Page 238 - Radiochemistry and nuclear chemistry

222 Radiochemistry and Nuclear Chemistry

O. 76

0
9 0.74 --...
-
I=1

~-i ~072 ,
I=1

O. 70 , , i ,
0 5 I0 15 20 25
Water content (7.)
FIG. 8.17. Quenching effect of water. The sample/standards ratio is the count rate ratio
between 4 ml scintillator cocktail (Beckman Ready-Solv HP) + water and Packard reference
standard.

Figure 8.16 is popular because of the high counting efficiency for samples introduced into
the well (~, > 0.9). For counting very large liquid volumes (e.g. environmental, water
samples) a specially designed sample vessel is used which fits over and around the
cylindrical detector arrangement (Marinelli beaker).

8.6. (~erenkov detectors

The (~erenkov effect described in w can be used for detection of high energy
/3-radiation because the velocity of the nuclear particle must exceed the ratio c/n, where n
is the refractive index of the absorber.

EB,h,,,ho ~ = 0.511 [--1 + {1 + (n 2 -- 1)-1} t/2 ] (MeV) (8.14)

The/3-threshold energy in lucite (n = 1.5) is 0.17 MeV, so lucite and similar plastics are
often used as particle absorbers in (~erenkov detectors. In order to detect the light emitted,
PMTs are placed in the direction of the emitted light. There are many similarities between
scintillation and ~erenkov detectors; however, the light pulse from the Cerenkov detector
is faster, - 10 -10 s, but smaller than from scintillation detectors. The advantage of the
Cerenkov detector is that aqueous or organic (uncolored) solutions can be used without the
need to add a scintillator, that soft/3-emitters and "),-emitters give little or no signal, and
that the angle of emitted light reveals the velocity of the absorbed particles. At energies <
10 MeV, only/3-particles are detected.