from the Lil. The photomultiplier produced a series of pulses, which were recorded
with a 256-channel pulse-height analyzer.
Since the channel numberof each event was proportional to the light output of the crys-
tal for that event, a quantity P(E,), representing the total light output at a given neutron
energy En, could he defined as:
256
P(Ey) = >>
.
Where:
jal
j my
j =channel number
my = number of counts in that channel
.
At higher neutron energies, reactions other than those listed above began to take place,
namely Li'(n, dn)He‘ and Li’(n, d)He', as well as gamma activity arising from the in-
elastic scattering of neutrons on iodine. All of these processes contributed to the light
output of the crystal and formed a continuous background, which increased with increas-
ing energy.
The total neutron flux, N, was determined with the aid of a calibrated long counter,
which subtended the same solid angle at the neutron source as the detector. The quantity
P(E,)/N was then proportional to the average light output perunit of neutron flux.
It was then necessary to establish a relationship between the crystal's light output and
the sensitivities of the photomultiplier and the photodiode. This determination was complicated by the fact that the spectral response of the two tubes was different. Therefore,
a single crystal was used as a light source for both, and X rays from the NRL 21 Mev
betatron were sufficiently intense to be used for excitation.
By measurement of the charge released by the photodiode when a high intensity beta-
tron pulse was incident upon the crystal and comparison of it with that produced by the
photomultiplier from a low-intensity pulse, it was possible to eatabliah the desired relationship between the tube sensitivities.
The calibration formula could then be expreased:
QPP
PD if,) =
The value Qe D (En)
Ypy and ypp
Pg
YPM
YPD
[{P(En)/N 1 gp
[P(En)
Pg
average charge released by the photodiode per unit of neutron
flux of energy E, striking the crystal;
radiation dosages per burst at the low and high levels, respectively;
height of the pulge produced by the photomultiplier for a lowintensity betatron burst;
QBD. charge released by the photodiode for a high-intensity burst
The value P(Ey)/N was defined above.
The curve shown in Figure 2.1 represents QFD (Ey) ag a function of energy for both
Li‘l and the normal crystal, LiNt. By use of the measured current outputs of the photo16