Simple nuclear reactions on Ca48 induced by high energy protons

I 2.A.I: 2.F

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Nuclear Physics 41 (1963) 504---510; (~) North-Holland Publishing Co., Amsterdam

I

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

SLMPLE NUCLEAR REACTIONS ON Ca 'ta INDUCED BY H I G H ENERGY PROTONS I. L E V E N B E R G , V. POK_ROVSKY and I. Y U T L A N D O V

Joint Institute for Nuclear Research, Dubna, U.S.S.R. Received 15 June 1962 Abstract: The reactions Ca 4s (p, pn), (p, 2n) and (p, n) are investigated for p r o t o n energies from 120 to 660 MeV. The cross section for the (p, pn) reaction depends weakly o n the proton energy and is about 105 mb. The cross sections for the (p, 2n) and (p, n) reactions decrease with proton energy from "~ 20 m b and ~ 8 m b for 120 MeV to ~ 6 mb and ~ 2 mb for 660 MeV, respectively. The dependence o f the ratios trp, 2a/trp, n and trp, pn/O'p, n on the mass number o f the target nucleus and proton energy is discussed.

1. Introduction

Among different nuclear reactions occurring in the interaction of fast particles with complex nuclei, the "simple" reactions with emission of 1 or 2 nucleons from the nucleus are of greatest interest. The explanation is the considerable difference of the experimental values of the cross sections for such reactions from those calculated on the basis of the Serber representations 1). Thus, for the reaction (p, pn), investigated in more detail than other reactions, the experimental values of the cross sections exceed 3 or 4 times the calculated ones (e.g. ref. 2)). To account for these discrepancies it was assumed that simple reactions of the type (p, pn), (p, 2p), (p, n) etc. occur mainly via direct interactions with nuclear diffuse surface nucleons and that the shell structure of the target nucleus has an essential effect on them 2). However, the experimental data available at present are insufficient for a reliable detection of this dependence. Besides, the results of the published investigations performed in different laboratories by different methods are often in poor agreement with each other. Therefore, we have undertaken a run of experiments to investigate simple reactions on magic and non-magic nuclei close to each other with respect to A and Z. This paper is concerned with the study of the (p, pn), (p, 2n) and (p, n) reactions on the nucleus 2oCaas. 2. Experimental Technique 2.1. T A R G E T A N D R A D I A T I O N C O N D I T I O N S

A natural mixture of Ca isotopes was used for irradiation. The target was pressed in the shape of a rectangular wafer ( 1 5 x 4 × 1.5 ram) from calcium carbonate. It 504

NUCLEAR REACTIONS ON Ca 4s

505

was concluded that the purity of the target substance was sufficiently high since no spallation products of possible heavy contaminations had been discovered in the irradiated target. The target was irradiated in the internal beam of the synchrocyclotron of the Joint Institute for Nuclear Research for 15 to 20 min. The proton beam was parallel to the 1.5 mm side of the target. The incident proton energy was varied by positioning the target at different radii in the accelerator chamber. The proton beam intensity was determined by the yield of the A127 (p, 3pn)Na 24 reaction. The target was wrapped in three layers of 20 # aluminium foil. To measure the Na 2. activity, middle layers on both sides of the target were used. The layers had been cut strictly to the size of the target. The difference in the activities of these foils did not exceed 10 ~o and amounted on the average to some 5 per cent. 2.2. CHEMICAL PROCEDURES The irradiated target was dissolved in a small amount of diluted HC1 with ~ l0 mg of Sc carrier. After CO2 has been removed by boiling the solution, scandium hydrooxide was precipitated by addition of NH4OH. Then the calcium and scandium fraction were purified radiochemically. 2.2.1. Calcium. The purification of the calcium fraction consisted in the repeated precipitation of calcium oxalate in the presence of magnesium retaining carrier and the subsequent removal of radioactive contaminations with iron and scandium hydro-oxides. Finally, the calcium was precipitated as oxalate (CaC204 • H20 ) filtrated on a disc of filtre paper, rinsed by water and ethyl alcohol, dried at 110°C and weighted. To avoid pulverization, the sample was sealed with transparent tape. 2.2.2. Scandium. The purification of the scandium fraction consisted of three operations: precipitation of scandium fluoride with Na2SiF6, precipitation of scandium hydro-oxide in the presence of calcium retaining carrier and extraction of scandium with tri-butil phosphate from concentrated HC1 solution. Finally scandium was precipitated as hydro-oxide, heated at ~ 1000° C until the weight was constant, weighed, deposited on a disc of filtre paper and sealed with transparent tape. 2.3. MEASUREMENTS OF ACTIVITY These were made with a scintillation ),-spectrometer having a NaI(TI) 40 x 40 mm crystal and a 128-channel AMA-3c analyser. The ratio of absolute activities of the reaction products and Na 24 was derived by comparing the intensities of characteristic ),-lines, taking into account the decay schemes and the efficiency of counting. The efficiency of the spectrometer to ),-rays of different energies was determined by means of the Ce 141, Hg 2°3, Cs 137, Na 22 and Co 6° sources the absolute activity of which was measured with a 47~ gas flow-type counter. The radioactive characteristics of isotopes used in the calculation of the cross sections for the reactions under study are given in table 1.

506

!. LEVENBERG et aL

Besides the usual corrections for the isotopic composition of the target, chemical yield, irradiation time, etc., the accumulation of Sc47 due to the decay of Ca 47 was taken into account in case of Sc.7. TABLE 1 Characteristic data of isotopes

Nucleus

Na n

15.0

Ca 47 Sc4~ So'*'

Energy o f characteristic y-line keV

Half-life

Number of ~,-quanta per decay %

Reference

h

1368

100

s)

4.53 d

1300

74

a, 4)

160 1315

67 I00

') s)

3.4 44

d h

2.4. CONTRIBUTION OF SECONDARY PROCESSES

Secondary neutrons in the accelerator chamber may lead to the imitation of the (p, pn), (p, 2n) and (p, n) reactions by the (n, 2n), (n, 2 n n - ) and (n, nT~-) reactions respectively. Since the reactions with emission of mesons have very small cross sections check experiments were made only to estimate the contribution of the (n, 2n) reaction to the cross section for the (p, pn) reaction. It was established that this contribution does not exceed a few percent and it was neglected in the subsequent calculations.

3. Results of Experiments The cross sections for the (p, pn), (p, 2n) and (p, n) in mb in an incident proton energy interval from 120 to 660 MeV are listed in table 2. The cross sections for the monitoring reaction 5) are given in the same table. The cross sections indicated are TABLE 2 Cross sections Ep, MeV

120

(p, pn) 118 ~ 2 O, 2n) 20.3~1.6 ~,n) 7.8~0.3 Al~(p, 3pn) 10.2

200

300

400

500

600

106 ± 1 0 18.6±0.6 4.7±1.2 9.1

106 ± 4 11.0~0.1 4.1~0.3 11.0

101 ± 4 8.7t0.3 3.6±0.1 11.3

1~ ~1 8.7~0.1 3.9~0.2 11.1

110 ± 8 6.2~1.0 2.2±0.2 !1.0

660 110 ~:2 5.7±0.3 2.6~0.1 10.9

averages of two or three measurements with quadratic mean errors. The general accuracy for the cross sections is about 15 % according to our estimate: Besides, the cross sections contain errors due to uncertainties in the decay schemes and in the cross section for the monitoring reaction.

NUCLEAR REACTIONS ON Ca48

507

The relevant excitation functions are represented in fig. 1. Note that the cross section for the (p, pn) reaction is somewhat overestimated due to the contribution from the reaction Ca 4s (p, 2p)K 47. The K 47 nucleus is unknown. Probably it is extremely unstable and decays into Ca47. 6~

t20i ~GO

I

N•}

p, pn

{

°------

20

,o

~--------2____~ i

,00

i

200

~

~____._ i

~00

i

s00

6~,

Ep,4v

Fig. 1. Excitation functions of the Ca 4a (p, pn), (p, 2n) and (!o, n) reactions.

4. Discussion of Results

It is noteworthy that the excitation functions behave differently (fig. 1) for the different reactions under study. The cross section for the (p, pn) reaction decreases in an interval of proton energies from 120 to 200 MeV and then remain practically constant, perhaps with a certain trend towards increase. The cross sections for the (p, 2n) and (p, n) reactions decrease monotonously with energy. A similar behaviour o f the excitation functions for simple reactions is observed for other nuclei as well (see refs. 6-8) for example). It can be supposed that the primary interaction of an incident proton with a nuclear neutron is the most essential mechanism of all reactions under study. The character o f this interaction determines the channel of a reaction. From this point o f view it is of interest to consider the ratios of the cross sections from simple reactions depending on the energy of incident particles. Fig. 2. represents the ratios of the cross sections for the (p, 2n) and (p, n) reactions for Ca "s, Ga 69 (refs. 8,9)) and y89 (ref. 1o)). It can be seen that in all cases O'p,2n/O'p,n depends very weakly on the energy of the incident protons. Note that for Ga 69 the quantity ap, 2°/ap,, remains practically constant up to Ep = 2.9 GeV. The latter may serve as an indication of the fact that the mechanism of the (p, n) reaction and the first stage of the (p, 2n) reaction are identical. Under this assumption

l, LEVENBERO et aL

508

it is necessary, to explain the constancy o f ap, 2Jap.., that the probability for the production of the residual nucleus with excitation needed for the evaporation of only one neutron should depend weakly on the incident proton energy. An indirect proof of this is the weak dependence of the average excitation energy of the residual nuclei after the (p, nucleon) cascade on Ep (ref. 1t)). Our assumption also means that the contribution of the p, 2n cascade to the cross section of the (p, 2n) reaction is small. Indeed, the average excitation energy of the residual nucleus after the p, 2n cascade is about 85 MeV when Ep >~ 200 MeV (A = 64-100), and the probability of the production of a residual nucleus with excitation < 10 MeV is close to z e r o i t ) . ~2n 6p, n

3F

'I

I

i

3Do

6~

i

I

i

,I i

too '

?~'

soo

~oo '

Eo ~ e Y

Fig. 2. Dependence of the ratio ap, 2./a.,,~ on incident proton energy for Ep > 100 MoV. Black circles correspond to Ca4s, black triangles to Gael and open circles to ys% The absolute values of the ratio °v, ~,,/o'p,,, strongly depend on the mass number o f the target nucleus (fig. 2). No less strong dependence on A is shown by the ratio ap, p . / a v , 2n which, besides, varies differently with the incident proton energy. The scarcity of experimental data makes the interpretation of these facts difficult. The ratio of the cross sections for the (p, pn) and (p, n) reactions shows an altogether different behaviour. Fig. 3 represents the ratios ap, v./op,, for Ca 4s, Ga 69 (refs. 8, 9)), y89 (ref. xo)) and T h 232 (refs. 12, za)). It is clear that the ratio ap, p./av . strongly depends on the incident proton energy and practically does not depend on the number of nucleons in the target nucleus. The latter can be interpreted in the following manner. According to the current concept the (p, pn) and (p, n) reactions occur in the peripheral region of the nucleus and each of them must be sensitive to the details o f the nuclear structure 14). It is

NUCLEAR REACTIONS ON Ca48

509

natural to suppose that the number of neutrons "available" * to the (p, pn) and (p, n) reactions is the same unless the nuclear reconstruction energy is taken into account. Hence it follows that the ratio ap, p,/a~,,,, must not depend on the number of available neutrons, i.e., on the properties of the target nucleus. ep.~,

,/

// /

/

/- // tI

/ //

t / .¢

/

/

/

//

¢00 '

20O .

.

300.

.

600

50o

d

o

ee

Ne 9

Fig. 3. Dependence o f the ratio crp, pn/~p, n on incident p r o t o n energy. Black circles correspond to Ca 4s, black triangles to G a 6°, open circles to y s s and crosses to T h ~3~.

For the given target nucleus the relative probability for the emission of both colliding particles (incident proton and nuclear neutron) increases with the increase of incident proton energy. At the same time the relative probability for the scattering of the incident proton through a large angle and its absorption by the nucleus (which must lead to the (p, n) reaction) decreases. This is what causes the substantial increase of the ratio at, , p./ap,, with incident proton energy. The contribution to the cross section of the (p, pn) reaction from the direct knock-out of a neutron when Ep > I GeV is in the estimate of ref. 14) no less than 95 Yo. The above peculiarities in the behaviour of the ratio ap, pn/ap,,) warrants the conclusion that this mechanism prevails already in the region of proton energies of the order of hundreds of MeV. t I.e., the ejection o f which does not lead to the transfer to the nucleus o f an excitation energy larger than the neutron binding energy.

510

I. LEVENBERG e t al.

Fig. 3 does n o t give the c o r r e s p o n d i n g ratios for C u 65 at Ep = 340 M e V 12) equal to 100, n o r for U 23a at Ep = 340 M e V 12) equal to 185 a n d at Ep = 680 M e V 26) equal t o 250. The large value o f Up, pn/trp, n for U 2as should p r o b a b l y be explained by the fact t h a t the a b s o r p t i o n o f a p r o t o n scattered t h r o u g h a large angle leads, in m o s t cases, to the fission o f the nucleus a n d hence to the decrease o f a cross section for the (p, n) reaction. As to Cu 65 the cause o f the discrepancy is n o t clear. It is possible t h a t it is due to a c o m p a r a t i v e l y low accuracy in d e t e r m i n i n g the cross sections in the first experiments on the interaction o f fast particles with c o m p l e x nuclei. Finally, note t h a t for C a 4s a n d T h 2a2 the e x p e r i m e n t a l cross sections include the c o n t r i b u t i o n f r o m the (p, 2p) reaction. It is quite p r o b a b l e t h a t t a k i n g this c o n t r i b u tion into a c c o u n t will lead to an even closer g r o u p i n g o f the ratios t~p,pn/ap,~ for different nuclei. The a u t h o r s are i n d e b t e d to L. I. Lapidus, M. G. M e s h c h e r y a k o v a n d A. N. M u r i n for their interest in the investigation a n d stimulating discussions a n d to Yu. V. N o r s e y e v a n d L. M. T a r a s o v a for aid in the chemical t r e a t m e n t o f the targets.

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