Cross-section measurements for the 75As(α,2n)77Br reaction

J. inorg, nucl. Chem., 1973, Vol. 35, pp. 3413-3416. Pergamon Press. Printed in Great Britain.

CROSS-SECTION MEASUREMENTS FOR THE 75As(~,2n)77Br REACTION S. L. WATERS, A. D. NUNN and M. L. THAKUR MRC Cyclotron Unit, Hammersmith Hospital, Ducane Road, London W l 2 0 H S (Received 6 December 1972) AImraet--Cross-sections have been determined experimentally for the 7SAs(0t,2n)77Br reaction for incident particle energies of 13.8 to 28.1 MeV using the MRC cyclotron at Hammersmith Hospital. The cross-sections have been tabulated and the excitation function plotted. Upper limits for the crosssection of the 7SAs(ct,3n)76Br and 7SAs(*t,2pn)76As reactions are also given for an incident energy of 28.1 MeV. INTRODUCTION

BEFORE a routine production schedule for a new radionuclide is established, various reaction parameters must be studied in order to find the most productive irradiation conditions. The particular problem of bromine-77 involved the irradiation of stable arsenic targets in the external He ion beam of the MRC cyclotron. Although the decay scheme of bromine-77 has been carefully studied[l], no data could be found in the literature concerning the reaction parameters. Consequently the spinning wheel target system which has been developed in our laboratory was used to determine the excitation function for the reaction, using the general methods which have been described in a previous publication[2]. These results were then used to predict the production yield of bromine-77, and of any contaminants, for different thicknesses of arsenic target material at different incident particle energies. EXPERIMENTAL Cyclotron target design For the normal production of 77Br for medical use, arsenic pentoxide or arsenic trioxide (in powder form) has been used as the target material[3]. However, both these targets are unsuitable for use with the spinning wheel apparatus, where thin uniform targets are required. Such targets were subsequently prepared in the laboratory by the decomposition of arsine under a stream of nitrogen[3]. Arsine is liberated from a warm mixture of arsenic pentoxide, zinc powder and sulphuric acid. It is flushed towards two tungsten electrodes with oxygen-free nitrogen. When a discharge (from a Tesla vacuum tester) is passed through the electrodes, arsenic is deposited onto an aluminium planchat positioned to catch it. Targets made in this way had weights of approximately 0.7 mg of arsenic distributed uniformly over an area about 2 cm 2. These were then used in the spinning wheel apparatus for the cross-section measurements. Irradiation and counting The wheel was prepared with eight arsenic targets and two of copper, the latter being used as a beam monitor, cf. the method of Lebowitz and Greene[4]. The arsenic targets were enclosed in aluminium 1. 2. 3. 4.

D. C. Sarantites and B. R. Erdal, Phys. Rev. 177, 1631 (1969). I. A. Watson, S. L. Waters, D. K. Bewley and D. J. Silvester, Nucl. Instrum. Meth. In press. A. D. Nunn, Nucl. Instrum. Meth. 99, 251 (1972). E. Lebowitz and M. W. Greene, Int. J. appl. Radiat. Isotopes 21,625 (1970). 3413

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S. L. WATERS, A. D. NUNN and M. L. THAKUR

envelopes, and aluminium absorbers were included to degrade the incident He ion beam energy. The wheel was bombarded for a period of up to an hour with a steady beam current of about 0.5 gA per target. After the bombardment, the wheel was left for some time to allow short-lived 7SBr (T½ 6.4 min) to decay. The wheel was then dismantled and the arsenic targets were left intact and the 77Br activity was assayed using a 15 cm 3 Ge(Li) detector. The reason for counting targets intact after irradiation was to include any of the 77Br activity which had recoiled out of the arsenic target material into the aluminium envelope. Various methods were used to estimate the weight of arsenic, which was of the order of 0.7 mg. Direct weighing or" the aluminium planchets before and after deposition of the target material was inaccurate, as the difference amounted to only 0.2 per cent of the weight of the planchet. Instead the arsenic was dissolved from the aluminium backing with 5N nitric acid or a solution of sodium hypochlorite, and made up to a standard volume with water. The samples were then analysed using the atomic absorption method. Simultaneously, attempts were made to determine the concentration of arsenic by cathode-ray polarography, but unfortunately this method is very dependent on the oxidation state of the arsenic, and precise determinations were not obtained. RESULTS AND DISCUSSION

The results of the experiments are tabulated in Table 1 and are plotted as an excitation function in Fig. 1. The reaction cross-sections (a) were calculated using the relationship a = A o x 3.7 x 1 0 4 N x ¢p X (1 - - e -~t) where A o = activity of 77Br in/~Ci at the end of bombardment; N = number o f target nuclei of arsenic; Table 1. Experimentally-determined cross-sections (a) as a function of helium ion beam energy for the reaction 75As(~,2n)77Br Incident beam energy (MeV)*

Cross-section (mb)f

28"1 27.8 27.4 26"3 25'1 23-9 22'7 21'4 20.1 18'6 17'1 15"5 13.8

670 _ 101 644 _ 96-6 706 _ 106 803 + 120 721 + 108 638 + 95.6 798 + 120 651 + 97'6 550 __+ 82"5 506 + 75"9 427 _ 64.0 321 + 48'2 80-3 + 12.0

*Incident beam energy estimated from experimental data of Bewley et al.[5] and the range-energy compi!ation of Williamson et al.[6]. fFor errors on cross-section measurements see text.

Cross-sections for the 7~As(Qt,2n)77Brreaction

3415

i0 3 9 8 7 6 5

,13

E

d

_o !

P

(J

I0 2 9 8 7 6 5

I

14

L

16

I

18

I

20

I

22

I

24

Helium ion beam energy.

1

26

l

28

I

30

MeV

Fig. 1. Cross-sections for the ~As(~t,2n)7~Br reaction.

= He ion flux; A = decay constant for 77Br (where T½ = 56 hr); t = duration of bombardment in hours. The He ion energies were based on measurements by Bewley e t al.[5] and the range-energy data of Williamson et al.[6]. The errors on the cross-section values are a combination of the precision obtained from the standard deviation of the mean of the cross-section for nine identical targets bombarded with He ions of the same incident energy and the random error obtained in the atomic absorption estimation of the arsenic content. The general shape of the excitation function for the (~,2n) reaction as shown in Fig. 1 is similar to that for the other odd-even nuclei close to arsenic in the periodic table, though the exact positions of the maxima, and the upper and lower ends are anomalous. For example, from energy calculations the threshold for the (~,2n) reaction is 14-2 MeV. It would be expected therefore that the probability of detecting 77Br below this incident energy is very small. The "tunnelling effect" might account for a very small percentage of the maximum cross-section below this energy, but would not explain the experimental value of 80.3 mb obtained at 13.8 MeV, which is l0 per cent of the maximum. This is more satisfactorily explained by the fact that the energy of the He ion beam is somewhat higher than that measured by Bewley e t a/.[5], and that the energy scale should be shifted by about 0.5 MeV. Above about 25 MeV the excitation function appears to decrease steadily; this is probably due to other higher-energy reactions taking place. Of these the (ct,3n) reaction, and (at,2pn) are the most likely, i.e. 5. D. K. Bewley, S. B. Field a,nd C. J. Parnell, Phys. Med. Biol. 12, 1 (1967). 6. C. F. Williamson, T. J. Boujot and J. Picard, CEA Report R3042, 1962.

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S. L. WATERS, A. D. N U N N and M. L. THAKUR

75As(ct,3n)76Br, T½ 16 hr 75As(~,2pn)V6As, T½ 26"5 hr

Q value MeV

Threshold MeV

-24.2 -21.0

25.6 22.1

In fact neither 76Br nor 76As were detected in any of the activations. Using the analytical method of Currie[7], the upper limits of 76Br and 76As for the highest incident He ion beam energy (28.1 MeV) were only 0.015 #Ci and 0.026 #Ci respectively. This would represent a cross-section of <25 mb for each reaction at this energy (8.3 mb and 21 mb respectively). Acknowledgements--We wish to thank Dr. D. J. Silvester; we also acknowledge the very helpful cooperation of Mr. G. Stone and his staff at the Laboratory of the Government Chemist, who made the

arsenic determinationsusingan atomic absorption apparatus. 7. L. A. Currie, Analyt. Chem. 40, 586 (1968).