Neutron activation cross sections at 14.4 MeV for Si and Zn isotopes

2.A.I ]!

Nuclear Physics A122 (1968) 679--683; (~) North-HollandPublishing Co., Amsterdam Not to be reproduced by photoprint or microfilmwithout written permissionfrom the publisher

NEUTRON

ACTIVATION CROSS SECTIONS

A T 14.4 M e V F O R Si A N D Z n I S O T O P E S N. RANAKUMAR, E. KONDAIAH t and R. W. FINK School of Chemistry, Georgia Institute of Technology, Atlanta, Ga. USA tt Received 9 August 1968 Abstract: Pure Si powder has been used to compare the cross section for the 28Si(n, p)2SA1(2.238 min) reaction with that of the 5~Fe(n, p)56Mn reaction. In the case of 3°Si(n, p)a°mA1 (72.5 sec), previously reported gammas were not found; an upper limit of 7 mb is obtained for this cross section. The cross section for ~0Zn(n, 2n)egmZn is reported. Three (n, 2n), six (n, p), and two (n, ~) cross sections for Si and Zn isotopes are measured by using the method of mixed powders and Ge(Li) gamma detection. E

1

I

NUCLEAR REACTION 27A1, 2a,2a,s°si, e4,ee,esZn (n, p), a°Si, 6aZn (n, ~) 64,es,~oZn (n, 2n), E ~ 14.4 MeV; measured a.

1. Introduction The m e a s u r e m e n t o f a c t i v a t i o n cross sections using the m i x e d p o w d e r m e t h o d with G e ( L i ) detectors was first d e v e l o p e d in this l a b o r a t o r y by R a o a n d F i n k a), a n d the m e t h o d has been f o u n d 2) to give r e p r o d u c i b l e results within 4.5 %. T o o b t a i n g o o d r e p r o d u c i b i l i t y , it is necessary to c o m p a r e activities o f similar half-lives so t h a t n e u t r o n b e a m fluctuations d u r i n g i r r a d i a t i o n d o n o t i n t r o d u c e large errors in c o m p a r i n g the cross sections. H i t h e r t o , there has been no well-determined n e u t r o n a c t i v a t i o n cross section at 14.4 M e V for short-lived activities t h a t m a y be used as a s t a n d a r d . W e have chosen the 2aSi(n, p)2SA1 (2.238 m i n ) r e a c t i o n as the s t a n d a r d a n d have carefully d e t e r m i n e d its cross section against the well k n o w n 3) s t a n d a r d cross section o f 100__+6 m b for the 56Fe(n, p ) 5 6 M n (2.576 h) reaction. T h e silicon r e a c t i o n has m a n y a d v a n t a g e s as a s h o r t half-life s t a n d a r d ; for example; (i) zone-refined elemental Si with u l t r a - h i g h p u r i t y ( 9 9 . 9 9 9 + % ) is c o m m e r c i a l l y available; (ii) 2aSi has a large n a t u r a l isotopic a b u n d a n c e (92.18 %); (iii) a single g a m m a o f 1780 k e V energy is e m i t t e d in 100 % o f the 28A1 decays; (iv) because o f the high g a m m a energy, the internal c o n v e r s i o n is negligible, and s e l f - a b s o r p t i o n in the s a m p l e s g e n e r a l l y used ( ~ 1 g / c m 2) also is negligible. L i t e r a t u r e values 4) for the silicon r e a c t i o n cross section v a r y f r o m 180 m b to t NSF Senior Foreign Scientist and Visiting Professor, School of Physics, Georgia Institute of Technology (1967/68); on leave from Tata Institute of Fundamental Research, Bombay, and Andhra University, Waltair, India. *t Work supported in part by the U. S. Atomic Energy Commission. 679

680

N. RANAKLrMARet

al.

365 mb, and they are not determined directly at the neutron energy of 14.4_ 0.3 MeV used extensively in our measurements s). There also are large variations in the few cross sections 4,6,7) reported for the 298i(n, p)ZgA1, a°Si(n, p)S°mA1, and a°Si(n, e) 27Mg reactions. Similarly, the cross sections in the literature 4) for Zn isotopes show large variations.

2. Experimental Short-lived activities are measured using a fast rabbit system (transit time < 800 msec) between the accelerator target and a 16 cm 3 coaxial Ge(Li) detector. To facilitate rapid data collection from short-lived sources on the rabbit system, a magnetic digital tape recorder-data manipulator has been installed on the R I D L 400-channel analyser. This device permits 400 channels of stored data to be recorded in 7 sec on tape. The analyser can then store another 400 channels and repeat the tape storage. Up to 36 such 400-channel spectra can thus be successively stored on one tape cartridge for later printout on a fast (40 lines/sec) Franklin printer or on an XY plotter. The system permits the simultaneous determination of gamma energies, relative intensities, and half-lives from each rabbit sample. This system was used for the 28Si(n, p)28A1 cross section determination. In all of the measurements, the neutron flux was monitored. The following experimental runs were performed: (i) A mixture of zone-refined elemental Si powder was well mixed with iron powder of the same grain size, and the mixture was irradiated for various times (2 to 5 min) and gamma spectra recorded with the 16 cm 3 coaxial Ge(Li)detector (resolution 3.5 keV F W H M at 1332 keV). The photopeak areas under the 1780 keV (28A1) and 847 keV (56Mn) gammas were determined and used to compute the zsSi(n, p) 28A1 cross section, taking the value for the 56Fe(n, p)S6Mn reaction 3) to be 100-t-6 rob. The equation given in ref. 5) was used to perform the computation. (ii) Mixtures of A1 and Fe powders were irradiated for various times (5 to 10 min) in a constant neutron flux, and the cross section for the ZTAl(n, p)Z7Mg (9.46 min) was determined to be 68___8 rob, by comparing the photopeak areas under the 1013 keV (27Mg) and 1811 keV (S6Mn) gammas. Then the silicon cross section was determined against this secondary standard 27Al(n, p)Z7Mg reaction cross section in an irradiation of a mixture of Si and A1 powders. In all, a dozen such measurements were made giving essentially the same result for the 288i(n, p)ZSA1 cross section: 252-t-15 mb at 14.4-I-0.3 MeV. Previously reported values 4) of this cross section vary from 180 to 365 rob. Cross sections for other Si and Zn reactions were determined against one or more of the following monitor reactions, the parameters for which are given below: 28Si(n, p)ZSA1 (2.238 min); E~ = 1780 keV, f d = 1.0 (ref. 8)), ~ = 0 and a = 252 + 15 mb (present value); 27Al(n, p)ZTMg (9.46 min); E~ = 842 keV, fa = 0.696 (ref. 8)), e = 0 and a = 68 +_8 mb (present value);

72.5 sec b)

9.46 rain

38.4 min

3°Si(n, p)36~A1

a°Si(n, g)~7Mg

64Zn(n, 2n)63Zn

1115 1480

1078

1038

1340

439

1115

669 962

842

2240 b)

1280

1780

Er (keV)

0.16 0.2465

0.923

0.0925

0.005

1.0

0.49

0.0881 c) 0.0696 e)

0.696

0.608 b)

0.94

1.0

fa 0'+e)

b) fa, half-life, and E~ taken from ref. 6).

2.56 h

~aZn(n, a)~Ni

n) Assumed.

5.1 min

30 sec

~SZnfn, p)~Cu

12.8 h

s4Zn(n, p)~Cu

~Zn(n, p)~Cu

13.8 h

~0Zn(n, 2n)~gmZn

245 d

6.6 min

6eZn(n, 2n)65Zn

2.238 min

~Si(n, p)Z°Al

Half-life

28Si(n, p)~SAl

Reaction

0.00018 0a)

0 ~)

0 ~)

0.00013 (eK/~')

0.053

0.00018

0.00052 0.00023

0 a)

0 a)

0 a)

0 a)

~ (e/y)

8

1

6

~Fe(n, p)~eMn ~TAl(n,o0~Na

~aSi(n, p)~aAl

~Al(n, p)aTMg

~6Fe(n, p)SeMn ~Al(n, o0~Na

68Fe(n, p)56Mn ~TAl(n, ~)~4Na

STAl(n,g)24Na

SVAl(n,p)~TMg S6Fe(n, p)S6Mn

2sSi(n, p)28A1

~sSi(n, p)S6Al

56Fe(n, p)S6Mn 2sSi(n, p)ZSA1

56Fe(n, p)56Mn

e) fa taken from ref. 1~).

9k

8 ~:: 1

65±

210± 20

600± 40

650±150

150± 12

68±

~7

130-4- 16

252± 15

Measured cross Monitor reaction section (rob) used

TABLE 1 Cross sections of Si and Zn isotopes for reactions with 14.4±0.3 MeV neutrons

8-51

25

34-110

177-386 154 (ref ~))

530-550

102-254

45-123 175 (ref. 7))

180 (ref. 6))

7.8

20

78

312

1005(mq-g)

483

201

50.3

60(m-kg)

240

101 22.7 (ref. 7)) 120

180-365

Literature Theoretical cross sections cross sections (mb) (mb)

0

P~

Z

Z

682

N. RANAKUMARet al.

56Fe(n, p)S6Mn (2.576 h); Er = 847 keV, fd = 0.9867 (ref. 8)), ~ = 0 and a =: 1 0 0 + 6 m b 3); 27Al(n, ~)24Na (14.96 h); E~ = 1369 keV, f d = 1.0 (ref. s)), ~ = 0 and a = l14+6mb* 3. Results and discussion

The cross sections were c o m p u t e d using the formula given in ref. 5) and are listed in table 1. The quoted errors are the r.m.s, errors and comprise the various errors mentioned in ref. 1o). These do not include possible uncertainties in a m, fd, ~, and halt-life, as discussed in ref. 1 o). The values of half-life, E~,fd, and c~ listed in table 1 are based on information in ref. 8), unless otherwise stated in the footnotes. A few cases requiring special explanation are discussed below: 3°Si(n, p)a°mA1 (72.5 sec). Peeters 6) reported the existence of a g a m m a activity with energies between 2.5 and 3.5 MeV, having a half life of 72.5 sec and attributed to 3 omA1 from activation of pure Si with 14 M e V neutrons with a cross section of 180 + 60 mb. We have looked for these high energy g a m m a s in our Si samples but could not find any with the reported half-life. F r o m our data, an upper limit to this cross section can be given as < 7 mb. 66Zn(n, 2n)65Zn (245 d) and 68Zn(n, 0065Ni (2.56 h). The 66Zn(n, 2n]6SZn reaction gives rise to a 1115 keV g a m m a which also arises f r o m the decay of 65Ni formed by the 6SZn(n, ct) reaction. The m e a s u r e m e n t of the 65Zn yield therefore was p e r f o r m e d after the death of 65Ni which was evidenced by the disappearance of the 1480 keV g a m m a of 65Ni. The cross section of the 6SZn(n, ct)65Ni reaction was calculated using the 1115 keV as well as the 1480 keV g a m m a s , with the contribution f r o m the long-lived 65Zn being taken into account in the f o r m e r case. Literature values quoted in table 1 are f r o m ref. 4); as there are a large n u m b e r of prior measurements in some cases, only the range of cross sections is shown. Recent measurements which yield values outside the range are quoted separately. The cross section of the 7°Zn (n, 2n)69mZn reaction is reported for the first time. Theoretical values reported by Pearlstein 11) for (n, 2n) and by G a r d n e r et al. for (n, p) (ref. 12)) and (n, ct) (ref. 13)) cross sections are listed for c o m p a r i s o n in the table. The ratio of experimental (n, p) cross sections of 64Zn : 66Zn " 68Zn is 1 : 0.31 : 0.04, whereas G a r d n e r ' s prediction 12) is 1 : 0.25 : 0.06. t Averaged from many precision values reported in the literature, see e.g. ref. 9). References

1) P. Venugopala Rao and R. W. Fink, Phys. Rev. 154 (1967) 1023 2) R. W. Fink, Proc. Conf. on Small Accelerators, April 8-10, 1968 (Oak Ridge Associated Universities, Oak Ridge, Tennessee) 3) H. Liskien and A. Paulsen, J. Nucl. En. 19 (1965) 73

NEUTRON CROSSSECTIONSFOR Si AND Zn

683

4) M. D. Goldberg, S. F. Mughabghab, S. N. Purohit, B. A. Magurno and V. M. May, Neutron cross sections, U.S. Atomic Energy Comm. Rep. BNL-325, 2rid Ed, Suppl. No. 2 (1966) 5) E. Kondaiah, N. RanaKumar and R. W. Fink, Nucl. Phys. A120 (1968) 337 6) E. Peeters, Phys. Lett. 7 (1963) 142 7) A. Pasquerelli, Nucl. Phys. A93 (1967) 218 8) C. M. Lederer et al., Table of Isotopes, 6th ed. (Wiley, New York, 1967) 9) H. Liskien and A. Paulsen, Euratom Report EUR-119e (1966) 10) E. Kondaiah, N. RanaKumar and R. W. Fink, Nucl. Phys. A120 (1968) 329 11) S. Pearlstein, Nuclear Data A3 (1967) 327 and U.S. Atomic Energy Comm. Rep. BNL-897 (T-365) (1964) 12) D. G. Gardner, Nucl. Phys. 29 (1962) 373 13) D. G. Gardner and Y. Yu, Nucl. Phys. 60 (1964) 49 14) D. de Fremme et al., Nucl. Phys. A103 (1967) 203 15) G. P. Vinitskaya et al., J. Nucl. Phys. (USSR) 5 (1967) 839