|Year : 2011 | Volume
| Issue : 1 | Page : 69-73
Automatic readout system for superheated emulsion based neutron detector
JP Meena1, A Parihar1, SG Vaijapurkar1, Anand Mohan2
1 Defence Laboratory, Ratanada Place, Jodhpur, India
2 Department of Electronics Engineering, IT- BHU, Varanasi, India
|Date of Web Publication||17-Mar-2012|
J P Meena
Defence Laboratory, Ratanada Place, Jodhpur
Source of Support: None, Conflict of Interest: None
The paper presents a microcontroller based automatic reader system for neutron measurement using indigenously developed superheated emulsion detector. The system is designed for real time counting of bubbles formed in superheated emulsion detector. A piezoelectric transducer is used for sensing bubble acoustic during the nucleation. The front end of system is mainly consisting of specially designed signal conditioning unit, piezoelectric transducer, an amplifier, a high-pass filter, a differentiator, a comparator and monostable multivibrator. The system is based on PIC 18F6520 microcontroller having large internal SRAM, 10-bit internal ADC, I 2 C interface, UART/USART modules. The paper also describes the design of following microcontroller peripheral units viz temperature monitoring, battery monitoring, LCD display, keypad and a serial communication. The reader system measures and displays neutron dose and dose rate, number of bubble and elapsed time. The developed system can be used for detecting very low neutron leakage in the accelerators, nuclear reactors and nuclear submarines. The important features of system are compact, light weight, cost effective and high neutron sensitivity. The prototype was tested and evaluated by exposing to 241 Am-Be neutron source and results have been reported.
Keywords: Micro-controller, piezoelectric transducer, superheated emulation, neutron detector, signal conditioning and processing; inter system programming
|How to cite this article:|
Meena J P, Parihar A, Vaijapurkar S G, Mohan A. Automatic readout system for superheated emulsion based neutron detector. Radiat Prot Environ 2011;34:69-73
|How to cite this URL:|
Meena J P, Parihar A, Vaijapurkar S G, Mohan A. Automatic readout system for superheated emulsion based neutron detector. Radiat Prot Environ [serial online] 2011 [cited 2020 Jun 2];34:69-73. Available from: http://www.rpe.org.in/text.asp?2011/34/1/69/93961
| 1. Introduction|| |
The superheated emulsion based detector invented by Apfel is based on bubble chamber principle (Glaser D.A., 1952, Apfel R.E., 1979). The drops of low boiling refrigerant are suspended in a host polymer matrix. When neutron interacts with these droplets, the nucleation (bubbles) occurs. The number of nucleation or bubbles can be correlated to neutron equivalent dose. The formation of bubble is accompanied with acoustic. M/s Apfel enterprise and M/s Bubble technology Industries has also been reported such type of neutron survey meter using acoustic technique (Apfel R.E., Roy S.C., 1983, Biro T et al, 1990).
Defence Laboratory, Jodhpur (DLJ) is actively engaged in developing superheated emulsion technology for neutron and gamma measurements and finally succeeded to establish this technology in the country using acoustic technique (Vaijapurkar S.G. et al, 1995, Vaijapurkar S.G. et al, 1997, Vaijapurkar S.G. et al, 2003, Vaijapurkar S.G. et al, 2008, Meena J.P. et al, 2008). [Figure 1] shows the photograph of DLJ developed superheated emulsion based neutron detector before and after the neutron exposure. After establishing the technology of superheated emulsion based gamma and neutron sensor, the laboratory has attempted to develop reader system indigenously using acoustic principle. The developed reader can be used as a neutron survey meter for detection and measurements of the neutron intensity in reactor and accelerator environment, medical cyclotron, neutron leakage in nuclear submarines and neutron flux measurements during blast of enhanced radiation weapons like suit case bomb or neutron bomb. The reader system finds extensive applications in research laboratory as a neutron area monitor for occupational workers.
The conventional Rem Counter used as a neutron area monitor are bulky, heavy and does not measure cumulative dose. Considering these disadvantages, the efforts have been made to develop a compact and light weight electronic instrument (a reader system) based on superheated emulsion detector. The developed reader system consists of high performance PIC 18F6520 microcontroller which has on chip large internal memory, ADC, serial communication and peripheral modules etc. The C code programme is written using MPLAB software development tool.
The paper discusses about the technical design details of the developed acoustic bubble reader system. The advanced electronic modules designed indigenously, fabricated and assembled. The developed proto type reader system has also been tested in neutron radiation environment and results are presented.
| 2. Sensor Device and Detection Technique|| |
The [Figure 2] shows the automatic reader system for counting the bubble. The superheated emulsion drop detector consists of polycarbonate tube of 4 cm 3 volume filled with sensor material and a pressure screw cap. The sensor material is liquid drops of refrigerants suspended in polymer matrix.
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 into bubble on interaction with neutrons. The number of bubbles is proportional to equivalent dose. The bubbles are fixed in elastic medium and can be subsequently counted optically or using acoustic technique. Optically it can be counted by manual method of visual counting or using image analysis.
The automatic bubble counting can also be carried out using imaging software or by acoustic technique.
The acoustic technique has an advantage that both dose and dose rate can be measured. The acoustic signal is generated when neutron energy is transferred to superheated droplet uniformly suspended in polymer matrix, forming a bubble. Re-compressing the detector materials transforms the bubble back in to droplet and detector can be reused. [Figure 3] shows the inside look of sensor and reader system.
| 3. System Description|| |
The block diagram of reader system is shown in [Figure 4]. It consists of PIC18F6520 micro-controller unit with I/O interfaces modules. The system has the following interfacing modules.
(i) Signal conditioning and anti coincidence circuit (ii) Temperature monitoring circuit (iii) Audio-visual signaling unit (iv) Real time clock (v) Display and keyboard interfaces (vi) Battery monitoring unit (vii) RS-232 Interface (viii) Power on reset circuit.
The acoustic generated in bubble formation is sensed by piezoelectric transducer (AP 48) and converted into electric signal. A signal conditioning circuit is developed to provide noise elimination, amplification and conversion of signal into TTL pulse. The signal conditioning circuit comprises Instrumentation amplifier, band pass filter, comparator and non-retrigable mono-stable multivibrator as shown in [Figure 5]. The band pass filter is designed to pass frequency band of 200 Hz to 120 KHz which further reduces the noise.
The output of the band pass filter is connected to comparator which eliminates noise below a set threshold 100mV. The output of the comparator is fed to mono shot whose time constant is set to 300μS such that ringing pulses are eliminated. [Figure 6] shows the output signal of the piezo-electric signal transducer which is generated during the one bubble event. The signal comprises of main pulse along with small pulses due to ringing effect. The signal has 30μs rise time and complete event takes 250μs time. The outputs of mono-stable multivibrators are TTL pulses applied to the microcontroller unit through anti coincidence circuit.
The anticoincidence is designed to avoid the false bubble counting due to external noise. (shock and vibrations) The anti coincidence circuit comprises of two piezoelectric transducers mounted on same plate, one in contact with superheated emulsion detector and other in contact with environment. The output of piezoelectric transducer is passed through signal conditioning units, and fed to XOR gate for coincidence removal of environmental noise. Finally the filtered signal will be fed to the PIC microcontroller for computing neutron dose and dose rate.
A temperature monitoring of the neutron sensor, is realized using temperature sensor (Pt100) which is in contact with bubble detector. The output of the temperature sensor is processed and fed to ADC of microcontroller for necessary corrections. The audio-visual alarm indication can be set at three different radiation levels. PWM of microcontroller generates different frequencies whenever neutron exposure exceeds the set dose level. Real Time Clock (RTC) DS 1307 is used to keep record of time and date while storing the readings of the reader systems. 2×4 key pad is used to interact with the system and connected to the interrupt pin of microcontroller. Liquid Crystal Display (LCD) is used to display information of the neutron dosimeter like bubble count, neutron dose, neutron dose-rate, temperature, elapsed time etc. The readily available 16×4 alphanumeric LCD Display Module from Oriole having built-in controller and driver was interfaced directly to the microcontroller.
| 4. Implementation|| |
The simulation of different circuits and their PCB designs were carried out using Virtual System Modeling (VSM) software along with PCB design from M/s Proteus, UK. The entire component has been mounted and soldered on the PCB. Functionality of the electronic modules have been tested and interfaced with the micro-controller unit.
The microcontroller firmware software was written in plain C code using development tool (MPLAB) that is provided by Microchip. The MPLAB package includes an assembler, a C compiler, a linker and series of debugging tools that allows easy development of applications. Provision in the software is made for user to initially select a type of detector depending on the type of refrigerant used. Provision is also made to select detector sensitivity depending on the quantity of refrigerants used (Lo Yuan-Chyuan and Apfel R E, 1988). To cover entire dose rate range from 0.01μSv/hr- 0.01Sv/hr., a provision is made in software to select from three types of detectors i.e. high sensitivity moderate sensitivity and low sensitivity. The non linearity in the sensitivity is corrected using interpolation technique.
| 5. Performance Evaluation|| |
Experiments were conducted for neutron response of superheated emulsion detector and the performance study of neutron reader system. The superheated emulsion detector device is inserted in the reader system such that it is always in contact with piezo electric transducer. The output of the transducers is connected through coaxial cable to the signal conditioning circuit of reader system. During the experiment the superheated emulsion detector device was irradiated using calibrated 1Ci Am-Be neutron source. The radiation source was procured from Amersham, U. K. in 1984 and has a half life of 470 years. The yield of the source is calibrated against Rem counter (Cardinal Health, Australia). The distance between the source and superheated emulsion detector device is varied for testing at different dose rates. To carry out neutron response study, superheated emulsion detector were exposed to 1Ci Am-Be neutron source at a dose rate from 17μSv/h to 115μSv/h and dose from 10μSv to 120μSv. The height of the neutron sensor and the neutron source is kept 1.5 meter above the surface to minimize the interference of surface scattering on the neutron response of the superheated emulsion detector.
| 6. Results and Discussions|| |
The performance study of microcontroller based reader system was carried out by exposing superheated emulsion detector to 1 Ci Am-Be neutron source. [Figure 7] shows comparison of acoustic and manual bubble counts. The straight line curve shows a linear relationship between the bubbles formed in superheated emulsion detector device and bubble displayed by reader system. The linearity persists up to 250 nos. of bubbles with in a measurement accuracy of ±20%. The neutron dose response studies of superheated emulsion detector using developed reader system has been carried out and results shows linear relationship (error ±20%) from 10μSv-120μSv for detector having sensitivity of 1bub/μSv as shown in [Figure 8]. Neutron dose rate response is shown in [Figure 9]. The curve shows linear relation ship from 17μSv/h to 115μSv/h. It indicates an acceptable correlation (error ±20%) between the expected neutron dose rate and observed neutron dose rate.
| 7. Conclusion|| |
The developed neutron reader system is light in weight and has high sensitivity as compared to conventional neutron survey meters. The system was tested with 241 Am-Be source and results confirms the suitability of developed electronic instrument for measurement of neutron dose and dose rate. The relationship between visual counting and acoustic counting of bubbles formed due to neutron exposure is within ±20%. The work is in progress to collect some more data using low and moderate sensitivity detectors. Further experiments will be conducted using other threshold energy neutron detectors. The development of neutron zone monitor with provision for automatic compression /decompression is in progress for continuous neutron monitoring in the restricted zone.
| 8. Acknowledgement|| |
We are thankful to Dr. Narendra Kumar, Director, Defence Laboratory, Jodhpur and Shri P. K. Bhatnagar, Head NRMA Division, Defence Laboratory, Jodhpur for their encouragement to carry out this work. The authors are also thankful to Shri K. R. Sanwer, technical officer 'C' for his support. We are also thankful to M/s ni2 Logic Pvt. Ltd., Pune for technical support.
| 9. References|| |
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- Vaijapurkar S.G., Senwar K.R, Hooda J.S. and Parihar A. (2008), The performance evaluation of gamma and Neutron sensitive superheated emulsion (bubble) detectors, Radiation Protection dosimetry, Vol.130, No.3, 285-290.
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]