Page 248 - Radiochemistry and nuclear chemistry

232 Radiochemistry and Nuclear Chemistry

A B +

E

F -I. j- RIGHT
LAMP
G "1~ A" WRONG
N-NN\\\\\\\\\
HOT PLATE
H ~ WRONG
1 r

FIG. 8.22. The preparation of thin, uniform samples of known surface density. A, filtering
arrangement; B, electrolytic plating system; C, centrifugation vessel; D-E, pipetting of a
solution into an evaporation tray (glass, stainless steel, platinum), yielding a correct even
deposit F or uneven samples G-H. The aluminum ring E should have a temperature only
slightly exceeding the boiling point of the liquid.

corresponds to approximately 20 % of the range of the B-particles in the backing material.
Scattering can also occur from the walls of the sample holder, but this is usually less
important than the backscattering from the sample backing material.
If the sample is thick, the count rate may be incr~ by internal backscattering, but the
decrease of the count rate due to self-absorption within the sample is a greater factor. This
sample self-absorption is inversely proportional to the B-ray energy and directly
proportional to the thickness of the sample. For 32p (1.72 MeV) sample thicknesses of 15
nag cm -2 show little self-absorption, while for 14C (0.15 MeV), the absorption for sample
thicknesses as small as 1 mg cm -2 is significant. The absorption factor can be determined
by counting a series of samples of different thicknesses but with the same total
disintegration rate, and then extrapolating that to zero sample weight.
The absorption factor ~kabs is related to the absorption of the particles after they leave the
sample by any coveting over the sample, by the air between the sample and the detector,
by the absorption in the detector window, etc. Again, this factor is usually determined by
experimental comparison with a sample of known absolute disintegration rate, a standard.