The neutron flux outside the shielding of accelerators is composed of neutrons with a wide range of energies. This flux includes thermal neutrons (which are neutrons with energy of about 0.025 eV), and neutrons with energies all the way up towards the operating energy of the cyclotron. It is always accompanied by gamma fields. For dosimetry purposes the neutrons are classified into two groups: Those with energies less than 10 keV called slow neutrons , and those with energy above 10 keV called fast neutrons .
The measurement of equivalent dose in tissue irradiated by neutrons is complicated by the many interactions which can occur between the tissue and neutrons of widely different energy. Instruments are accurate only for a specific range of neutron energy and the instrument described below (Andersson Braun monitor or `Snoopy') is used in the range 0.025 eV (thermal) to 15 MeV.
The detector of this instrument is a proportional chamber in the centre of a polyethylene cylinder. The incident neutrons with energy greater than thermal are moderated through elastic scattering in the polyethylene, and a perforated boron sleeve around the detector is used to shape the response.
The chamber is filled with boron trifluoride gas
(BF 
). Thermal neutrons
absorbed by the
B nuclei in the gas undergo a nuclear reaction which
results in energetic alpha particles and
Li nuclei being ejected. These
`heavy' nuclei produce intense ionisation
and their pulse is easily
distinguished from the much smaller pulses that are produced by Compton and
photoelectrons resulting from gamma rays
.
The construction of the instrument is shown in Fig. 6.5
. The design of the moderator , absorber,
detector unit is such that it is approximately tissue equivalent
for neutrons in the energy range 0.025 eV to 15 MeV.
The instrument has four accurate linear ranges of 20, 200, 2000, and
full scale.
Figure 6.5: Neutron equivalent dose rate meter
A discriminator selects only the pulses produced by the
B(n, 
) Li reaction and
rejects any pulses due to gamma background. The instrument will remain gamma
compensated in fields of up to
.
Long signal cables may be attached between the detector and ratemeter enabling remote surveys to be done.
The main drawback of this instrument is that it has
greatly reduced sensitivity
to the higher energy neutrons generated by the 500 MeV cyclotron facility.
Independent measurements indicate that up to approximately half of the total
equivalent dose in these neutron fields is due to neutrons with energy above
about 20 MeV. The sensitivity to high-energy neutrons can not be easily
increased without substantially increasing the amount of material in the
already heavy moderator (Fig. 6.6
). To estimate the possible dose
to workers, the reading on this
monitor should therefore be multiplied by 2 to account for the high-energy
neutron component which is missed. This is not necessary when measuring
neutron fields near the radioisotope production cyclotrons because no
high-energy neutrons are generated by these machines.
Figure 6.6: Neutron equivalent dose rate meter