Boron trifluoride type

The boron trifluoride (BF3) proportional counter de­tects neutrons by the n-a boron-10 reaction and con­sists essentially of a fine wire concentric with, and insulated from, a thin copper or aluminium tube to form a vessel filled with BF3, as shown in Fig 2.47.

The electrons produced in the primary ionised tracks are attracted to the central anode wire and, with a sufficiently high positive potential (1.5 to 2 kV), sec­ondary ionising collisions occur and the final collected charge is thus amplified by a gas multiplication pro­cess. The BF3~gas is free from electron capturing impurities and the counter body is thoroughly out — gassed to prevent subsequent contamination of the gas and deterioration of the counting characteristics. Good vacuum characteristics are obtain. J by using oxygen-free copper and fluxless brazing techniques for all joints.

Modern counters also use aluminium extensively and this gives a better performance than copper, par­ticularly for life in high neutron fluxes.

For a 12EB40 counter filled with enriched boron triflouride to 400 mm Hg, the slow neutron sensi­tivity is about 3.5 counts/s/n/cm2/s. This type of counter will work satisfactorily in a у flux of about 2 Gy/h and the maximum counting rate possible is about 5 x 104 counts/s because of pulse length. Proportional counters are about 100 times more sen­sitive to slow neutrons than fission counters of simi­lar size, but unfortunately their 7 sensitivity is also considerably greater and it is not possible to discri­minate between 7 and neutron pulses of similar amplitude.

With a gas multiplication factor of 40, the life of a BF3 counter is about 1010 counts at below 100°C whilst above 150°C counters deteriorate rapidly. Counters are not operated in neutron fluxes above about 2 x 107n/cm2/s to avoid difficulty in discri­minating between neutron pulses due to space charge effects.

The BFi proportional counter is used extensively in the thermal columns of magnox stations with arrangements for withdrawal during power raise and insertion during the shutdown phase, as discussed in Chapter 3.

Fission type

By the use of the fission process, detectors can be made that operate at higher temperatures and neu­tron fluxes than the BF3 type without suffering from

pulse pile up.

A typical uranium-235 fission counter consists of a stainless-steel container with cathode liner concen­tric with, and insulated from, the cylindrical anode. A thin layer of fissionable material is deposited on

the electrode surface.

The fission counter detects neutrons by the U-235-n reaction; the resulting fission fragments cause ionisa­tion of the filling gas. The coating of uranium is limited to about 1 mg/cm2 to limit energy lost by fission fragments emerging from the coating. The neutron-induced fission pulses, relative to the 7 pulses in this type of counter, are much larger than in the proportional counter; they are also much shorter in duration, and these two effects enable the fission counter to operate satisfactorily in 7 fluxes up to 103 Gy/h. The sensitivity of the P7 counter, shown in Fig 2.48, is in the range 0.01 to 0.1 counts/s/n/cm2/s with rates up to 2 x 105 pulses/s with 10% counting losses.

The operating potential required is between 200 and 400 V. Since inert gases are used for filling and there_is no gas multiplication, fission counters are less sensitive to impurities and may be operated at temperatures of up to 550°C and with containment. up to 40 bar The life of the counter is 1019 nvt and it will operate in a neutron flux of about 1011/ cm2/s before j3/pulse ‘pile-up’ becomes serious, thus limiting the use of the detector at low powers after

irradiation. The burn-up at 10n n/cm-/s is 0.2ro per annum. Further details of fission counters type P7 are given in Table 2.5.

Another chamber, P8, is available specially devel­oped for Campbell channels, described in Section 5.2.12 of this chapter.