Radioactive beams by fragmentation and ISOL techniques - CERN

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for example Si and 38 P observed with an 40Ar beam with sufficient yield to measure
their massgl.
product
(5)
c) Slowing down of energetic reaction products
For several domains of physics, it is necessary to have low energy products. For
example, interesting phenomena have been predicted in the fusion of neutron rich
nuclei9). Energies of 1-5 MeV/nucleon are therefore necessary. These may be obtained by
slowing down the beams before the reaction. For a study of fusion of Be isotopes with
an U target we measured the loss of intensity due to this slowing down]
Slowing down in a thick degrader increases the absolute width of the momentum
distribution. Writing a taylor serie to first order for Bpy as a fonction of Bp,
Bp{=Bp{()+(dBpf/dBpi)ABri, we see that the initial width GA;)/p will be multiplied by
(dBpf/dBpi
The transmission of the spectrometer is given by Ap/p, so even for a constant
width a loss of transmission of Bpi/Bpf will result. Thus the loss factor fi will be the
Fig. 3 shows that this relation holds experimentallyl
An achromatic degrader could in principle give better results than a thick target. In
practice, we found a yield 3 times lower than in the thick target case. The main reason is
the limitation of spectrometers : with a thick target the straggling will add to the finite
distribution of the nuclear reaction, and at least in the case studied here, the straggling is
small as compared to the width of the nuclear reaction in the (9,q>, Ap/p) space. With an
achromatic degrader, the Hrst part of the spectrometer already makes a severe cut in this
space. A very thick degader will enlarge this distribution once again due to straggling.
and it will be larger than the acceptance of the second part of the spectrometer, which is in
most existing devices identical to the one of the first part. The conclusion may be
different if the second part of the spectrometer has a very large acceptance. For the
spectrometer LISEwe U) concluded, that it is more easy, more efficient and gives lower
emittance beams if a thick target is used for simultaneous production and slowing down
instead of the use of an achromatic degrader.
For this case, we can use the analytical formulas of J.P. Dufour et allil. to
estimate the loss . We get OCR Output
f, 2 _
dBp{ Bpi
&Ea