A study of protons from In115 and Au197 with 14.8 MeV incident neutrons

Nuclear Physics 47 (1963) 473--480; ( ~ North-HoUand Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

A S T U D Y OF P R O T O N S F R O M In 11s and An 197

WITH 14.8 MeV INCIDENT NEUTRONS H. S. HANS t and R. K. M O H I N D R A tt

Department of Physics, Muslim University, Aligargh U.P. India Received 12 May 1963 Abstract: This work describes measurements of the energy spectra of protons from In lx5 at 0 °, 45 °, 105 ° and 135 ° and from Au 1D~ at the forward direction only when bombarded with 14.8 MeV neutrons. The experimental spectra have been compared with the calculated ones on the basis of the statistical model and the theory o f volume direct interaction. The agreement is fair qualitatively except at the extreme forward angles. The angular distribution plots have been given and the behaviour of nuclear temperatures has been studied at various angles for the ease o f In ~5.

1. Introduction

To test the validity of the statistical theory of nuclear reactions and to estimate the direct effects, a large number of experiments have been done on the energy and angular distribution of (n, p) reactions from medium and heavy weight nuclei ttt. In medium weight nuclei, (n, p) spectra on the low energy side are contaminated by (n, np) protons and comparison with the theory becomes difficult 1). In the case of heavy elements the low energy (n, np) protons are inhibited by the Coulomb barrier and also the (n, p) cross section is decreased considerably 2). For heavy elements the (n, p) cross section is mainly due to higher energy protons emitted directly. The study of the elements in this region can be useful for understanding the higher energy region of proton spectra. Furthermore, for heavy elements some discrepancies have been found between the earlier evaporation theory and the experimental data concerning the energy spectra of emitted protons 3). (i) The main differences are as follows: The energy distribution curve is not smooth as expected theoretically but two or more peaks are observed particularly in the case of heavy elements. Nemeth 3) has explained this to be due to the oscillations of the compound nucleus. (ii) The total cross sections for heavy nuclei are found to be higher than those predicted by the evaporation theory s). (iii) The angular distributions are more anisotropic 3) than predicted by the evaporation theory. The present work describes the energy spectra of protons from In 115 at 0 °, 45 °, 105° and 135° and at the extreme forward direction only, for A u 197. T h i s study was undert Present address: Dept. o f Physics, Texas A and M, College Station, Texas, U.S.A. tt Present address: Dept. of Physics, Punjab University, Chandigarh, India. ttt See refs. a, ~) and further references in the latter. 473

474

rl. S. HANS AND R. K. MOHINDRA

taken as a part o f an attempt to see h o w the theory o f volume direct interaction and the statistical model behave in this region. The energy distribution o f protons f r o m In 115 has been reported by E u b a n k et aL 4) using a scintillation spectrometer. Peck et al. 5) reported both the energy and angular distribution by using emulsion plates. The energy spectra o f mu 197 at the forward angle, has been reported by Colli et al. 6) using scintillation technique and by Hailing 7) with emulsions. The shapes o f the two spectra are quite different and the energy spectra have not been given in terms o f the differential cross sections. The observed spectra in our case have been c o m p a r e d to the calculated ones o n the basis o f the statistical and volume direct interaction models.

2. Experimental Method The counter telescope constituted o f two proportional counters in coincidence with a CsI(TI) scintillation counter plus a p r o p o r t i o n a l counter in anticoincidence; the associated electronic circuitry used in the present experiment was described earlier a). The experiment was carried out and the data analysed essentially in the same m a n n e r as previously 9). The target in In 115 was 32.8 m g / c m 2 thick and 3 cm in diameter. The energy spectra o f emitted protons were studied at 0 °, 45 °, 105 ° and 135 °. The distance between the I n 115 target and the neutron source was kept 12 cm in all the observations. The A u 197 target was 45.3 m g / c m 2 thick and 3 c m in diameter. The (n, p) cross section o f gold is too low and therefore in order to have a higher flux at the gold target, it was placed at a distance o f 4.5 c m from the neutron source. The spectrum was studied at the forward angle only. The p r o t o n spectra have been corrected for the absorption loss due to the finite thickness o f the target lo). The calculations have been carried out on the basis o f the statistical model 11) and the theory o f volume direct interaction as p r o p o s e d by Brown and Muirhead 12). In the statistical model calculations, N e w t o n ' s shell dependent formula 13) has been used. The m e t h o d o f calculation is exactly the same as reported earlier 14). The values o f the various constants used are given in table 1. TABLE 1

Separation energies Nucleus

Sp a) (MeV)

Sn b) (MeV)

Ag112

7.8 10.4 6.8 6.4 8.2 5.7

6.4 6.1 9.0 6.1 5.7 7.5

Cd n5

In 115 Ir xg' Pt 197 Au 197

V = 42 MeV (ref. aa)), Xnp/Xn a = 0.3 (ref. 14)), tri(ln 115) = 1940 mb (ref. is)), ai(Au19:) = 2475 mb. a) Ref. 15), b) Ref. lo), e) Ref. 17).

S c) (MeV) 4.3 4.5 3.6 --1.5 -- 1.4 --2.0

ENERGY

SPECTRA

475

OIF ] P R O T O N S

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3.5

2.0

3.0 45 °

~:

2.s

o ~ "~

2.0

1.5

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1.5

0.0

1.0

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6

7

8

9

10

11

12

13

14

lS

in Me'V"

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/

/

I

"

/

Calculated

" " ~" ~ " ~ .

0I

O 0

1

2

3

4

S

6

7

8

9

10

PROTON ENERGY CM.

.35

11

12

13

14

15

(MeV)

"~ .30 (a) 0 .25 *\ .20 .15

,

C o m p o u n d only i

.10

•

Newton's |ormula

I

,05

I i

0

1

2

5

6

7

i

g

J

8

9

10

P R O T O N Eh'ERGY CM.

i

11

12

r

13

14

15

(MeV)

Fig. 1(a). Energy spectra of protons from In 11s at 0 ° and 45 °. The uppermost curves are the conventional statistical theory plots where the abscissa is the energy of the outgoing channel ~ and the ordinate is log W, where W is the yield divided by the energy t~ and the cross section for the compound nucleus formation aop. The dotted curves in the lower two figures denote the calculated spectra (i.e., compound and direct)• Newton's level density formula was used in the compound nucleus calculations. The compound nucleus calculations included (n, p) as well as (n, np).

476

H.S.

HANS

AND

R. K. M O H I N D R A

4.0-

3.0

3,5-

-2.5

3.0-

-2.0 -1.5

2,5-

N o~ ,-4

2.0-

~

1.5-

o "~ -0.5

-1.0

1.0"

-0

0.5-

--0.5

0~

--1.0

-0.5

6

+

6

~

i'o fi

/2

1'3

l;t

15

> ~ in MeV

0.30 " (d) 135"

0.250.20-

x

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I

/i

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0.05-

/

/

/

,,

\\

2,,

Calculated

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0.25 r 0.20 0.15

I

/

-

/

\

",

O.lO0.05 -

///

Calculated

.1

2

" " -~

........ 3

4

5

6 7 8 9 1 iI ~.- PROTON ENERGY CM. (MeM)

?_ , ~ o o 12

13

14

1

Fig. l(b). The energy spectra at 105° and 135°. See fig. 1(a).

Figs. l(a) and (b) show the observed and the calculated spectra for In 11s due to the statistical model plus the volume direct interaction. The spectra for A u 197 are s h o w n in fig. 4. In the case o f A u 197, the (n, np) contribution has been found to be negligible.

477

ENIIR.GY S P E C T R A OF P R O T O N S

.32 ,28 .24

It

.20

o b. . . .

d

.16 .12 .08 .04 0

i

Proton Energy CM, (MeV}

Fig. 2. Comparison between the proton spectra from In lz5 in the forward angle as observed

the authors and Eubank e t

5-7

1.2

MeV

1,2

.1.0

by

al.

7-9 MeV

1.0

e o.e-I

~0.8

0.4

o.2]

0

0

~o io

~

0

~o' l~o 1~o l;o 16oo o

~o

~ ~o

1.2

~o

~o ~-

!~- ANGLE (CMS)

•

io

l~o 1'2o 1~o l~o °

ANGLE (CMS)

> 9 MeV

1.0 0.8

i

i

0

i

160

i

180 °

ANGLE (CMS)

Fig. 3. The angular distribution o f protons from In 115 for the various energy intervals. The smooth curves are the angular distributions calculated by adding the compound and direct contributions at various angles.

478

H. S. HANS AND R. K. MOHINDRA

3. R e s u l t s

3.1. The In11s SPECTRA Fig. 1 shows the energy spectra of protons from In 115 at 0 °, 45 °, 105 ° and 135 °. The dotted lines show the calculated spectra. The observed spectra are flatter than in the case of Co 59 9). This is expected as the (n, np) yield of low energy protons is further reduced because (i) there is in In 115 as compared to Co 59 a stronger competition between the second neutron in (n, 2n) and proton emission in (n, np) as the difference between neutron and proton binding energies ( 9 - 6 . 8 = 2.2 MeV) is less than in the case of Co 59 ( 1 0 . 5 - 7 . 4 = 3.1 MeV); (ii) the Coulomb barrier is higher thus the low energy protons are inhibited. Earlier, Eubank e t al. 4) reported the energy spectrum on In 115 at the forward direction only. The present spectrum follows the general trend of their spectrum except that the low energy yield in our observed spectrum is higher. Also the above authors get a slight peak at the highest energy end, while in the present experiment, only a slight upward trend is indicated. The resolution of their spectrometer is approximately the same as ours. Fig. 2 shows the comparison. Here the statistical model calculations have been performed by using only Newton's shell dependent formula 13) which gives good agreement 9) for the case of A127 and Co 59. The calculated direct interaction contribution at the exterme forward direction is too high, and for other angles all the computed spectra are deficient in high energy protons. Fig. 3 shows the angular distribution plots of protons f r o m In 115. There is some anisotropy in the 5-7 MeV region and the anisotropy increases in the higher energy regions. The anisotropy in the highest energy region is not so marked as in the case of Co 59. It is expected that for high Z elements the spectrum should be dominated by direct protons as most of the low energy protons due to compound nucleus formation are inhibited by the Coulomb barrier. The nuclear temperature shows a similar behaviour. The value 25) of Q(n, d) = 4.5 MeV, thus the m a x i m u m energy of deuterons is 10.2 MeV, which according to our estimates may contaminate the proton spectrum below 8 MeV though an attempt has been made to check the deuterons by using a blocking circuit. Table 2 gives the value of the parameter a of the level density formula co(u) = Ce 2"/" corresponding to the nuclear temperature obtained for the backward angle spectra. TABLE 2

Parameter a of the level density formula to(u) = Ce~/a"

Element

013~o (MeV)

& (MeV)

(2np (MeV)

Ur~k (MeV)

Int15

0.98

-- 1.35

--0.67

6.7

aexpt=

6.98

Up.k 02

A/a

16.5

The quantity U~ak = E l n - - Q n p - - S c - - E p , is the excitation energy corresponding to the peak of the proton spectrum, Eln is the incident energy, and Sa is the pairing energy as given by Cameron ~).

SPECTRA O F P R O T O N S

ENERGY

479

The value of A/a = 16.5, obtained in this case should be compared to A/a = 10.6 for A127 and A/a = 17.2 for Co 59 in the previous work 9). PROTONS FROM Au x97

3.2. E N E R G Y DISTRIBUTION OF

The energy spectrum for A u 197 at 0 ° is shown in fig. 4 with the experimental spectrum due to Colli et al. ~), which has been normalised to our spectrum at 1 1.2 MeV. The general trend is in agreement but CoUi et al. observed broad peaks in their spectrum while the present spectrum is quite smooth. The contribution of the protons in the region of 10-14 MeV is quite high in the observed spectrum as compared to Colli et al. According to our estimates deuteron contamination may be present up to about 9 MeV. The shape of the observed spectrum is quite similar to the calculated one o n the basis of the volume direct interaction but the computed yield is too high for the extreme forward angle as in other cases. The contribution due to the compound nucleus formation is negligible. The shape of the spectrum dearly shows the effect of the Coulomb barrier below 6 MeV.

•

A u 1~7 - ....

,i

.18 ,

i

f

\

,~,,/'

Calculated

.45

'\.

/

/"

.16 •

.S0

.

.40

'\

(Comp 4- dir) .14

• .35

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.10

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d o

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...',

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6

.

7

8

.

.

.

9 I~

. 10

.

".tk

. ll

Proton Energy CM.

• .05

,,, ,-,,

12

13

14

15

(Me'V)

Fig. 4. The observed and calculated proton energy spectra from Aul°7, at forward angles. The spectrum observed by the authors is compared with the one obtained by Colli et aL The scale on the right only applies to the calculated spectrum. The authors are grateful to Professor P. S. Gill for his kind interest and encouragement throughout this work. They would like to thank the Isotope Division, Atomic Energy Research Establishment, Harwell for preparing the In li5 enriched target for us. One of the authors (R.K.M.) is thankful to the Department of Atomic Energy Government of India for the award of a Senior Scientific Research Assistantship.

480

n.s.

HANS AND

R.

K. MOH1NDRA

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19)

D. L. Allan, Nuclear Physics 24 (1961) 274 L. Colli et aL, Nuovo Cim. 13 (1959) 730 J. Nemeth, Nuclear Physics 6 (1958) 686 H. P. Eubank et aL, Nuclear Physics 10 (1959) 418 R. A. Peck, Jr., Phys. Rev. 123 (1961) 1738 L. Colli et aL, Nuovo Cim. 7 (1958) 400 R. K. Hailing, R. A. Peek and H. P. Eubank, Phys. Rev. 106 (1957) 971 R. K. Mohindra and H. S. Hans, Ind. J. Phys. 36 (1962) 93 R. K. Mohindra and H. S. Hans, Nuclear Physics 44 (1963) 597 R. K. Mohindra, P h . D . thesis, Muslim University Aligarh (1962) S. Hayakawa, M. Kawai and K. Kikuchi, Prog. Theor. Phys. 13 (1955) 415 G. Brown and H. Muirhead, Phil. Mag. 2 (1957) 473 T. W. Newton, Can. J. Phys. 34 (1956) 804 R. K. Mohindra and H. S. Hans, Ind. J. Phys., in the press Masami, Y and Z. Maturnoto, J. Phys. Soc. Japan 16 (1961) 1497 D. M. Van Patter and W. Whaling, Revs. Mod. Phys. 26 (1954) 402 P. A. Seeger, Nuclear Physics 25 (1961) 1 E. Erba et aL, Nuovo Cim. 22 (1961) 1237 A. G. W. Cameron, Can. J. Phys. 36 (1958) 1040