|Year : 2010 | Volume
| Issue : 3 | Page : 94-99
Bubble detector for neutron and gamma discrimination
Head Radiation Dosimetry and Processing, Defence Laboratory, Jodhpur, India
|Date of Web Publication||22-Oct-2011|
S G Vaijapurkar
Head Radiation Dosimetry and Processing, Defence Laboratory, Jodhpur
Source of Support: None, Conflict of Interest: None
Depending on neutron energy, a number of personnel neutron dosimeters such as NTA film, Albedo TLD dosimeter and Nuclear Track Detectors (CR39) have been developed and reported so far. At present, CR-39 is in use as personnel neutron dosimeters for neutron monitoring of occupational workers in most of the nuclear installations due to non-availability high sensitive real time gamma insensitive personnel neutron dosimeters as per ICRP recommendations. The Superheated Emulsion Detector or bubble detector assess the magnitude of detrimental effects on health of person exposed to neutron radiation in terms of absorbed dose or equivalent dose even in presence of high gamma radiations flux in real time unlike nuclear track detectors. Bubble Technology Industries (BTI), Canada and Aphel Enterprises, USA have commercialized these type of bubble detectors for neutron and gamma measurements. The paper will elaborate the basic principle, special features, mechanism of bubble formation, national and international status including it's applications in accelerator Physics, Medical sciences, nuclear submarine and neutron /gamma dose measurements in nuclear reactors. These detectors may be most popular in due course among health physicists due to it's reusability and precise measurements for neutron measurements in reactor environment, pulse neutron measurements and personal neutron dosimetery.
Keywords: Bubble detectors, neutron and gamma discrimination, mechanism, applications
|How to cite this article:|
Vaijapurkar S G. Bubble detector for neutron and gamma discrimination. Radiat Prot Environ 2010;33:94-9
| 1. Introduction|| |
Nuclear radiations are hazardous for the human health. It produces a significant biological damage due to the absorption of the radiation energy. Among the gamma and the neutrons, the neutrons produce more biological damage as compared to the same amount of the gamma energy absorbed in the tissues. Therefore the measurement of the neutron at low doses is very important particularly from the point of view of it's higher relative biological effectiveness. One of the technical limitations of the Nuclear Track Detector identified in the early 1980s was the relatively high energy threshold for fast neutrons which was approximately 1 MeV. This made the Nuclear Track Detector insensitive to a large faction of the lower energy neutrons. The identification of Columbia resin CR39 or CR-39 led to some improvements in the lowering of the energy threshold to about 100keV. The lower limit of neutron detection is 200μSv. But the modifications in earlier concept of quality factor for neutrons in ICRP-60 recommendations the detector detection threshold for neutron equivalent dose must be decreased by factor of 5 to 10 so as to meet the requirement of detection threshold about 0.1 mSv per month for personnel neutron dosimeters (Portal and Dietz, 1992).
Since the work of Glasser in 1952 on the bubble chamber, many studies have been performed on the response of superheated liquid to ionizing radiation (Glasser, 1952). The use of superheated emulsion detectors was first investigated by Apfel in 1979, and later patented as Superheated Drop Detector (Apfel, 1979). In this study superheated droplets of CCl 2 F 2 were suspended in a highly viscous material and nucleation of these droplets was observed due to neutron exposure. Ing and Birnboim in 1984 have proposed the use of polymers as a host medium and successfully made reusable detectors (Ing and Birnboim, 1984). This novel technology is also protected under worldwide patent owned by AECL, Canada. A universally accepted name for the above two type of detectors is Superheated Emulsion based detector. The above detectors are also known as superheated Emulsion detector by health physics community.
Since then, researcher worldwide i.e. USSR, China etc. have reported various types of detectors based on this principle (Schulze, Rosenstock and Kronhoz, 1992 & Guo Shi-Lun, 2006). In India, Bose Institute Calcutta, IGCAR, Kalpakkam and Defence Laboratory, Jodhpur is also engaged in such type of development since long. Defence Laboratory, Jodhpur has succeeded to develop a reusable type bubble dosimeter device (Vaijapurkar and Paturkar, 1995). The improvement of detector device design technology at Defence Laboratory, Jodhpur is now comparable with the commercially available detectors developed by Bubble Technology Industries, Canada. The substitutes of CFC compounds, which are environmental friendly, have also been identified (Vaijapurkar and Senwar, 2003). Defence Laboratory, Jodhpur has also developed gamma bubble detectors. The developed detectors have been tested at Radiation Standardization Section, Bhabha Atomic Research Centre, Mumbai and Raja Ramanna Centre for Advanced Technology, Indore. The test reports were found satisfactory for their use in neutron and mixed field (neutron and gamma) monitoring (Vaijapurkar and Senwar, 2004; Vaijapurkar and Senwar., 2008).
Due to it's quick visual response, high sensitivity and zero sensitivity to gamma radiation for superheated Emulsion based neutron detector, the superheated Emulsion based detectors have wide applications in Nuclear Research laboratories, Nuclear power plants, Medical sciences, and in Defence Establishments.
| 2. Basic Principle|| |
Liquid raised to a temperature higher than it's boiling point without vaporization is known as superheated liquid. The superheated state of liquid is a meta-stable state; it changes it's liquid state and undergoes vaporization/gasification with deposition of very small amount energy (trigger energy), such as the energy deposition caused by an incident neutron. The droplets of superheated liquid suspended in a suitable host medium are used for measurement of neutrons and gamma radiations
The counting of number of nucleation may be carried out by optical means or with piezoelectric sensor having enough sensitivity to detect the small acoustic disturbance caused during bubble formation. The number of nucleation caused by neutron interaction is related to equivalent dose.
2.1 Mechanism of bubble formation
When a neutron with energy E n interacts with a nucleus of atomic weight A, the maximum energy that this nucleus can receive from the neutron through head on collisions is given by,
The nucleus receiving this energy knock out the orbital electrons of the surrounding atoms and shuttles through the liquid depositing energy until the electron collision charge capture brings it to rest. One can explain qualitatively the mechanism of vapor bubble nucleation using Seitz's thermal spike model (Seitz, 1958).The critical radius i.e. R c is calculated for spherical bubble
Where, γ(T) is the surface tension of liquid at temperature T.
P v (T) is the vapor pressure of the superheated liquid at temperature T, and P 0 is the pressure of surrounding medium. The minimum possible energy to nucleate a bubble can be estimated (Lo et al, 1988).
The effective neutron detection threshold (MeV) or minimum possible energy to nucleate a bubble can also be expressed in terms of degree of superheat. The degree of superheat is defined as
T b is boiling or saturation temperature of the droplets at temperature T. Seeking a unified parameterization of experimental data; a new non-dimensional quantity reduced superheat is proposed which is defined as
S is called as reduced superheat. It was found that an exponential trend correlates effective thresholds and reduced superheat as shown in semi-logarithmic plot [Figure 1]. These correlations have general validity for halocarbons as it covers fluorocarbons, chlorofluorocarbons and halogenated hydrocarbons. The photon sensitization occurs for S ≥0.51, i.e. at the mid point between boiling and critical temperatures of the emulsions (d'Errico, 1999) [Figure 2].
|Figure 2: Effective neutron thresholds of superheated Emulsions of R-12, R-142B, C-318 and R-114 plotted against reduced superheat16|
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| 3. Thermal Neutron Response|| |
The thermal neutron response of CCl 2 F 2 based superheated emulsion detectors can be explained by
The chlorine ion interaction with incident neutron is responsible to cause the bubble nucleation, when Freon-12 is used as superheated liquid.
| 4. Superheated Liquids for Neutron and Gamma|| |
The commonly used low boiling point refrigerants for superheated Emulsion based neutron and gamma detectors are CCl 2 F 2 , C 2 Cl 2 F 4 , C 2 H 3 ClF 2 , C 4 H 10, C 4 F 8, etc. The physical properties of these refrigerants at 25°C are shown in [Table 1]. In addition to the liquids mentioned above, mixtures in various proportions of two refrigerants, HCFC, HFC and Fluorocarbon compounds can also be used to develop multiple threshold neutron and gamma detectors. Three such compounds i.e, CF 3 CH 2 F, C 2 HClF 4 and C 3 F 6 have been reported. CF 3 CH 2 F and C 2 HClF 4 based detectors are sensitive to neutron and can be alternative droplet materials which are environmental friendly. C 3 F 6 compound based superheated emulsion detector is sensitive to neutron and gamma radiation. These refrigerants may also be useful to develop a wide range neutron spectrometer.
|Table 1: Physical properties of Superheated Emulsion Detector liquids at 25°C|
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| 5. Superheated Emulsion Based Detector|| |
The sensor material dispersed in the polycarbonate tube is 4cm 3 of firm transparent elastic polymer medium suspended with liquid drops of refrigerants. When the pressure on the detector matrix is released by unscrewing the top of the detector, the liquid droplets become superheated. These superheated droplets vaporize when they interact with neutrons. The numbers of bubbles are proportional to equivalent dose. The bubbles are fixed in elastic medium and can be subsequently counted visually or with the help of macro lens. Re-compressing the detector material transforms the bubble back in to droplet and detector can be reused (Vaijapurkar and Senwar, 1995). [Figure 3]
| 6. Applications|| |
6.1 Neutron dosimetry
Many groups worldwide are using bubble detectors for routine monitoring of personal neutron doses (Vanhavere et al, 2007) . The monitoring is extremely important streaming through nuclear reactor, nuclear submarine or neutron camera or pulse neutron accelerator where neutrons fields are high and localized. Bubble Technology Industries improvised the earlier bubble detector i.e.BD-100 with addition of temperature compensation by modifying the design of device. The temperature compensated Bubble Detector is useful for personal neutron dosimetry and named as BD-PND. The above BD-PND has been tested at Defence Laboratory, Jodhpur in terms of energy dependence, temperature dependence, repeated use, reliability, reproducibility, dynamic range etc. (Vaijapurkar and Senwar, 2000). These detectors have extremely high neutron sensitivity (at least an order of magnitude more sensitive than any other passive device). The detectors have isotropic response, immediate visual dose record and completely insensitive to gamma radiations. The earlier technical limitations of bubble detector were resolved by introduction of temperature compensation by introduction a special proprietary material on the top layer of suspended polymer medium. A new bubble detector has also been introduced for thermal neutron monitoring using 6 Li compound dispersed throughout the polymer medium (Ing et al., 1997).
Active electronic device developed by Framework scientific LLC, USA is well suited for area monitoring applications around high energy accelerators (Taylor et al, 2006). A novel instrument (RAISA) based on bubble detectors have also been developed to meet the needs for radiation monitoring for long term space mission. RAISA uses two bubble detectors, but at any time, only one of the detectors is used as the radiation sensor. When a maximum number of bubbles have accumulated in this detector, a microprocessor turns on the second detector and compresses the first detector. By switching back and forth RAISA can operate continuously for many years. RAISA can obviously be used for radiation monitoring in any nuclear installations (Ing et al., 1999). RAISA has an enormous dose and dose rate range and counting of bubbles is done electronically as the bubbles are formed. Defence Laboratory, Jodhpur has also developed active electronic devices based on superheated emulsion detector. A continuous area monitoring electronic acoustic device has also been developed for monitoring of neutron levels in vicinity of nuclear installations (Meena et al. 2004). New acoustic instrumentation for superheated drop detector has also been developed using high quality electret microphone and adaptive electronics (Felizardo et al., 2008).
6.2 Neutron spectrometry
By varying the formulation, bubble detectors having different neutron energy thresholds have been made by BTI, Canada (Buckner et al. 1994). The developed bubble detectors are having neutron energy thresholds of 10, 100, 600, 1000, 2500 and 10,000 keV. The bubble detector spectrometer has been used by various groups for characterization of neutron fields in nuclear utilities, handling of fissile materials and accelerator laboratories (see for instance, Rosenstock et al. 1995). The developed bubble detectors are completely insensitive to gamma radiations. Defence Laboratory, Jodhpur has also succeeded to prepare bubble detectors having threshold 1.2, 100, 500 and 5000 keV (Vaijapurkar et al. 2004). The DLJ developed bubble detectors have also been tested at RRCAT, Indore for monitoring streaming through shielding in INDUS-1 (Verma, et al, 2009).
6.3 Space, Aviation crew, high energy accelerator and heavy ion research
The interesting applications of the bubble detector spectroscopy are measurement of the neutron spectrum in space. The bubble spectrum using above detectors have been measured in the Russian satellites BIO-COSMOS (BION) and Russian space station MIR (Ing. et al., 1997). Despite the different orbital parameters and times, the neutron levels were quite consistent in the order of 100 μSv per day. There appeared to beam large number of neutron above 10MeV which were originally unexpected by some theorists and neutrons around a few MeV were expected because they were the "evaporation neutron" (Ing, 1997). High energy neutrons were produced by high energy protons.
Bubble detectors are also popular in the monitoring commercial and military aviation crews (Green et al. 2006). The survey conducted among commercial aircrew of continental North America and Caribbean flights showed annual exposure levels >1mSv. The bubble detector spectrometer is also popular among high energy accelerator from MeV to GeV range to estimate photo-neutron contributions in intense photon field. Superheated emulsions are attractive for the search of weakly interacting massive particles. Recently two international multi-collaborative experiments have developed arrays of superheated emulsions modules that are deployed in underground laboratories with extremely low muon and neutron background levels. These detectors present energy thresholds <10keV for nuclear recoils along with virtually complete beta and gamma background discrimination: therefore, they are expected to permit a significant improvement in cold dark matter sensitivity (d'Errico, 2006).
6.4 Gamma dosimetry
Gamma sensitive bubble detectors (BD-GAMMA) by BTI, Canada and gamma sensitive superheated drop detectors by Aphel Enterprises, USA have also been developed. BTI, Canada has successfully applied temperature compensation to the gamma detector similar to that done for the neutron detector. These detectors may be in use for 3-D dosimetry of photon-emitting brachytherapy sources (d' Errico F., 2006). Another interesting application of bubble detector pertains to the detection of dark matter (Azuelos, et al. 2006). Defence Laboratory, Jodhpur has also developed Gamma bubble detector which has been tested and calibrated at RSS, RSSD, BARC, Mumbai(Vaijapurkar et al,2008)
6.5 Defence applications
The detector has wide applications in Defence particularly leakage neutron measurements in nuclear submarine, neutron shielding efficacy of armored vehicles and neutron measurements during process of transportation of fissile materials. These sensors may be one of fastest indicator of presence of neutron Initial Nuclear Radiation zone after the Atom bomb explosion. The presence of fast neutrons having average energy 2.0MeV is a very good indicator in INR zone for fission based nuclear explosion. Whereas for fusion based explosion based on D-T and T-T reaction the threshold based superheated Emulsion detector can play vital role to confirm the type of fusion device. For discrimination of fission and fusion type nuclear weapon devices in any nuclear emergency such detector may play vital role.
The Gamma detector and neutron detectors can be used by educators, police, Nuclear Emergency response team particularly in nuclear fall-out or dispersion of radioactive materials in case of dirty bomb explosion. The radiation surveyors and radiation workers if any one of them needs to know instantly presence of gamma radiation. These sensor devices are user friendly and more suitable as compared to any other existing sensors. Any illiterate person can also be able to know the presence of gamma and neutron radiations using such detector devices. Due to extremely high sensitivity and fast response time of the detector even very low radiation levels can also be detected instantly. The sensor does not require any power and light in weight.
| 7. Conclusion|| |
The Superheated Emulsion Detectors for neutron and gamma radiations, in terms of temperature range of operation, neutron response, stability, real time response is matured technology. One of the most impressive aspects of these detectors is that their characteristics can be varied easily to match specific applications e.g., changing of composition of superheated emulsion materials. One can make an extremely sensitive detector capable of detecting well below 1mrem.
| 8. Acknowledgements|| |
The authors are grateful to Dr. Narendra Kumar, Director Defence Laboratory, Jodhpur for their support to pursue the work. The authors are also grateful to all the team members for their technical support.
| 9. References|| |
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[Figure 1], [Figure 2], [Figure 3]