These detectors are still being used in real time applications like monitoring delayed Neutron activity in Pressurized Heavy Water Nuclear Reactors for failed fuel detection, nuclear waste assay monitoring systems, special nuclear material detection systems.
This article reviews the physics behind the BF3 gas filled thermal neutron detectors and their characteristics.
INTRODUCTION
Boron-10, an isotope of Boron has got a reaction cross-section of about 3840 barns for thermal neutrons. 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. Boron-Tri- Fluoride 10BF3 gas is widely used for radiation sensors for the purpose of thermal neutron detection.
PRINCIPLE OF OPERATION
The basic configuration of any gas filled detector with assumption of cylindrical geometry is as shown in following Fig.
The modes of operation of gas filled detectors have been classified into three major regions derived from the plot of applied voltage vs output pulse height (shown in below Fig).
They are
i) Ionization region ii) Proportional region and iii) Geiger Mueller region.
10BF3 gas filled detectors are operated in the proportional region and hence are classified as proportional counters.
Proportional region is the applied voltage region of the detector during which the output variation will be proportional to the incident radiation flux. These detectors usually operate in pulse mode unlike ionization chambers as number of incident events information is required.
To produce enough charge, good amount of charge is required which couldn’t be generated in primary ionization. Hence, high potential is applied to these detectors which could amplify the amount of charge generated. This process is called as 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(M). 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 called as 10BF3 gas filled proportional counters.
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 reaction products “Li++” & “He++” are
ionized and hence possess charge and share the energy resulting in ionization
by electrostatic interaction.
DESIGN
ANODE
For a cylindrical shaped detector, the electric field at a radial
distance ‘r’ measured from the axis is given by
Where, V is the positive potential applied to central wire(anode) of radius ‘a’ and ‘b’ is radius of outer cylinder kept at ground potential. It is evident that the lesser the radius of anode wire, the more the intensity of electric field due to potential on anode. Hence, Tungsten wire with diameter of few microns usually 1mil or 2 mil wire is used as anode to generate strong electric field for same applied voltage.
Materials
Aluminum is a desirable material due to its low absorption crossection for thermal neutrons.
But all commercial aluminum was found to have high background count when exposed to high neutron flux due to induced radioactivity in the impurities.
Copper has got good strength but has high absorption cross-section for neutrons.
Both Aluminum and Copper are not suitable for high vacuum degassing process.
Hence, Stainless steel of grade 304L is generally used for body of the detectors.
For electrical isolation of electrodes, High pure ceramic to metal seals are used.
PERFORMANCE
The quality of the 10BF3
detector could be estimated by using pulse height analyser. Typical output
pulse height distribution from a 10BF3 counter for
thermal neutrons is as shown in Fig-3. Here, pulse heights are
expressed in units of the energy deposited in the counter.
The full width at half maxima (FWHM) of the peak will determine the resolution and thus quality of collection of charge in the detector.
The pulse height(Vo) at the detector output is given by
Where, Q is secondary charge created near anode due to
avalanche formation; ‘n’ is no. of primary ion pairs created by ionization of gas by Li
and He; Cf is feedback capacitance of charge sensitive amplifier.
CHARACTERISTICS
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 in Fig-4.
Plateau
length (Operating voltage range) 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 less. In other words, slope shall be
minimum.
Plateau slope per volt is given by
Good slope indicates that the
pulse height distribution is good.
OPERATING VOLTAGE
This is the voltage which is selected 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.
SENSITIVITY(S)
S = counts per second at operating voltage/ incident neutron flux
Theoretical expression for sensitivity is
V is sensitive volume of detector
EFFICIENCY
The efficiency of the detector along radial direction is given by
ξ = 1-exp(-Nσt)
‘N’ is number of converter nuclei per unit volume;
σ is neutron absorption crossection
‘t’ is gas gap thickness
EFFECT OF PRESSURE AND TEMPERATURE ON PERFORMANCE OF 10BF3 DETECTORS
The pulse height distribution (PHD) in the detector gets affected by temperature. Broadening of the PHD is observed with increase in pressure, however with increase in efficiency of detection. The PHD width decreases with decrease in temperature. Thus to obtain good efficiency a high pressure detector could be used maintaining low temperature.
CONCLUSION
Literature pertaining to BF3 counters was reviewed and insight is given to the basic principle of operation, design parameters and the functional characteristics. This article would help readers to understand the physics behind the operation of 10BF3 counters and understand the functionality.
REFERENCES
[1] G F Knoll, Radiation Detection & Measurement, John Wiley & Sons
[2] I.L. Fowler and P.R.Tunnicliffe, Boron Trifluoride Proportional Counters; The review of Scientific Instruments; Volume-21, Number 8
[3] I.L. Fowler, Very large Boron Trifluoride Proportional Counters; The review of Scientific Instruments; Volume-34, Number 7, July 1963
[4] Shraddha S.Desai, Mala N. Rao; Effect of Temperature on performance of Boron Trifluoride Neutron Proportional Counters; Radiation Measurements, 144(2021)106593
[5] SS Kapoor, V S Ramamurthy, Nuclear radiation Detectors, New Age International Publishers
