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474
MEASUREMENT
AND
DETECTION
OF
RADIATION
however, cannot exceed the range of the alphas produced. The advantage of
boron-lined over the BF, counters is the flexibility of using a gas more appropri-
ate than
BF,. Then the operating voltage is less and the counter is less sensitive
to gamma rays. For this reason, boron-lined counters may be used to detect
neutrons in intense gamma fields.
Both BF3 and boron-lined counters are mainly used for the measurement of
the number (not energy) of thermal neutrons.
14.2.3
6~i
Counters
Neutron detection by 6~i is based on the reaction
The cross section for this reaction is of the l/v type up to 10 keV, with a value
of 937
b
at 0.025 eV. Lithium is used either as LiF or as the component of a
scintillator.
A
neutron spectrometer based on
L~F'
consists of a thin slice of 6~i~
(-
30
x
low5
kg/m2
=
30 pg/cm2) sandwiched between two surface-barrier
silicon detectors. When neutrons strike the LiF, charged-particle pairs (4~e-3~)
are produced and are detected simultaneously by the two detectors. The pulses
from the detectors are amplified and then summed to produce a single pulse,
which is proportional to the energy of the neutron plus the
Q
value of the
reaction.
6
There are many inorganic scintillators based on lithium. LiI(Eu) has been
used for neutron energy measurements from
1
to 14 MeV with 10 percent
energy reso~ution.~ It has good efficiency for low-energy neutrons, but activation
of iodine creates some problems. The most widely used lithium scintillator was
developed by Ginther and Schulman3 and Voitovetskii et
al.4 It is a cerium-
activated scintillating glass containing Li20. The proportion of the cerium
activator affects the efficiency of luminescence.
A
series of measurements of
many properties of commercially available glasses has been reported recently by
Sp~wart.~,~ Today one can buy these glasses in a large variety of thicknesses
(0.5-25 mm), sizes (up to 125 mm in diameter), Li contents (up to
11
percent),
and 6~i enrichments (up to 95 percent). The efficiency of 6~i glass as a function
of neutron energy is shown in Fig. 14.2.
6
To increase the efficiency, Li glass scintillators with thickness about 13 mm
and diameter 110 mm have been optically coupled to one or more photomulti-
plier tubes through light To avoid moderation of the incident neutrons,
the light pipe should not contain hydrogenous material. One problem with such
thick scintillators is considerable scattering of the incident neutrons. The
scattered neutrons add an exponential tail to the primary neutron signal, a tail
that should be included in the time resolution function of the instrument in
time-of-flight measurements.