ray spectrum into its components is possible i.e ( p ,Yq ), (p»Y^) etc contributions may be estimated individually.

In document Nuclear reaction studies (Page 44-51)

The earliest application of the inverse photonuclear

reaction technique to giant resonance studies in light nuclei

was nade by Gemmell et al. (Ge 59a, Ge 59b) using a proton beam

from a 7.7 MeV cyclotron, with energy variation obtained by the

use of aluminium absorbing foils. Of the target nuclei bombarded,

7 11

only Li and B had sufficiently high Q-values for radiative

proton capture (17.25 MeV and 12.13 MeV, respectively) for the

yield measurements to extend beyond the peak of the giant

resonance. The cross-section curve for the B (p,Y0)C

reaction suggested possible structure within the resonance. A

more recent and accurate measurement by Gove et al. (Go 6l) 12 confirmed that there was no pronounced splitting of the C

giant resonance.

T 8

A thick target (280 keV) Li (p,Y)Be yield curve was o

measured at 90 by Gemmell et al. and gave no indication of

structure, but resolution was considered to be no better than

300 keV. The combined yield of high energy Y-rays from this

reaction showed a broad resonance with a maximum at a bombarding

energy of 5.8 MeV, in agreement with the prediction by

Wilkinson (Wi 54).

Two other measurements have been reported for the

7, s 8

Li (p,Y)Be reaction yield at comparable energies. Bair et al.

(Ba 52), using a 20 keV target, showed that at a fixed angle the

high energy Y-ray yield increased smoothly beyond Ep = 3 MeV.

up to 10 MeV and claimed that broad and completely structureless

resonances in yield at 90° of Y-rays to the ground and first 0

excited states of Be displayed maxima at about 5 and 8 MeV

proton energy, respectively. Pew experimental details and no

graphical results were given but agreement with the work of

Gemmell et al. was considered to be "not good".

Remark has been made of the 2.6 MeV difference in 8

excitation corresponding to maxima of the Be Yq- and Y^-ray

yield curves, as quoted by Thomas et al. This is to be

0

compared with the 2.9 MeV energy difference between the Be 0+

i ' J. _V. V

ground state and 2+ first excited state (Aj 59)* Other workers

have offered evidence for giant resonances built upon excited

states of nuclei.

Penfold and Garwin (Pe 59a) extracted cross-sections for

the elastic and inelastic scattering of Y-rays out of a

16

bremsstrahlung beam incident on 0 . An interpretation of the

peaks at 22 MeV in the elastic scattering cross-section, and at

29 MeV in the inelastic cross-section, was given in terms of

strong resonance dipole absorption by the 0+ 0 ground state

and 0 + or 2+ excited state 6 or 7 MeV higher, respectively i.e.

16 16 16 16*

giant resonances for 0 (Y,y)0 and 0 (Y,Y*)0 were

t

suggested .

t

In fact, the data required that a number of isolated

resonances be involved in the elastic scattering cross-section. Absorption at 22 MeV was particularly strong, however.

27 28

The giant resonances obtained for A1 (p»YQ )Si and

Al2^(p,Y.

)si28*

cross-sections by Gardner and Gugelot (Ga 61)

displayed a relative displacement of about 2 MeV, as did the

resonance envelopes from the work of Gove et al. (Go 6l). The

data of Gove also suggested an energy difference of about 4 MeV

*11 12 11 12*

in excitation for the maxima in the B (p,YQ)C and B (p>Y<|)C

yields*

Inspection of the yield curves presented by Gove et al.

illustrated how much detail had been obscured, for example, in

the B (p»y)C data of Gemmell et al. and the Al (p,Y)Si

data taken by Gardner et al* It seemed quite possible that the

cyclotron beam study of the Li (p,Y)Be reaction had smeared out

structure by poor resolution* The evidence given by copious

photonuclear data (To 60) for multiple splitting of the giant

dipole resonances in many light nuclei posed the question of 8

whether there really was no structure in the Be giant resonance,

as experimental results had suggested to that time.

In summary, it is pointed out that, in favourable cases,

proton radiative capture studies permit a precise investigation 8

of the giant dipole resonance region for light nuclei* The Be

nucleus is unstable and therefore cannot be used as a target for

photodisintegration studies* The Li (p,Y)Be reaction is favoured

by a high Q-value so that the giant resonance may be reached

comfortably with available proton energies. Since agreement

disputed and no structure in the curves had been found, a

clarification seemed worthwhile. More precise values were

desired than those tentatively quoted for the excitation

7/ n 8 7/ \ 8*

energies at which Li (p,YQ )Be and Li CP»7^;Be reactions

exhibited maximum yield in the giant resonance region.

The improved resolution in proton beam energy and the

much lower background offered by the Tandem Van de Graaff

machine encouraged a repetition and extension of the earlier

work of Gemmell et al. The experiment was undertaken,

therefore, to provide thin target excitation functions and

7 8

angular distributions for Y-rays from the Li (p,YQ )Be and

7, v 8* Li (p,Y-j)Be reactions. ) { ' {

:

,

2

r) X 3

-

$ \ 3.2 Experimental Procedure 3.2*1 Target Preparation. *T

Thin targets of natural Lithium (92.6$ L i f) were

evaporated from a tantalum boat onto 0.010” thick tantalum

backings and mounted on a frame located axially in a

cylindrical, perspex target chamber.

To confirm that excessive oxidation or nitrogenation of

the Lithium target did not occur during transfer from the

evaporator, the following test was performed*

A crude target chamber was constructed which could be

transferred from the evaporator unit to the beam line without

c o n d i t i o n s showed no s i g n i f i c a n t d i f f e r e n c e i n t h i c k n e s s , a s m e a s u r e d by t h e ( p , Y ) y i e l d , b e f o r e and a f t e r a d e l i b e r a t e , s h o r t e x p o s u r e t o a i r , n o r was t h e u s e f u l l i f e o f a t a r g e t r e d u c e d n o t i c e a b l y . C o n s e q u e n t l y , t h e b u l k i e r s y s t e m was a ban do ne d i n f a v o u r o f t h e s i m p l e p e r s p e x a s s e m b l y . L i t h i u m was a l s o d e p o s i t e d on t h i n , s e l f - s u p p o r t i n g , c a r b o n f o i l s . No d i f f e r e n c e c o u l d be o b s e r v e d i n t h e Y-ra y s p e c t r a r e c o r d e d d u r i n g p r o t o n bombardment o f c a r b o n - and t a n t a l u m - b a c k e d t a r g e t s f o r beam e n e r g i e s up t o 9 MeV. Most n e u t r o n s were p r o d u c e d i n t h e l i t h i u m t a r g e t so t h a t t h e s m a l l r e d u c t i o n g a i n e d i n b a c k g r o u n d d i d n o t w a r r a n t t h e u s e o f t h e f r a g i l e , c a r b o n f o i l s , a t l e a s t i n t h e r a n g e 2 . 5 t o 9 MeV p r o t o n e n e r g y .

T a r g e t s were k e p t t h i n - a p p r o x i m a t e l y 10 keV f o r 6 MeV p r o t o n s - t o e l i m i n a t e e x c e s s i v e p i l e - u p o f p u l s e s due t o n e u t r o n c a p t u r e i n t h e d e t e c t i n g c r y s t a l s . L i t t l e c o u l d be 7 7 done a b o u t t h e p r o l i f i c y i e l d o f n e u t r o n s fr om t h e L i ( p , n ) B e r e a c t i o n b u t a r e a l r e d u c t i o n c o u l d be a c h i e v e d i n t h e r e l a t i v e number o f h i g h e n e r g y n e u t r o n s p r o d u c e d i n t h e t w o - s t a g e r e a c t i o n L i ^ ( p , a ) H e ^ , L i ^ ( a , n ) B 1^ , s i n c e t h e l a t t e r c o n t r i b u t i o n 2 i s p r o p o r t i o n a l t o t h e s q u a r e o f t h e number o f t a r g e t n u c l e i / c m ' .

F r e s h t a r g e t s were mounted i n t h e c ha mbe r w h e n e v e r beam- s p o t m a r k i n g o r d e t e r i o r a t i o n e f f e c t s became a p p a r e n t .

3.2.2 The Detection Equipment.

Gamma-rays from the reaction were detected by two lead-

shielded 5” diameter by 4” long Nal(Tl) scintillation

spectrometers. One was free to rotate in a horizontal plane

about the target, while the other was fixed in position to act

as a monitor during angular distribution measurements.

Pulses from the DuMont 6363 photomultipliers were fed

through Franklin preamplifiers and non-overloading amplifiers

and displayed by a RIDL 400-channel and RCL 512-channel pulse-

height analyser.

3.2.3 Line-Shapes.

Gamma-ray line shapes for the purpose of spectrum

analysis were measured under conditions of identical geometry

using the proton beam of the 1.2 MeV Cockcroft-Walton

accelerator. The mono-energetic Y-rays from the T^(p,Y)He^

(Q = 19.8 MeY) reaction at Ep — 1 MeV and 16 MeV Y-rays from

11 12

the B (p,y)C ' reaction were detected and found to have similar line-shapes, confirming the findings of Mainsbridge (Ma 60a)

that Y-ray line-shape changes little with Y-ray energy above

about 16 MeV.

The Y-ray line-shape was considerably improved by

reducing the angle of acceptance for Y-rays entering the crystal,

about a factor of two reduction being observed in the low energy

cylinder housed in a 4” x 13” x 13” steel block, the whole

assembly being interposed in the 6-J- inches separating the

target spot from the front face of the movable crystal* The

o collimating aperture was tapered with a half-cone angle of 14 •

The improved tritium Y-ray line-shape was used as a

7 8

standard and fitted to the Y -line in the Li'(p,Y )Be

o o

(

q = 17.2 MeV) Y-spectrum obtained for Ep = 441 keV. The

resultant Y^ line-shape compared favourably with those obtained

at higher proton energies where Y^ radiation is dominant;

agreement with results obtained by Mainsbridge (Ma 60a) in an

earlier and similar work, was good.

3.2.4 Photomultiplier Gain Stability.

It was found that the photomultiplier gains were sensitive

to count rate i.e. to the Y-ray and particularly neutron flux.

The proton beam intensity was reduced, therefore, as reaction

yields increased, to minimize gain changes. A close check was

kept on gain drifts by frequent calibration with 2.62 MeV Y-rays

M

from the ThC source and high-energy proton capture Y-rays from

1 1 1 2 11 7 8

the B (p»Y)C ** reaction, using a B target. The Li (p,Y)Be

In document Nuclear reaction studies (Page 44-51)