Introduction:
There are few isotopes which have
good reaction crossection for neutrons. B-10, an isotope of Boron has got a
crossection of about 3840 barns for thermal neutrons and hence has got good
importance in the field of radiation detectors. The abundance of B-10 is only
about 19%. Elemental 10B in
powder form can be used as sensitive medium directly by coating onto cathodes
of detectors, but the limitation is that they have low sensitivity & poor
pulse height characteristics. Hence the idea of using 10B in gas
form has emerged. 10BF3 gas is widely used for radiation
sensors for the purpose of neutron detection.
Principle:
When a thermal neutron impinges on to the detector filled
with 10BF3 gas, the probability of nuclear reaction is
high and may result in an event which happens by following reaction:
The gas acts both as sensitive medium and ionizing medium. The
products Li & He are ionized and hence possess charge and energy resulting
in ionization by electrostatic interaction.
Design:
The shape of 10BF3 filled counters is
usually cylindrical in shape. The scheme of the detector will be as below. The
outer shell acts as cathode and the tungsten wire at centre acts as anode.
Operation:
The 10BF3
gas filled detector is operated in proportional region of gas filled detectors.
The anode is kept under influence of high potential such that enough electric
field is created to create gas multiplication.
Let us first describe that what we mean by gas multiplication
“Gas multiplying means
multiplying charge carriers created due to primary ionization of gas by
incident radiation in the gas. The charge carriers i.e. electrons drift towards
the anode at positive potential. The electrons gain momentum and get accelerated
to high energies which can transfer to other atoms resulting in emission of
further electrons called secondary electrons. The multiplication occurs more
near the regions of anode wire as there is very high electric field intensity
in the vicinity. The phenomenon of multiplication of charge in gas under
influence of external high voltage is called as gas multiplication. Gas multiplication depends on pressure,
voltage and mass of charge carriers.
According to Diethron, the
expression for ‘M’ is
where
‘M’ is gas multiplication factor;
‘V’ Is applied voltage;
‘a’ is anode radius;
‘b’ is cathode radius and
‘P’ is gas pressure.
K = E/P ; where ‘E’ is electric field and ‘P’ is
pressure inside the detector
The high voltage region in which
this gas multiplication is linear and the collected charge is proportional to
initial ionization i.e. number of original ion pairs created by incident
radiation is called as proportional region.
10BF3 counters are operated in this
region and are hence often called as 10BF3 gas filled
proportional counters.
HV Plateau curve:
The curve is derived by recording the variation of average counts
observed in a preset time with high voltage applied to anode. A sample curve is
shown below.
Following are the physical parameters which can be obtained
from the curve:
i)
Plateau length (Operating voltage range):
This is the range of high voltage over which counter can be operated in
proportional mode. The gas multiplication almost remains constant over this
range of operation. The region is selected in such a way that the difference of
count rate is minimum. In other words, slope shall be minimum.
ii)
Plateau slope: The slope in general for
BF3 counters, shall be <3% per 100V.
iii) Operating voltage: This is the voltage
which is selected by the user based on the sensitivity required and stability
of high voltage power supply. This shall be noted that the count rate at lower
end of plateau region will be low as compared to count rate at high end of
plateau. Hence deciding operating voltage will also decide sensitivity of
detector.
