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Development of a high-range gamma ion chamber for space critical applications
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 584 (2008) 374–376 www.elsevier.com/locate/nima
Development of a high-range gamma ion chamber for space critical applications Mary Alex, P.K. Mukhopadhyay Electronics Division, BARC, Mumbai, Maharashtra 400 085, India Received 26 June 2007; received in revised form 24 October 2007; accepted 24 October 2007 Available online 4 November 2007
Abstract The development of a gamma ionisation chamber as an alternative to Geiger Mueller (GM) counters for space critical applications has been described. The chamber has an overall diameter and length of 27 and 53 mm, respectively. It has a gamma sensitivity of 10 pA/R/h and its response is linear within 1% from 80 to 9600 R/h as compared to the GM counters, which have a non-linear response (the percentage loss of counts is 70% at 1000 R/h) at high gamma fields. The chamber has a uniform energy response from 660 keV to 1.25 MeV. The special technique of nickel-plating used to convert one of the surfaces of an alumina insulator into an electrode has resulted in the indigenous development of a compact and mechanically robust ionisation chamber. r 2007 Elsevier B.V. All rights reserved. PACs: 29.40. n; 29.40.Cs; 07.85. m Keywords: Radiation detectors; Ionisation chambers; Gamma-ray instruments
1. Introduction High-intensity gamma radiation is encountered in many locations in nuclear facilities. These include particle accelerators such as electron linear accelerators used in radiotherapy, some areas in nuclear reactors, interior portion of ‘hot cell’ facilities in which highly radioactive materials are manipulated by remote-handling devices, etc. In some situations the radiation intensity rises from relatively low levels (o1 R/h) to 10,000 R/h or higher. In isotope laboratories where high levels of gamma activity is brought in and sealed with the help of remotehandling equipment, gamma exposure levels of the order of 200 R/min are routinely encountered. Special devices are used for the on-line detection of this high-intensity radiation since in most of these applications the space available for locating the detector is limited. For instance the device used for the measurement of high gamma activity near the fuelling machine in a power reactor should be small enough to be placed adjacent to a 1-in. duct line. Corresponding author. Tel.: +91 22 25595060.
E-mail address: [email protected] (M. Alex). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.10.039
Small devices such as Farmer–Baldwin ionisation chambers typically having sensitive volumes of the order of 0.6 cm3 are used near radiotherapy machines [1]. Miniature ionisation chambers and gamma thermometers are used for monitoring radiation in reactors. Geiger Mueller (GM) counters are used for monitoring high gamma activity near the fuelling machine in a power reactor [2]. While GM counters, are robust, have small size, high sensitivity and simple circuitry, they need to be replaced often since their operational life is short. Their operation is usually limited to a total count life of about 5 1010 counts. Other problems with GM counters include the nonlinearity in response in high fields. The percentage loss of counts of Type ZP1300 counter made by M/s Philips for example is 70% at 1000 R/h [2]. Small gas-filled ionisation chambers have the advantage of replacing GM counters in these space critical applications on account of the various advantages which include simplicity, ruggedness, long operational life, wide dynamic range, long-term stability in performance and resistant to radiation damage. The paper describes the indigenous development of a gamma ionisation chamber with dimensions comparable to
ARTICLE IN PRESS M. Alex, P.K. Mukhopadhyay / Nuclear Instruments and Methods in Physics Research A 584 (2008) 374–376
that of a GM counter and which can replace GM counters in certain applications for measuring gamma fields from 1 R/h to 104 R/h.
375
there is no possibility for cantilever type of vibrations. The specifications of the ionisation chamber are given in Table. 1. The photograph of the chamber is shown in Fig. 2.
2. Mechanical construction of the ionisation chamber 3. Performance tests The schematic diagram of the ionisation chamber is shown in Fig. 1. It consists of a pair of electrodes acting as the high voltage (HV) and signal electrodes. The signal electrode is the conductor of ceramic to metal (C/M) feedthru, which is also used as the gas-filling tube. The HV electrode is a highpurity alumina ceramic tube of 15 mmID 20 mmOD 36 mm length made conducting by nickel-plating on the inner diameter of the ceramic tube. The nickel-plating was carried out by Materials Processing Division of BARC [3,4]. The ceramic tube is rested at one end on a stainless steel (SS) end plate while the other end of the tube is in contact with SS disc, which is connected to C/M feedthru. The two electrodes are housed in a cylindrical SS outer housing, which is filled with argon gas at 13 atm pressure. In this design, since the outer surface of the ceramic tube is in contact with the inner surface of the detector housing
To study the performance of the ionisation chamber, it was tested at the calibrated cobalt-60 Teletherapy machine in the Radiation Standards Section, BARC in gamma fields ranging from 80 to 3000 R/h. Since the chamber is designed to measure exposures of the order of 104 R/h, tests at higher dose rates up to 104 R/h were carried out at the 60Co irradiation facility at Food Technology Division, BARC. The gamma exposure rate at the chamber location was estimated with the help of a calibrated in-core gamma detector used in reactor instrumentation. The sensitivity of the standard detector was known with an accuracy of 75%. The HV electrode of the ionisation chamber was connected to the power supply and the signal electrode was connected to the Keithley614 Electrometer amplifier. The chamber response was measured by varying the voltage. The voltage–current characteristics of the ionisation chamber at various gamma exposure rates showed that a voltage of 1 kV is needed to produce saturation current at 9600 R/h (Fig. 3). The response of the chamber to various gamma fields was also studied in the range 80–9600 R/h. Fig. 4 shows the linearity of the response and it was observed to be within 1%. The energy response of the ionisation chamber to three sources (60Co (1.25 MeV), 137 Cs (0.66 MeV), and 226Ra (0.8 MeV)) was measured and found to be uniform from 660 keV to 1.25 MeV (Table 2). 3.1. Experiments at Apsara reactor
Table 1 Specifications of the ionisation chamber Overall length (mm) Overall dia (mm) Electrode gap (mm) Sensitive volume (cm3) Gas fill and pressure Gamma sensitivity (R/h)
53 27 6.3 5.4 Ar at 13 atm 10 pA
The ionisation chamber was used to estimate the gamma field at the Apsara Thermal column. The Thermal Column of Apsara reactor is an assembly of graphite blocks provided at one end of the reactor pool for the irradiation 9600R/h 400R/h 80R/h
1.00E-06
2900R/h 150R/h
1.00E-07 Current (Amp)
Fig. 1. Schematic diagram of ionisation chamber assembly.
1.00E-08
1.00E-09
1.00E-10 0
Fig. 2. High-range gamma ionisation chamber.
200
400
600 Voltage
800
1000
1200
Fig. 3. V/I characteristics of the ionisation chamber at various gamma exposures.
ARTICLE IN PRESS M. Alex, P.K. Mukhopadhyay / Nuclear Instruments and Methods in Physics Research A 584 (2008) 374–376
376
1.00E-08
1.00E-06
Current (Amp)
Current (A)
1.00E-07
1.00E-08
1.00E-09
1.00E-10
1.00E-09
1.00E-10 1.00E+01
1.00E+03 1.00E+02 Gamma field (R/h)
1.00E+04
Fig. 4. Linearity of chamber response at various gamma fields.
Table 2 Gamma sensitivity of the ionisation chamber for different gamma sources Gamma source
Gamma energy (MeV)
Gamma sensitivity (pA/R/h)
60
1.25 0.8 0.66
10 10 10
Co 226 Ra 137 Cs
and experiments with highly thermalised neutrons. It is used for the calibration of neutron detectors for reactor applications. However, in the calibration of neutron detectors, since the ionisation chamber responds to both neutron and gamma radiation, to evaluate the precise gamma response of the neutron detector, it is necessary to estimate the gamma field. The gamma chamber was used to evaluate the gamma field at the detector location. A suitable fixture was developed to guide the detector into the 70-in. long hole and to align the detector along the axis of the Thermal Column. Before installing the chamber in the Thermal Column, the leakage current of the chamber was measured as 1.7 pA. After installation the chamber current was measured at reactor shut down as well as at various power levels (Fig. 5). From the current values and the gamma sensitivity of the field the gamma field at the various reactor power levels was measured. The gamma field varied from 352 to o1 R/h at reactor power levels ranging from 300 kW to reactor shut down. Thus the
1.00E-11 1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02 1.00E+03 Reactor Power (kW) Fig. 5. The signal measured at various reactor power levels at Apsara Thermal Column.
experiment has shown that the gamma field at various power levels is significantly lower and does not contribute to the total current in the neutron detector. 4. Conclusions The indigenous development of the ionisation chamber was carried out as a replacement for the GM counters for various applications in BARC. The design and fabrication of the detector with special techniques has resulted in a product, which is far more efficient and has many superior features. The chamber response is linear within 71% from 80 R/h to 9600 R/h as compared to the GM counters, which have a non-linear response (the percentage loss of counts is 70% at 1000 R/h) at high gamma fields. The technique of nickel-plating used to convert one of the surfaces of an alumina insulator into an electrode has resulted in the indigenous development of a compact and mechanically robust ionisation chamber. References [1] J.W. Boag, J. Currant, Br. J. Radiol. 53 (1980) 471. [2] Geiger-Mueller Tubes, Philips Data Handbook, Book T6, 1986. [3] S.N. Athavale, P.B. Shrivastva, M.K. Totlani, Direct bonding of metals to ceramics: copper and nickel to sintered alumina, in: Proceedings of the BRNS Symposium on ‘Sintering and Sintered Products’, BARC, Bombay, 1979, 677pp. [4] M.K. Totlani, S.N. Athavale, N.C. Soni Ram Prasad, C.K. Gupta, Bull. Ind. Vac. Soc. 23 (3) (1992) 21.
Nuclear Instruments and Methods in Physics Research A 584 (2008) 374–376 www.elsevier.com/locate/nima
Development of a high-range gamma ion chamber for space critical applications Mary Alex, P.K. Mukhopadhyay Electronics Division, BARC, Mumbai, Maharashtra 400 085, India Received 26 June 2007; received in revised form 24 October 2007; accepted 24 October 2007 Available online 4 November 2007
Abstract The development of a gamma ionisation chamber as an alternative to Geiger Mueller (GM) counters for space critical applications has been described. The chamber has an overall diameter and length of 27 and 53 mm, respectively. It has a gamma sensitivity of 10 pA/R/h and its response is linear within 1% from 80 to 9600 R/h as compared to the GM counters, which have a non-linear response (the percentage loss of counts is 70% at 1000 R/h) at high gamma fields. The chamber has a uniform energy response from 660 keV to 1.25 MeV. The special technique of nickel-plating used to convert one of the surfaces of an alumina insulator into an electrode has resulted in the indigenous development of a compact and mechanically robust ionisation chamber. r 2007 Elsevier B.V. All rights reserved. PACs: 29.40. n; 29.40.Cs; 07.85. m Keywords: Radiation detectors; Ionisation chambers; Gamma-ray instruments
1. Introduction High-intensity gamma radiation is encountered in many locations in nuclear facilities. These include particle accelerators such as electron linear accelerators used in radiotherapy, some areas in nuclear reactors, interior portion of ‘hot cell’ facilities in which highly radioactive materials are manipulated by remote-handling devices, etc. In some situations the radiation intensity rises from relatively low levels (o1 R/h) to 10,000 R/h or higher. In isotope laboratories where high levels of gamma activity is brought in and sealed with the help of remotehandling equipment, gamma exposure levels of the order of 200 R/min are routinely encountered. Special devices are used for the on-line detection of this high-intensity radiation since in most of these applications the space available for locating the detector is limited. For instance the device used for the measurement of high gamma activity near the fuelling machine in a power reactor should be small enough to be placed adjacent to a 1-in. duct line. Corresponding author. Tel.: +91 22 25595060.
E-mail address: [email protected] (M. Alex). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.10.039
Small devices such as Farmer–Baldwin ionisation chambers typically having sensitive volumes of the order of 0.6 cm3 are used near radiotherapy machines [1]. Miniature ionisation chambers and gamma thermometers are used for monitoring radiation in reactors. Geiger Mueller (GM) counters are used for monitoring high gamma activity near the fuelling machine in a power reactor [2]. While GM counters, are robust, have small size, high sensitivity and simple circuitry, they need to be replaced often since their operational life is short. Their operation is usually limited to a total count life of about 5 1010 counts. Other problems with GM counters include the nonlinearity in response in high fields. The percentage loss of counts of Type ZP1300 counter made by M/s Philips for example is 70% at 1000 R/h [2]. Small gas-filled ionisation chambers have the advantage of replacing GM counters in these space critical applications on account of the various advantages which include simplicity, ruggedness, long operational life, wide dynamic range, long-term stability in performance and resistant to radiation damage. The paper describes the indigenous development of a gamma ionisation chamber with dimensions comparable to
ARTICLE IN PRESS M. Alex, P.K. Mukhopadhyay / Nuclear Instruments and Methods in Physics Research A 584 (2008) 374–376
that of a GM counter and which can replace GM counters in certain applications for measuring gamma fields from 1 R/h to 104 R/h.
375
there is no possibility for cantilever type of vibrations. The specifications of the ionisation chamber are given in Table. 1. The photograph of the chamber is shown in Fig. 2.
2. Mechanical construction of the ionisation chamber 3. Performance tests The schematic diagram of the ionisation chamber is shown in Fig. 1. It consists of a pair of electrodes acting as the high voltage (HV) and signal electrodes. The signal electrode is the conductor of ceramic to metal (C/M) feedthru, which is also used as the gas-filling tube. The HV electrode is a highpurity alumina ceramic tube of 15 mmID 20 mmOD 36 mm length made conducting by nickel-plating on the inner diameter of the ceramic tube. The nickel-plating was carried out by Materials Processing Division of BARC [3,4]. The ceramic tube is rested at one end on a stainless steel (SS) end plate while the other end of the tube is in contact with SS disc, which is connected to C/M feedthru. The two electrodes are housed in a cylindrical SS outer housing, which is filled with argon gas at 13 atm pressure. In this design, since the outer surface of the ceramic tube is in contact with the inner surface of the detector housing
To study the performance of the ionisation chamber, it was tested at the calibrated cobalt-60 Teletherapy machine in the Radiation Standards Section, BARC in gamma fields ranging from 80 to 3000 R/h. Since the chamber is designed to measure exposures of the order of 104 R/h, tests at higher dose rates up to 104 R/h were carried out at the 60Co irradiation facility at Food Technology Division, BARC. The gamma exposure rate at the chamber location was estimated with the help of a calibrated in-core gamma detector used in reactor instrumentation. The sensitivity of the standard detector was known with an accuracy of 75%. The HV electrode of the ionisation chamber was connected to the power supply and the signal electrode was connected to the Keithley614 Electrometer amplifier. The chamber response was measured by varying the voltage. The voltage–current characteristics of the ionisation chamber at various gamma exposure rates showed that a voltage of 1 kV is needed to produce saturation current at 9600 R/h (Fig. 3). The response of the chamber to various gamma fields was also studied in the range 80–9600 R/h. Fig. 4 shows the linearity of the response and it was observed to be within 1%. The energy response of the ionisation chamber to three sources (60Co (1.25 MeV), 137 Cs (0.66 MeV), and 226Ra (0.8 MeV)) was measured and found to be uniform from 660 keV to 1.25 MeV (Table 2). 3.1. Experiments at Apsara reactor
Table 1 Specifications of the ionisation chamber Overall length (mm) Overall dia (mm) Electrode gap (mm) Sensitive volume (cm3) Gas fill and pressure Gamma sensitivity (R/h)
53 27 6.3 5.4 Ar at 13 atm 10 pA
The ionisation chamber was used to estimate the gamma field at the Apsara Thermal column. The Thermal Column of Apsara reactor is an assembly of graphite blocks provided at one end of the reactor pool for the irradiation 9600R/h 400R/h 80R/h
1.00E-06
2900R/h 150R/h
1.00E-07 Current (Amp)
Fig. 1. Schematic diagram of ionisation chamber assembly.
1.00E-08
1.00E-09
1.00E-10 0
Fig. 2. High-range gamma ionisation chamber.
200
400
600 Voltage
800
1000
1200
Fig. 3. V/I characteristics of the ionisation chamber at various gamma exposures.
ARTICLE IN PRESS M. Alex, P.K. Mukhopadhyay / Nuclear Instruments and Methods in Physics Research A 584 (2008) 374–376
376
1.00E-08
1.00E-06
Current (Amp)
Current (A)
1.00E-07
1.00E-08
1.00E-09
1.00E-10
1.00E-09
1.00E-10 1.00E+01
1.00E+03 1.00E+02 Gamma field (R/h)
1.00E+04
Fig. 4. Linearity of chamber response at various gamma fields.
Table 2 Gamma sensitivity of the ionisation chamber for different gamma sources Gamma source
Gamma energy (MeV)
Gamma sensitivity (pA/R/h)
60
1.25 0.8 0.66
10 10 10
Co 226 Ra 137 Cs
and experiments with highly thermalised neutrons. It is used for the calibration of neutron detectors for reactor applications. However, in the calibration of neutron detectors, since the ionisation chamber responds to both neutron and gamma radiation, to evaluate the precise gamma response of the neutron detector, it is necessary to estimate the gamma field. The gamma chamber was used to evaluate the gamma field at the detector location. A suitable fixture was developed to guide the detector into the 70-in. long hole and to align the detector along the axis of the Thermal Column. Before installing the chamber in the Thermal Column, the leakage current of the chamber was measured as 1.7 pA. After installation the chamber current was measured at reactor shut down as well as at various power levels (Fig. 5). From the current values and the gamma sensitivity of the field the gamma field at the various reactor power levels was measured. The gamma field varied from 352 to o1 R/h at reactor power levels ranging from 300 kW to reactor shut down. Thus the
1.00E-11 1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02 1.00E+03 Reactor Power (kW) Fig. 5. The signal measured at various reactor power levels at Apsara Thermal Column.
experiment has shown that the gamma field at various power levels is significantly lower and does not contribute to the total current in the neutron detector. 4. Conclusions The indigenous development of the ionisation chamber was carried out as a replacement for the GM counters for various applications in BARC. The design and fabrication of the detector with special techniques has resulted in a product, which is far more efficient and has many superior features. The chamber response is linear within 71% from 80 R/h to 9600 R/h as compared to the GM counters, which have a non-linear response (the percentage loss of counts is 70% at 1000 R/h) at high gamma fields. The technique of nickel-plating used to convert one of the surfaces of an alumina insulator into an electrode has resulted in the indigenous development of a compact and mechanically robust ionisation chamber. References [1] J.W. Boag, J. Currant, Br. J. Radiol. 53 (1980) 471. [2] Geiger-Mueller Tubes, Philips Data Handbook, Book T6, 1986. [3] S.N. Athavale, P.B. Shrivastva, M.K. Totlani, Direct bonding of metals to ceramics: copper and nickel to sintered alumina, in: Proceedings of the BRNS Symposium on ‘Sintering and Sintered Products’, BARC, Bombay, 1979, 677pp. [4] M.K. Totlani, S.N. Athavale, N.C. Soni Ram Prasad, C.K. Gupta, Bull. Ind. Vac. Soc. 23 (3) (1992) 21.