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lEA N.° 297

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CROSS SECTION OF THE UÍT.n)^^^ U REACTION NEAR THRESHOLD

0 . Y . MAFRA, M. F. CESAR, S. KUNIYOSHI e J. GOLDEMBERG

PUBLICAÇÃO lEA N.°

Junho — 1973

297

mSTITUTO DE ENERGIA ATÔMICA Caixa Postal 11049 (Pinheiros)

CIDADE UNIVERSITÁRIA "ARMANDO D E SAlOiES OLIVEIRA'' SAO PAULO — B R A S I L

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CROSS SECTSON O F T H E ^ ^ * U(y,n)^ ^ ' U R E A C T I O W WEAR T H R E S H O L D

O.Y. M a f r a * , M.F. Cesar*, S. K u n i y o s h i * e J . Goldemberg'

Divisão de Física Nuclear Instituto de Energia A t ô m i c a

São Paulo - Brasil

Publicação I EA W9 2 9 7 Junho - 1973

i n s t i t u t o de Energia Atômica - São Paulo.

Instituto de Física da Universidade de São Paulo.

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Instituto de Energia Atômica Conselho Superior

Eng? Roberto N. Jafet - Presidente Prof.Dr.Emilio IVIattar - Vice-Presidente Prof.Dr.Jose Augusto iViartins

Dr.Affonso Celso Pastore Prof.Dr.Milton Campos Eng9 Hélcio Modesto da Costa Superintendente

Rómulo Ribeiro Pieroni

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CROSS S E C T I O N OF T H E ^ ' * U(T,n)^ ' ' U R E A C T I O N N E A R T H R E S H O L D

O.Y. M a f r a * , M.F. Cesar*, S. Kuniyoshi*and J . G o l d e m b e r g * *

Abstract

The ^•'*U(7,n)'^''^U reaction cross section has been measured by activation at excitation energies from 6.07 to 9 MeV, The (J,n) cross section thus obtained has shown the same structure of the photofission cross section for the same element what w i l l imply in the same structure in the total photoabsorption cross section.

1. Introduction

The (7,f) reaction at energies near threshold has been intensively Investigated in recent years because of its theoretical implications. Particularly the ^ ^ * U photofission cross section has been measured by several authors due t o the possibility of using gamma radiation w i t h resolution of the order of KeV or even eV^

The structure observed ' ' ' in this cross section measured even by us in a previous led us t o the study of the c o m p e t i l

emission in function of the excitation energy^'.

paper^' led us t o the study of the competition between photofission and photoneutron

For that we have measured the total photoneutron emission cross section ^ using as neutron detector a 47T Halpern type long counter^' and the photofission cross section w i t h a fission chamber.

In order t o get the photoneutron emission we have made some hypothesis about the number of neutrons emitted in photofission. Near threshold, we have

Assuming f o r t> an equation of the type

Instituto de Energia Atómica — São Paulo Instituto de Física da Universidade ae São Paulo.

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V = /\ 0.160 E^'

and using the A value equal t o 1.3 (measured f o r ^ ^ U w i t h gamma radiation f r o m anihilation of positrons in flight) we got the photoneutron emission cross section a,^ ^.

The ^ cross section thus obtained has shown the same structure of the ^ in the same element. This intermediate structure is somewhat surprising because in the (y.f) cross section it has been attributed t o the small number of states leading t o fission. To find the same structure in the (7,n) cross section indicates that the same states are involved in this process.

Although we have used monochromatic gamma ray lines obtained f r o m neutron capture reaction in a reactor, the results of our experiment were confirmed by low resolution experiments using gamma rays of a continuous variable energy f r o m a Compton scattering m o n o c h r o m a t o r ^ ' .

However since the ^ cross section determination was strongly dependent on the value attributed t o the average number of neutrons emitted per fission v, we decided t o repeat the (y,n) measurement in an activation experiment.

2.. Experimental Arrangement

The gamma radiation employed was produced in several elements used as targets and placed near the I E A - R 1 2 M w reactor core. This experimental arrangement produces monochromatic gamma lines f r o m 5.43 t o 11.83 MeV w i t h a low neutron background and has been described in details in previous p a p e r s ^ ' ^ ° ' .

Before the irradiation the uranium was radiochemically treated by the Chemical Engineering Department in order to clean the sample f r o m its descendents mainly ^ • ' ' ' T h w i t h a 24.1 days half life w h i c h emits gamma rays w i t h energy near the ^ ^ ^ U lines.

The uranium was then transformed in U3O8 pellets by the Metallurgy Nuclear Division.

This pellets were shielded w i t h paraffin and boron and cadmium for the irradiation in a position where the gamma f l u x is of the order of l O ' ^ y / c m ^ / s , The shielding was used t o prevent the thermal fission of the ^ ^ ' ' U and the (n,2n) reaction in ^ • ' ^ U . The experimental arrangement f o r irradiation is in fig. 1.

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1 0 0 cm

^ 5 0 cm

À

^ = l 2 7 c m [ X ] Alvo(n,jc) Canal T a n g e n c i a l

Í ^ a r a f l n a - B o r o

_ £ á d m i o

Alvo de U r a n i o

C a r o ç o do R e a t o r

Fíg. 1 - Experimental arrangement for irradiation. w

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Tantalum foils were used as f l u x monitor.

A f t e r the irradiation the 7-rays f r o m the ^ ^ ' ' u were measured w i t h a 18.2 cc GeLi detector. A n experimental spectrum of the irradiated uranium w i t h all the lines identification is shown in f i g , 2.

o .

5

>

i

s

<M

>

« 10

n 10

>

10 to

n N

>

it

ao O

M

01 0>

8

>

«

CD CM CM

E N E R G Y

Fig. 2 — Uranium spectrum after irradiation.

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5 For a comparison in table 1 tfie mainly lines of " ^ U , " ^ T h , ^ ^ ^ U and " ' U are s h o w n ' ^

Table 1

y (MeV) relative yield

0.143 11 %

0.185 5 4 %

0.204 5 %

2 3 - U

y (MeV) relative yield

0.026 2 %

0.060 3 6 %

0.165 2.0 %

0.208 2 3 %

0.267 0.76 %

0.332 1.4 % doublet

0.370 0.17 % doublet

2 3 9 y

0.044 4 %

0.075 51 %

^ " T h

0,063 3.5 %

0.093 4 %

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A f t e r the irradiation the activity of the ^ ^ ^ U was measured by the cascade 60 - 208 KeV after waiting approximately 14 hours in order t o allow the short lived fission products t o decay. The fission products could be eliminated after the irradiation using a new radiochemical treatment but this w o u l d bring problems related t o the loss of material during the processing, geometry variation for the various samples and weight variation due t o different drving conditions.

With the GeLi detector the ^ ^ ^ U lines can be very well identified and its activity is proportional t o the counts under the photopeaks.

The fission products and other reaction products that occurs during the irradiations have gamma lines of clearly distinguishable energies, the only disturbing reaction being (n,2n) that also leads t o ^ ' ' ^ U . To estimate the c o n t r i b u t i o n of the (n,2n) reaction an aluminum f o i l was irradiated w i t h each target and f r o m the threshold reaction (n, a) the fast neutron f l u x at the uranium sample position was measured. With this i n f o r m a t i o n i t was possible t o estimate (n,2n) c o n t r i b u t i o n that was, at the irradiation position, in the worst conditions of the order of 30%.

The fast f l u x had t o be measured each time because the different targets produced a different f l u x depression and neutron scattering.

The ^ ^ ^ U activity was also measured by a strobbed coincidence system where the 60 KeV line was used t o trigger a multichannel analyser permiting the 208 KeV line t o be analysed w i t h GeLi detector.

The gamma flux was mesured in each case outside the beam hole by a N a l ( T I ) 3 " x 3 "

crystal. The f l u x at the sample position was obtained using the law of variation of the f l u x along the beam hole obtained by the activation of tantalum foils.

3. Results and Discussion

The cross section for each irradiation can be obtained through equation (1) if the target emits just one line.

A

a e x p = — 7 r- — (1) K P 0 m _ N o (1 _ e - A t | ) e - A t g

A where

A^ is the target absolute activity

KF extrapolation factor t o get the f l u x at the irradiation position 0 ^ is the measured f l u x

is the Avogadro's number A the mass number

A the disintegration constant t the irradiation time

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t the time elapsed after irradiation before measurement

Since each target emits a number of secondary lines (eventhough w i t h the low relative intensity w i t h respect t o the principal line) it is necessary t o correct for their c o n t r i b u t i o n using a linear equation system.

^, = '^exp (2)

Where r^ are the relative intensities and a- are the cross sections for each line. Using successive approximations it was possible t o get the cross sections.

The results obtained are in f i g . 3 and show the same structure we got previously which means the same structure is present in the (y.r\) and (7,f) cross section. The errors signed includes statistics and calibration.

T o f i n d the same structure in the (7,n) and {y,f) cross section indicates that the same states are involved in the t w o process w h a t is rather unexpected because the states observed in fission should be the saddle point ones and the states observed in photoneutron emission are the states of the compound nucleus.

The proéminent peak at 6.73 IVleV in (y,n) and (7,f) cross section i m p l y in a predominant peak in the total photoabsorption cross section because

Since is small above the neutron separation e n e r g y ' ^ ' ,

The same behaviour of (y) and the reemission cross section w o u l d imply that absorption and reemission are not independent process dependent only in the energy, angular m o m e n t u m and parity.

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2 0

10

4 0

2 0

<iV,f(mb) " <lV.f C R O S S S E C T I O N ( M a f r a et al )

I I

r

f y . n l ' n b ) ' ^^.n '^"OMiTy.N DATA ( M a f r a e f a l ) Jr T H I S E X P E R I M E N T

i s

E N E R G Y C M « V )

Fig. 3 - Comparison of a^. ^ and ^ cross section in U

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Resumo

A seção de choque para a reação ^^*U{7,n)'^"u foi medida através da ativação do urânio, para energias de excitação de 6,07 a 9 iVleV. A seção de choque (y.n) assim obtida mostrou a mesma estrutura da seção de choque de fotofissão para o mesmo elemento, o que implica no mesmo comportamento para a seção de choque total Qe fotoabsorção

Résumé

La section efficace pour la reaction ^•'*U(7,n)'^^U a été mesurée a travers de l'activation de l'uranium pour les energies d'excitation de 6,07 à 9 IVleV, La section efficace pour ia reaction (7,n) ainsi obtenue a présentée la même structure de la section efficace pour la photofission pour (e même element. Ce veux dire que la même structure será présenté dans la section efficace pour photoabsorption totale.

Acknowledgements

The authors w o u l d like t o t h a n k :

The Chemical Engineering Department by the development of the radiochemical techniques and by processing the samples during the experiment running,

The Nuclear Metallurgy Division, specially t o Dr. Helinton Motta Haydt by the uranium samples and (n,7) target.

Ana Maria P. Kuniyoshi and Eüti M. Tanaka f r o m the photodisintegration group f o r the help in the calculations.

References

1. Manfredini, A . et al. - Nucí. Phys. A l 2 7 (1969) 687.

2. Khan, A . M . and Knowies, J . M . - Nucl. Phys. A 1 7 9 (1972) 333.

3, Rabotnov, N.S. et al. - Sov. J . Nucl. Phys. 11 (1970) 285.

4. Mafra, O.Y., Kuniyoshi, S. - Acad. Bras, de Ciências, 4 3 , n? 3, 1 9 7 1 . 5, Mafra, O.Y. et al, - Nucl. Phys. A 1 8 6 / 1 (1972) 110.

6, H.M. Gerstenberg and E.C. Fuller - U.S. Department of Commerce - National Bureau of Standards NBS-416 (1967) - Hopkins, J.C, and Diven, B.C. - N u c l . Phys. 48 (1963) 4 3 3 . 8. J.W. Knowies and A . M . Khan - A t o m i c Energy of Canada Limited - Report AECL-2535

(1966).

9. Mafra, O.Y. - Master Thesis - S. Paulo 1969.

10. Cesar, M.F. et al, - l E A n? 251 - 1 9 7 1 .

1 1 . C M . Lederer, J . M . Hollander and J. Perlman — Table of Isotopes (John Wiley and Sons, Inc., New Y o r k , 1967).

12. J.E. Lynn - The Theory of Neutron Resonance Reactions (Claredon Press, O x f o r d 1968) p. 384.

Referências

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