EQUIPMENT AND EXPERIMENTAL TECHNIQUE

In document Alpha capture to the giant dipole and quadrupole resonances (Page 36-40)

Excitation functions were measured for 58Fe(a,y)80N i ,

48Ti (a,y ) 52C r , 40Ar (a,y)44Ca and 38Ar(a,y)42Ca reactions over the bom­ barding energy ranges 8-17.6 MeV, 6-12 MeV, 5.5-11.4 MeV and 6-16 MeV respectively. The 4He++ beam was obtained from the A.N.U. EN tandem accelerator and typical target currents were 0.5-1.0 yA. The excitation functions were generally measured in 50 keV beam energy steps, although certain regions of interest were investigated in finer detail using 25 keV steps. At high excitation energies where the cross section

became very small, thicker targets were used and the excitation function measured at either 100 keV or 200 keV intervals.

The gamma-rays were detected at 90° to the beam direction by the A.N.U. 10" x 10" Nal (Tl) spectrometer (B1 71). The energy resolution of the spectrometer was approximately 6.5% FWHM for 15 MeV gamma-rays. This allowed the transition to the ground state (y ) to be reasonably well

0

resolved from the transition to the first excited state (y ), whereas y

1 1

was generally not resolved from lower energy gamma-rays. The yields in the photopeaks for y^ and when possible y were obtained using a linear least squares fitting procedure, described in section 2.9. The spectro­ meter, collimators and associated electronics are described in sections 2.6 - 2.7. The preparation of the targets and the methods by which the target thicknesses were determined are described in sections 2.3 - 2.5. Angular distributions were measured at 20 bombarding energies for the

38Ar(a,y)42Ca reaction and at 10 bombarding energies for each of the other reactions. In addition 10 angular distributions were measured for the 28Si(a,y)32S reaction over the bombarding energy range 6-11.5 MeV.

23.

Prel iminary measurements of the 56Fc(a,y) r>0Ni and 40Ar (m,y) 44C;i

reactions showed that the 90° differential cross sections were approximately 1 yb. This small cross section resulted in a low count rate which could only be increased to a satisfactory level by using large beam intensities, since the maximum target thickness and the maximum solid angle subtended by the Nal(Tl) crystal were limited by resolution considerations described below.

The low-lying levels of the residual nuclei studied are shown in figure 2.1 where it can be seen that the first excited state lies

1.156-1.524 MeV above the ground state; the second excited state is 0.312-0.936 MeV higher in energy. The resolution of the Nal(Tl) spectro­ meter varies between 6.5 and 10%, depending upon the collimation geometry. For 20 MeV gamma-rays, and 6.5% resolution, peaks separated by greater than 1.3 MeV can be resolved. This is approximately the energy separation between y and y . The gamma-ray transitions to the ground state and first

0 1

excited state could therefore only be resolved by using collimator 4

(described later) which gives the best resolution but has the disadvantage of small solid angle. In addition the target thickness had to be limited to < 200 keV so as not to contribute significantly to the energy resolution of the spectrometer. Consequently, the only method available for obtaining a satisfactory count rate was to use large 4He++ beam intensities. Un­ fortunately, at the higher energies, pile-up of low energy gamma-rays into the region of interest limited the maximum beam current that could be utilized.

2.1 THE ALPHA PARTICLE BEAM

At the time of starting this series of experiments the 4He+ + target currents that could be obtained from the A.N.U. EN tandem accelerator were typically 100-200 nA. With such small beam intensities, the measurement of a single excitation function would have taken a year. Obviously, the above

2 7 5

4+

2 6 6

2+

2

648

0+

4+

2

423

2+

2-507

2 2 8

4+

2 3 6 9

4+

2-286

0+

188

2-158

2+

1836

0+

0+

1524

2+

1156

2+

1-433

2+

1-332

2+

Figure 2.1. Low-lying levels of the residual nuclei following the

2 4.

m easurem ents c o u ld n o t have b een c o m p lete d w i t h i n a r e a s o n a b l e p e r i o d o f

t i m e , and i t was t h e r e f o r e e s s e n t i a l t o i n c r e a s e t h e q u a n t i t y o f beam on

t a r g e t . T h i s was a c h i e v e d m a in ly by i n c r e a s i n g t h e o u tp u t o f t h e duo-

p l a s m a t r o n HVEC n e g a t i v e io n s o u r c e . However, t h e t r a n s m i s s i o n th r o u g h

t h e tandem a c c e l e r a t o r was improved by t h e u s e o f an E i n z e l le n s i n

c o n j u n c t i o n w i t h t h e m a g n e tic q u a d r u p o le t r i p l e t t o im prove t h e f o c u s s i n g

a t t h e low e n e r g y end o f t h e m ach in e. The f o l l o w i n g f o u r f a c t o r s c o n ­

t r i b u t e d t o improved o u t p u t o f t h e i o n s o u r c e :

( i ) C l e a n l i n e s s - t h e io n s o u r c e magnet a s s e m b ly , f i l a m e n t

and a p e r t u r e were c l e a n e d i n a g l a s s bead s h o t b l a s t e r

b e f o r e each r u n .

( i i ) A p e r t u r e s i z e - t h e s i z e o f t h e io n s o u r c e a p e r t u r e was

o p ti m iz e d by t r i a l and e r r o r . I t was found t h a t an

a p e r t u r e w ith a d i a m e t e r a p p r o x i m a t e l y 0 .0 3 2 " gave n o t o n ly a l a r g e o u t p u t b u t a l s o a good e m i t t a n c e which r e s u l t e d i n a p p r o x i m a t e l y 25% t r a n s m i s s i o n t h r o u g h t h e m ach in e. ( i i i ) The l i t h i u m exchange - a f r e s h s u p p ly o f c l e a n e r l i t h i u m was p u r c h a s e d .

( i v ) T an talu m i n s e r t s were u sed t o c o l l i m a t e t h e beam i n t o and

out o f t h e l i t h i u m exchange c a n a l r e d u c i n g t h e s p l u t t e r i n g

o f m e ta l on t o t h e s p h e r i c a l i n s u l a t o r s .

T h ese ch an g e s improved t h e o u tp u t a t t h e low e n e r g y F a r a d a y cup from

< 1 yA t o 3-4 yA and t y p i c a l c u r r e n t s on t a r g e t were 0 . 5 - 1 yA w ith a

maximum o f 1 .5 yA.

The e n e rg y o f t h e beam was s e l e c t e d by a s i n g l e f o c u s s i n g 90°

a n a l y z i n g m ag n et. In o r d e r t o m in im ize d i f f e r e n t i a l h y s t e r e s i s e f f e c t s ,

a t t h e s t a r t o f each r u n t h e magnet was c y c l e d t h r e e t i m e s b etw een 0 '1 5 amps

25.

substantially reduce the beam energy, the magnet was recycled in a similar manner. By this method it was possible to reproduce the energy to

± 5 keV. Object slits before the magnet were usually set to ± 0.150" and image slits after the magnet to ± 0.075". Beyond the image slits, the beam was deflected a further 25° by the switching magnet and focussed on to the target by a magnetic quadrupole doublet lens. The size of the beam spot was not critical because of the very large size of the Nal(Tl) detector. As a result, a single collimator, 0.200" in diameter was used 20 cms from the target. Its main purpose was to prevent beam striking the target frame. The integrated beam was digitized using an Ortec 439 charge

digitizer and the pulses counted on a preset-stop scalar which controlled the data collection on either the IBM 1800 computer or the Nuclear Data ND1024 multichannel analyzer.

2.2 TARGET CHAMBERS 2.2.1 The D-chamber

This chamber was used in conjunction with transmission targets. The cylindrical wall faced the spectrometer and produced a 6% attenuation

of 20 MeV gamma-rays. It had a 1” diameter entrance port and a similar

In document Alpha capture to the giant dipole and quadrupole resonances (Page 36-40)