowecl two interpretations: a simplex spectrum with endpoint at 800 ke\- or a complcs one \vith endpoints at 675 and 850 keV. The two corresponding decay schcmcs are discussed. Four new electron lines were detcctcd, which wcrc ascribed to L- and M-conversion lines of a 13 Ice\’ y-ray from 73As, and to K-L, L and I<--L., M Augerlines.

1. Infrod,l/ction. In 1939 S a g a n c et al. 1) found a positonactivity with a half-life of 50 hours, when Ge was irradiated by deuterons of 3 MeV. By absorption measurements the maximum energy of this radiation proved to be 0.6 MeV. They ascribed this activity to 7’As. Hopkins and Cunningham2) in 1948 found an activity of 52 minutes in As irradiated with 190 MeV deuterons, which they also ascribed to “As. According to N uc 1 e a r D a t a 3) they have also found a 60 hours’ activity. M c-C o w n et al. “) determined the half-life of the positon and K-X-ray activity to be 50 hours with a ratio K/P+ of 2 : 1. By mass-spectrometric measurements B r a c h e r and C r a t h o r n “) determined the mass number of the active As isotope with a half life of 50 hours to be 7 1. In the course of measurements on 72As, obtained by bombarding Ga with a particles, M e i et al. “) found an electron line of 162 keV decaying with a half-life of 60 hours. They supposed this y-ray to be emitted by 7’As. *) Sow

at the Potchcfstrootn

University

-

for C.H.E.,

279 -

Potcbefstroom,

Sonth-Africa.

280

P. H.

STOKER

AND

ONG

PING

HOI<

As beta-spectrometric measurements of the positon radiations have not yet been done, we have made some measurements concerning the half-life (section 3), the positon spectra (section 4) and the conversion lines in the negaton spectra (section 5). In section 6 the possible disintegration schemes of 7’As are discussed. 2. Preparation of the radio-active sozLyces. The investigations were performed with radioactive As, obtained by bombarding GeO, with deuterons in the cyclotron of the Institute for Nuclear Research at Amsterdam. By this irradiation the following radioactive isotopes will be formed, viz. 71As (60h), 72As (26h), ‘jAs (76d) and 74As (175d). The 73As-activity is a pure K-capture and so does not interfere with measurements of the electron activities. The 71As isotope is formed by a (d, n) reaction whereas 72As and 74As are also produced by (d, 2n) reactions, especially at higher deuteron energies. Therefore a relative high yield of 7’As was obtained by bombarding with 14 MeV deuterons, while 26 MeV deuterons were used to obtain a larger 72As activity. The active As was separated from Ge by precipitating it as a phosphate, a small quantity of non-active As serving as a carrier. Then the As was distilled as AsCl, and dissolved in HCl. After addition of H,O, or HNO, it was concentrated in a few drops, brought on a pep film ‘) and dried in vacuum. The source thus obtained was about 0.4 mg/cm2 thick. For As sources to be used in vacuum the non-volatile As,S, is commonly used. According to our experience As dissolved in acqua regia gives a non-volatile component, when it is dried in vacuum. This oxidizing agent prevents the formation of the volatile AsCl,. 3. The half-life of 7’As. A determination of the half-life of 7’As was made by following the decay of the 163,9 keV electron line in the beta-spectrometer. In fig. I the log of the area of the electron line is plotted versus time. The points of two separate measurements were normalized to the intensities of the corresponding 7’As spectra, found by the analysis as described in section 4. From this graph we deduced a half-life of 59.5 f 2 hours. This value was used for the decay correction of the positon spectrum and gave satisfying results. Measurements of the half-life were also performed by Mr. J. W e b e r in our laboratory by measuring the annihilation radiation

THE

BETA-RAY

SPECTRUM

OF

7’ARSENIC

241

and y-rays with a y-counter. However his result of 65 hours was not so certain due to the strong annihilation radiation from 72As.

Fig.

1

The

decay

of the

163.9

keV

electron

line

of “As.

4. The positon spectrum of 7’ j 4s . The measurements were performed in a 16.2 cm double-focusing magnetic P-ray spectrometer, working at 1.5% resolution.

IS00

r

-

HP A

Fig.

2. The

positon Ge by

spectra deuterons

--c

lip 6

of the As isotopes 1;2 day after irradiation of 26 MeV (-4) and 14 MeV (B).

of

In order to separate the p-‘--spectrum of “As from that of 72As and 74As, use was made of two sources, obtained from irradiations by 26 MeV and 14 MeV deuterons respectively. From both sources the spectrum of 74As was determined after the other activities had decayed. From the first source the spectrum of 72As can be obtained

282

P. H. STOKER

kND

ONG

PING

HOK

fairly accurately, since in this case the spectrum of 72As is much more intense than that of 7’As. These intensities will be more equal after some time owing to their different half-lives. So the spectrum was remeasured after 5 days and by a method of successive approximation the spectrum of 72As was found. From the second source the spectrum of 7’As was obtained by subtracting the spectra of the two other activities, fitting the 72As spectrum of the first source to the high energy side of the positon spectrum. In fig. 2A and 2B the total positon spectrum and the analysed partial activities of the different Asisotopes are represented, 14 day after irradiation of Ge by 26 and 14 MeV deuterons respectively.

Fig.

3.

The

positon

spectrum

of 7’As spectrum.

and

a

Fermi-ICurie

plot

of

the

In fig. 3 the final result of the 7’As spectrum as well as the FermiKurie plot is given. For this plot the j-function was calculated by the approximation of B e t h e and B a c h e r “) and corrected by the correction factors of R e i t z “) for screening of the Coulomb field. The straight line through the experimental points gives an endpoint of the spectrum at 800 f 20 keV. However the possibility of a complex spectrum is not ruled out, because of the existence of the well known 175 keV y-ray of 7’As. Indeed experimental points are found above the background up to 850 keV, though they are rather uncertain, since they have been found after subtraction of two other spectra. When they are considered to be real, the spectrum may also be interpreted as consisting of two components with endpoints at 850 and 675 keV. The deviations of the points from the straight line below 150 keV may be due to the thickness of the source.

THE

-

BETA-RAY

SPECTRUM

OF 7’ARSENIC

283

5. Internal conversion lines : Some of the conversion lines found in the negaton spectrum of the first source, two days after irradiation, are shown in fig. 4. In table I the energies of all the conversion lines found have been tabulated. They were ascribed to the various Asisotopes on account of their rate of decay. The four lines of smallest energy have not been reported before, and the energies of the others were determined more accurately. $2650

IOCJO-

, ‘3 t K2

0

’ 300

Fig.

n

4.

“II’I’JIII’ 400 -.

Low

f[II’ IO00

1500

energy

conversion

lines

TABLE Conversion Isotope

e--line (fig. 4)

lines of the radio

of the

As isotopes

Iokzation cnrrgy of Ge-atom (IieV)

’

(0-cn1)

(kc\‘)

1470 I520 3608

163.9 173.7 686

I I.1 1.4 II.1

Energy of y-ray (Ice\‘) 175.0, 175.1 / 697

705.0 785.1

42.0 51.5

11.1 1.4

53. I > 52.9

365.3 383. I

I12.8 I.6

1.4 0.2 transition

Auger

-

As isotopes.

I active

Energ!,

I-Ie L

2000

HP

306.2 327.9

/

8.2 9.4

1

Ii + L, L I< + L, iv1

13.0 13.0 > Calc. Mean Ewrgy (‘WV) 8.4 9.6

284

I’.

H.

STOKER

AND

ONG

PING

HOK

The lines K, and L, are due to the conversion of the 175 keV y-ray of 7’As. From the e;/e,,,f ratio of 8.3, we concluded this y-ray to be E2, M 1, M2 or M3 lo). The ratio eLIpi was 0.14. The K-conversion line of the 697 keV y-ray of 72As was first found by M i t c h e 11 et al. ll). in the negaton spectrum of 72Ga and later by M e i 6) in the positon disintegration of 72As. For the ratio eK/p+ we found (9.0 f 0.2). 10e3, as the mean of measurements from both sources, while M e i found 12.10-j. K, and L, are conversion lines of a 53 keV y-ray of 73As, previously found by E 11 i o t and D e u t s c h 12). In accordance with their result we found for e,/e,,,, : 5.2, indicating a M3 radiation lo). The L, and M, lines have not been found before *) and can be attributed to a 13.0 keV y-ray also of 73As. A, and A, may be K -+ L, L and K + L, M Augerlines on account of their energy. In all As-isotopes intense K-Augerlines may occur since the K-fluorescence yield is about 0.50. A rough estimate of the intensities of the K -+ L, L and K --f L, M Augerlines, corrected for counter window absorption 13) gives for their ratio 1 : 0.6, agreeing with the theoretical calculations of P i n c h e r 1 e r4) (1 : 0.58) but not with the experimental results of F e r e n c e r5) (1 : 0.31). Between 450 and 1200 keV the negaton spectrum had several points above the continuous spectrum, which could be due to conversion lines of low intensity. 6. Disczdssion of the decay scheme. For all nuclei with an odd number of protons or neutrons between 29 and 37 a spin 312 has been found, with exceptions of ,8:Rb and $Zn, which have a spin 512. According to the shell-model the ground states of these groups of nuclei are pslz and f5,,,respectively 16). Since ~:As has a fi3,* ground state and since the ground state of i:As is also determined by the 33rd unpaired proton, it can be expected that 7’As has the same ground state; an f5,*state however is not excluded. For the spin of the ground state of :AGe, the decay product of 7’As, no experimental data are available, since the spin of a nucleus with 39 neutrons is not yet known. 39 89Y having 39 protons may be described in its ground state by a p,,* term; so a fill2 term can also be expected for j;Ge. In fig. 5 two possible disintegration schemes are given with the *) came

Since this note to our notice,

was written a paper which affirm this

of S. J o h a II s s o n (A&iv Fysik result, but gives an energy of 13.5

4 (1952) 273) -& 0.3 lte\‘.

THE

BETA-RAY

SPECTRUBI

285

OF “ARSENIC

calculated ratios K//3’, based on the tables of F e e n b e r g and T r i g g 17). In the case of a simplex spectrum (fig. 5A) the log ft of the positon disintegration is 5.7 and therefore is allowed. From the experimental value for e,/Pi and the calculated K/p+ ratio it follows that the K-conversion coefficient of the 175 keV y-ray must be 0.042. According to the tables of R o s e et al. l*) this transition must be 80% Ml and 20% E2, giving d1 = 1 (no). If the ground state of “Ge is $I!? the first excited level will be p:l:8. For a forbidden ground to ground state transition 7’As must have a fjlB-level.

Fig.

5.

Possible

disintegration

schemes

for

7’As.

From this follows that for a complex positon spectrum the ground state of 7’As will be f13,* (fig. 5B). In this case the relative intensities of the positon components were calculated assuming E2 for the y-radiation, from which follows that the excited level of 7’Ge is fhiz. From the theoretical K-conversion coefficient ‘8) and the experimental value of e;//?’ the intensity ratio of the partial positon spectra was estimated. For the log/t of the positon transitions to the ground state and the excited state of “Ge the values 5.9 and 6.4 were found respectively. The f5,?term for the excited level makes the positon transition to this level L-forbidden in accordance with the higher logft value, while the transition to the ground state is allowed. Howelrer the excited level may also be a pa,, state, and the y-transition a mixture of Ml and E2. Our experimental results were not accurate enough for a determination of the relative intensities of the positon components, so the intensities of each of these radiations in the y-transitions can not be calculated. It is difficult to decide between these two decay schemes. The complex spectrum has in its favour the more probable level-assignment according to the shell-model, while the experimental facts are

286

THE

BETA-RAY

SPECTRUM

OF

“ARSENIC

not in contradiction with it. The simplex spectrum has a somewhat stronger experimental background but a f5,2ground state of 7’As, although not excluded, is not very probable. Experiments with a pure 71As source will decide between these two possibilities by an accurate measurement of the endpoint or b>. @- y coincidence measurements. These measurements will be made as soon as possible. 7. Ack~aozaledgentents. It is a pleasure to thank Prof. Dr G. J. S i z o o, and Prof. Dr C. C. J o n k e r for their valuable help ancl the many discussions on this problem. We are indebted to Prof. Dr A. H. W. At e n Jr. for suggesting this problem and for his collaboration in the preparation of the activity at the Institute for Nuclear Research, and to Mr. B. V e r k e r k for the chemical separations. Thanks are also due to Messrs. L. D o r s m a n and H. W. H o r em an for their assistance and to Mr. W. Jon gs m a for his technical assistance. Rrceivrtl

2-l-53. lIEFl:l
1) 2) 3) 4)

9) ‘0) ‘1)

1-3 13) 14) 15)

16) 17) ‘8)

S n g 3 11 c, Ii., Ii 0 j i m a, S., 11 i y a m 0 t 0, G. and I Ii a w a , M., Proc. phys. math. SW. Japnn dl (1939) 660; S a g a II e, I<., >I i y n 111 0 t 0, G. and I k a w a, bl., I’hys. Rrv. 3!1 (1941) 904. H o p k i II s Jr., H. IH. mcl C u II II i II g 11 a m, B. B., Phys. Rev. 7:I (1948) 1406. Xuclrnr Data, Sntionnl I%nwnu of Stnlldard’s Circular 449 (Sept. 1, 1950). M c C o \v II, D. A., W o o d \I n r d, 1.. L. and I’ o o 1, >I. I~., l’hys. Rev. 7’1 (1948) 1311, 1315. U r n c II e I-, 1). b’. and C r a t h o I‘ II, A. R., Sature IW (1952) 364. 81 e i, J. Y., 31 i t c h c 11, A. C. C. and H u d d 1 e s t o II, C. \I., Phys. Rev. 79 (1950) 19. H c c r s c h R p, 11. and S t o k e I-, P. H., to lx published in Physica. R c t 11 e, H. A. and B a c h e r, R. F., Rrv. mod. Phys. U (1936) 184. 1: e i s t c r, I., Phys. Iicv. 70 (1950) 375. Ii c i t z, J. R., Phys. Kcv. 77 (1950) IO. G o 1 d 11 n b e r, 11. aud S u II y ;, r, A. W., Phys. Rev. 8:j (1951) 906. Jl i t c he 1 I, A. C. G., Z n f f n r a n o, 1). J. and Ii c r II, B. I)., Phys. Rev. i.? (1948) 1424. E 1 1 i o t, C. D. and I:, c u t s c h, M., Phys. Rev. (;:I (1943) 457. S a s o II, Il., 1’11~s. Rev. 81 (1951) 639. Pincherle, L.,NuovoCimento1~(1935) 81. F e r c n c c, AI. Jr., Phys. Rev. r,l (1937) 720. I< o p f e r m a n n, H., Die Saturwiss. 37 (1951) 29; K or d 11 c i m, L. \\‘., Phys. Rev. 75 ( 1949) 1894. F e e II b e r g, E. and T r i g g, G., Rev. mod. Phys. ‘9 (1950) 399. Rose, M.E., Gocrtzel, G. H., Perry, c. L., K-shell 1ntem. conv. Coeff., Revised tables, Oak Ridge (June, 25, 1951); Phys. Rev. 83 (1951) 79.