Page 553 - Instrumentation Reference Book 3E

Detectors 535

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Gap &, I
Cliaphragrn Y Y
Guard Guard
electrode Collector electrode
Figurse 22.9 Freeair ionization chamber

or liquid scintillator. By operating the solid-state
detector in anti-coincidence with the annular
scintillation detector, Compton-scattered photons
from the primary detector into the anti-coinci-
1.17 MeV 1.33,MeV dence shield (as it is often called) can be electro-
nically subtracted from the solid-state detector's
energy response spectrum.

22.2.4.4 The detection of neutrons
The detection of nuclear particles usually depends
Ge (Li) detector
on the deposition of ionization energy and, since
neutrons are uncharged, they cannot be detected
directly. Neutron sensors therefore need to incor-
porate a conversion process by which the incoming
Channel number-) particles are changed to ionizing species and
Figure 22.10 Comparison of energyresolution by nuclear reactions such as zisU (fission)+
differeiit detectors.
N 200 MeV or "B(n, a)Li7 + - 2 MeV are often
used. Exothermic reactions are desirable because
of the improved signal-to-noise ratio produced by
ber of disadvantages which prevent this detector the increased energy per event, but there are limits
superseding the scinti.llation detector. First, the to the advantages which can be gained in this way.
prese:nt state of the art limits the size of solid-state In particular, the above reactions are not used for
detectors to some 100 cm3 maximum, whereas neutron spectrometry because of uncertainty in
scintillation crystals of NaI (Tl) may be grown to the proportion of the energy carried by the reac-
sizes of 76cm diameter by 30cm, while plastic tion products, and for that purpose the detection
and liquid scintillators can be even larger. Second, of proton recoils in a hydrogenous material is
solid-state detectors of germanium have to be often preferred.
operated to liquid-nitrogen temperatures and iii There are also many ways of detecting the result-
VQCUO. Third, large solid-state detectors are very ant ionization. It may be done in real time with:
expensive. As a result, the present state of the for example, solid-state semiconductor detectors.
art has tended towards the use of solid-state scintillators, or ionization chambers, or it may be
detectors when the problem is the determination carried out at some later, more convenient time by
of energy spectra, but scintillation counters are measuring the activation generated by the neu-
used in cases where extremely low levels of activ- trons in a chosen medium (such as 56M from
ity are required to be detected. A very popular com- 55 Mn). The choice of technique depends on
bined use of solid-state and scintillation detectors the information required and the constraints
is embodied in the technique of surrounding the of the environment. The latter are often imposed
relatively small solid-state detector in its liquid by the neutrons themselves. For example, boron-
nitrogen-cooled and evacuated cryostat with a loaded scintillators can be made very selisitive and
suitable scintillation counteri which can be an are convenient for detecting low fluxes, but scintil-
inorganic crystal such as NaI (Tl) or a plastic lators and photomultipliers are vulnerable to