Page 554 - Instrumentation Reference Book 3E

536 Nuclear instrumentation technology

damage by neutrons and are sensitive to the gamma well chosen this activation can produce residual
fluxes which tend to accompany them. They are not signals analogous to gamma signals and seriously
suitable for the high-temperature, high-flux appli- shorten the dynamic range. The electrodes and
cations found in nuclear reactors. envelopes of ion chambers are therefore made
Neutron populations in reactors are usually from high-purity materials which have small acti-
measured with gas-filled ionization chambers. vation cross-sections and short daughter half-
Conversion is achieved by fission in 23sU oxide lives. Aluminum is usually employed for low-tem-
applied as a thin layer (-1 mg cm-’) to the perature applications, but some chambers have to
chamber electrode(s). The ‘OB reaction is also used, operate at -550°C (a dull red) and these use
natural or enriched boron being present either as titanium and/or special low-manganese, low-
a painted layer or as BF3 gas. Ionization cham- cobalt stainless steels. Activity due to fission pro-
bers can be operated as pulse devices, detecting ducts from fissile coatings must also be considered
individual events; as d.c. generators, or in which and is a disadvantage to fission chambers. The
events overlap to produce current; or in the so- choice of insulators is also influenced by radiation
called “current fluctuation” or “Campbell” mode in and temperature considerations. Polymers dete-
which neutron flux is inferred from the magni- riorate at high fluences, but adequate performance
tude of the noise present on the output as a con- can be obtained from high-purity, polycrystalline
sequence of the way in which the neutrons arrive alumina and from artificial sapphire, even at
randomly in time. Once again the method used is 550 “C. Analogous problems are encountered
chosen to suit operational requirements. For with cables and special designs with multiple co-
example, the individual events due to neutrons axial conductors insulated with high-purity coni-
in a fission chamber are very much larger than pressed magnesia have been developed. Electrode/
those due to gammas. but the gamma photon cable systems of this type can provide insulation
arrival rate is usually much greater than that of resistances of order lo9 0 at 550 “C and are con-
the neutrons. Thus, pulse counters have good figured to eliminate electrical interference even
gamma rejection compared with d.c. chambers when measuring microamp signals in bandwidths
at low fluxes. On the other hand, the neutron- of order 30MHz under industrial plant condi-
to-gamma flux ratio tends to improve at high tions. Figure 22.11 shows the construction of a
reactor powers whilst counting losses increase boron-coated gamma-compensated d.c. chamber
and gamma pulse pile-up tends to simulate neu- designed to operate at 550 “C and 45 Bar pressure
tron events. D.c. operation therefore takes over in the AGR reactors. The gas filling is helium and
from pulse measurement at these levels. The cur- ,O activity from the low-manganese steel outer case
rent fluctuation mode gives particularly good is screened by thick titanium electrodes. The dia-
gamma discrimination because the signals depend meter of this chamber is 9 cm; it is 75 cm long and
on the mean square charge per event, Le., the weighs 25 kg. By contrast, Figure 22.12 shows a
initial advantage of neutrons over gammas is parallel plate design, some three hundred of which
accentuated. Such systems can work to the high- were used to replace fuel “biscuits” to determine
est fluxes, and it is now possible to make instru- flux distributions in the ZEBRA experimental fast
ments which combine pulse counting and current reactor at AEE Winfrith.
fluctuation on a single chamber and which cover a Boron trifluoride (BF3) proportional counters
dynamic range of more than 10 decades. are used for thermal neutron detection in many
The sensitivity of a detector is proportional to fields; they are convenient and sensitive, and are
the probability of occurrence of the expected available commercially with sensitivities between
nuclear reaction and can conveniently be 0.3 and 196s-’ (unit flux)-’. They tend to be
described in terms of the cross-section of a single much more gamma sensitive than pulse-fission
nucleus for that particular reaction. The unit of chambers because of the relatively low energy
area is the barn, Le., 10-*‘cm’. ‘OB has a cross- per event from the boron reaction, but this can
section of order 4000 b to slow (thermal) neutrons be offset by the larger sensitivity. A substantial
whilst that of 235U for fission is only N 550 b. In disadvantage for some applications is that they
addition, the number of reacting atoms present in have a relatively short life in terms of total dose
a given thickness of coating varies inversely with (gammas plus neutrons), and in reactor applica-
atomic weight so that. in principle, ‘OB sensors tions it may be necessary to provide withdrawal
are much more sensitive than those which depend mechanisms to limit this dose at high power.
on fission. This advantage is offset by the lower Proton-recoil counters are used to detect fast
energy per event and by the fact that boron is neutrons. These depend on the neutrons interact-
burnt up faster at a given neutron flux, Le., that ing with a material in the counter in a reaction of
such detectors lose sensitivity with time. the (n,p) type, in which a proton is emitted which,
Neutrons generate activation in most elements, being highly ionizing, can be detected. The mater-
and if detector constructional materials are not ial in the counter can be either a gas or a solid. It