The Radiophotoluminescence property of some materials makes it possible
to use them for dosimetry purposes. The silver activated metaphoshate glass 1 as
an RPL dosimeter when exposed to X. or y radiations , forms
fluorescent centers with long lived states ( metastate ). The silver ions (Ag+) in
unirradiated state produces vacancies when energy is supplied by irradiation
the release of electrons leads to the formation of RPL centers. By stimulating
the crystal by the ultra - violet light 1 without destroying the RPL centres I
.an orange fluorescence is produced proportional to the absorbed dose. These
glasses can also be used for long term personal radiation monitoring. One of
the advantages of this type of crystals is that it has linear dose sensitivity
over a long range and show no fading' The relative RPL intensity is slightly
dependent on radiation energy. and completely independent on dose rate.
Another advantage is that the dose can be added up.
By heating up the crystal the fluorescent centres can be erased.
The crystal is also sensitive to neutrons. thermal neutrons are detected
through (n 1 a) reactions in Boron ,Lithium and (n, y) reaction in Silver and
Phosphorus. The intermediate and fast neutrons are detected through elastic
: scattering in Lithium anp Oxygen and through (n, p) reaction in Ag and
Phosphorus !
By covering a crystal with cadmium to supress thermcl neutrons gamma
dose can be measured. Cadmium attenuates low' energy (Kev) gamma rays.
Sn has the same attenuation factor for low energy gamma rays and has very low
absorption cross section for thermal neutrons.
In a series of experiments around the Tehran University Research
Reactor core the following measurements , using FD - RI -. I .RPL glasses and
Fluoroglass dosimeter type FGD - 3B as a measuring unit , were carried out :
1) Integral gamma decay measurement of fission products after 2 to 5
hours operation at high power (3MW).
2) Gamma intensity measurements at the, specified distances from the
edge of the core in ten (10) holders fixed. to an alluminium bar (fig 1) .
,3) The; study of heating up the crystal !(FD-RI-I) up' to 400~C. The
experimental results show that the FD ~Rl- 1 crystal loses about 10% of its
total dose after 10 minutes being at 200°0 , about 40% at 320°0 and 100%
at ,400°0 for the same heating time (Fig 2).
4) The sensitivity of a fission chamber to gamma radiation from the
fission fragments in the core was determined. The identical fission chamber is
used for starting up the reactor. The ordinary counting system and three FDRI
-1 crystals , one eovered with lmm thick cadmium one covvered with lmm
thick Sn and third bare, were placed at the same place as the fission chamber
for neutron and gamma dose measurements. From these measurements it was
found that the fission chamber has a sensitivity of only O. 1% to the core
gammas as compared to the fission neutrons travelled 30 cm in water.
)