Abstract
This paper presents the design and development of area radiation monitor for the measurement of gamma radiation in and around the research reactor. The gamma dose rate is displayed on a linear scaled panel meter and it covers dose range from 0– 100µ Sv/h for X1 ranges, 0-1mSv/h for X10 ranges and 0–lOmSv/h for X100 ranges respectively. The status of radiation is indicated by four radiation levels such as low level (green LED), medium level (yellow LED), high level (red LED) and over high level (red LED). The monitor also provide the background radiation indicator and 220V audible alarm for over high-level of radiation. Remote display unit is also included to objective the status of the radiation and is capable of measuring the intensity of radiation from• the remote place. The radiation level can be adjusted at any value within the fill-scale deflection. High voltage power supply has been developed for the purpose.
Introduction
Area monitor is very convenient to detect the radiation in a source room, in and around the nuclear reactors, hot cells, irradiators and other facilities handling radioactive materials or X-rays. The designed area radiation monitor comprises of the following units:
(i) Low voltage power supply
(ii) High voltage power supply
(iii) Detector circuit
(iv) Pulse shaping and remote range indicator circuit
(v) Meter and response time circuit
(vi) Radiation level indicator circuit
(vii) Background radiation detection circuit and
(viii) Remote display circuit.
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The specifications of the designed area radiation monitor [I] are given in Table 1.
Table 1. Specifications of the Area Radiation Monitor
Feature | Specification |
Mains voltage | 230 volts, 50Hz |
Detector | ZP 13 24 (energy compensated gamma detector) |
Detector cathode thickness | 32 – 40 mg/cm2 |
Detector material | Chrome iron |
Detector sensitive length | 27 mm, |
High voltage | 575 volts d.c. |
Measurement ranges | 0 -100µ Sv/h for X1, 0 -1mSv/h for X10 and O‑10 mSv/h for X100 ranges |
Control system | Full automatic electronic circuit control system by IC |
Display | Linear rate meter is used |
Status indicator | Low, medium, high and over high range LED indicator |
Alarm setting range | Alarm can be set at any value within the full‑scale deflection and it is range dependent. |
Alarm | Internal buzzers for background and high level radiation and external 220V alarm for over high |
Power consumption | I OVA (approximately) |
Indication | Linear rate meter (0-100µA f, s.d.). |
Principle of Operation
The simplified block diagram of the area radiation monitor is shown in Fig. 1. The high voltage power supply unit provides the biasing voltage (575 volts) to the G.M. (Geiger Muller) tube. The designed high voltage (575 volts) output is derived from a 6volts power supply. The common cathode detector circuit is used to collect nuclear radiation around the nuclear reactor. The detector pulses are feed to the pulse shaping circuit. The pulse shaping circuit is used to provide a uniform current pulse for each pulse input.
The designed pulse shaping circuit has two outputs; one of the outputs is connected to the input to the meter and time response circuit and the other output is connected to the input to the background radiation detection circuit. The meter and time response circuit is used to measure the intensity of radiation in µ Sv/h. The '+' terminal of the meter is connected to the input of the radiation level indicator circuit. This radiation level indicator circuit can indicates four radiation levels such as low level, medium level, high level and over high level. The background radiation detection circuit used to indicates the presence of back ground radiation. When the background radiation is present, a green LED emits light and simultaneously a buzzer gives audible click. The remote display circuit use to measure the intensity of radiation from the remote distance i.e. from the control room of the nuclear reactor.
Meter and Time Response Circuit
The meter and time response circuit is used to measure the intensity of radiation in µ Sv/h. It consists of R12, R17, R18, VR7, C8, D8, D9, IC6 (3140) and meter MI.
The output pin 6 of IC5 is connected to the noninverting input pin 3 of IC6. Since the meter is placed in parallel to the capacitor C8. Therefore, when the capacitor is charged it causing the meter deflection. The amount of deflection is proportional to amount of charge, which, in turn is proportional to the average current. Thus the meter reads the average current, which is proportional to the pulse rate and width [7].
Radiation Level Indicator
The radiation indicator circuit is used to indicate the radiation level. It consists of R19 – R38, VR8 – VR10, D10 – D13, LED 4 – LED7, Q2 – Q5 (BC337), IC7 – IC9 (339), buzzerl and relay. This indicator circuit indicates four radiation levels, such as low level, medium level, high level and over high level. The green LED (i.e. LED4) indicates low-level intensity of radiation. The yellow LED (i.e. LEDS) indicates medium level of radiation. The red LED (i.e. LED6) indicates high level of radiation and the other red LED (i.e. LED7) indicates over high level of radiation. When the radiation level less then low level set value, the green LED (LED4) indicates light; this indicates the presence of low level radiation. Similarly when the radiation level exists between the medium level set values, the yellow LED (LEDS) indicates light; this indicates the presence of medium level radiation.
When the radiation level exists between the high level set values, the red LED (LED6) indicates light and the buzzerl is activated. Similarly, when the radiation level goes to higher then the over high level set value, another red LED (LED7) indicates light and the 220V alarm circuit is activated.
Background Radiation Detection Circuit
The background radiation detection circuit is used for indicate the presence of background radiation. It consists of R39 – R43, C9 – C12, LED8, Q6, Q7 (BC337), ICIO (555), IC11 (555) and buzzer2 When the background radiation present it gives audible alarm and the LED8 gives light. The output pin 7 of IC5 is connected to the triggering input pin 2 of IC 10. The IC 10 is connected for monostable operation. The duration of high state can be calculated by the following equation .
TH = 1.1 x R39 x C9 (in seconds)
Meter and Time Response Circuit
The meter and time response circuit is used to measure the intensity of radiation in µ Sv/h. It consists of R12, R17, R18, VR7, C8, D8, D9, IC6 (3140) and meter MI.
The output pin 6 of IC5 is connected to the noninverting input pin 3 of IC6. Since the meter is placed in parallel to the capacitor C8. Therefore, when the capacitor is charged it causing the meter deflection. The amount of deflection is proportional to amount of charge, which, in turn is proportional to the average current. Thus the meter reads the average current, which is proportional to the pulse rate and width [7].
Radiation Level Indicator
The radiation indicator circuit is used to indicate the radiation level. It consists of R19 – R38, VR8 – VR10, D10 – D13, LED 4 – LED7, Q2 – Q5 (BC337), IC7 – IC9 (339), buzzerl and relay. This indicator circuit indicates four radiation levels, such as low level, medium level, high level and over high level. The green LED (i.e. LED4) indicates low-level intensity of radiation. The yellow LED (i.e. LEDS) indicates medium level of radiation. The red LED (i.e. LED6) indicates high level of radiation and the other red LED (i.e. LED7) indicates over high level of radiation. When the radiation level less then low level set value, the green LED (LED4) indicates light; this indicates the presence of low level radiation. Similarly when the radiation level exists between the medium level set values, the yellow LED (LEDS) indicates light; this indicates the presence of medium level radiation.
When the radiation level exists between the high level set values, the red LED (LED6) indicates light and the buzzerl is activated. Similarly, when the radiation level goes to higher then the over high level set value, another red LED (LED7) indicates light and the 220V alarm circuit is activated.
Background Radiation Detection Circuit
The background radiation detection circuit is used for indicate the presence of background radiation. It consists of R39 – R43, C9 – C12, LED8, Q6, Q7 (BC337), ICIO (555), IC11 (555) and buzzer2 When the background radiation present it gives audible alarm and the LED8 gives light. The output pin 7 of IC5 is connected to the triggering input pin 2 of IC 10. The IC 10 is connected for monostable operation. The duration of high state can be calculated by the following equation .
TH = 1.1 x R39 x C9 (in seconds) (3)
Meter and Time Response Circuit
The meter and time response circuit is used to measure the intensity of radiation in µ Sv/h. It consists of R12, R17, R18, VR7, C8, D8, D9, IC6 (3140) and meter MI.
The output pin 6 of IC5 is connected to the noninverting input pin 3 of IC6. Since the meter is placed in parallel to the capacitor C8. Therefore, when the capacitor is charged it causing the meter deflection. The amount of deflection is proportional to amount of charge, which, in turn is proportional to the average current. Thus the meter reads the average current, which is proportional to the pulse rate and width [7].
Radiation Level Indicator
The radiation indicator circuit is used to indicate the radiation level. It consists of R19 – R38, VR8 – VR10, D10 – D13, LED 4 – LED7, Q2 – Q5 (BC337), IC7 – IC9 (339), buzzerl and relay. This indicator circuit indicates four radiation levels, such as low level, medium level, high level and over high level. The green LED (i.e. LED4) indicates low-level intensity of radiation. The yellow LED (i.e. LEDS) indicates medium level of radiation. The red LED (i.e. LED6) indicates high level of radiation and the other red LED (i.e. LED7) indicates over high level of radiation. When the radiation level less then low level set value, the green LED (LED4) indicates light; this indicates the presence of low level radiation. Similarly when the radiation level exists between the medium level set values, the yellow LED (LEDS) indicates light; this indicates the presence of medium level radiation.
When the radiation level exists between the high level set values, the red LED (LED6) indicates light and the buzzerl is activated. Similarly, when the radiation level goes to higher then the over high level set value, another red LED (LED7) indicates light and the 220V alarm circuit is activated.
Background Radiation Detection Circuit
The background radiation detection circuit is used for indicate the presence of background radiation. It consists of R39 – R43, C9 – C12, LED8, Q6, Q7 (BC337), ICIO (555), IC11 (555) and buzzer2 When the background radiation present it gives audible alarm and the LED8 gives light. The output pin 7 of IC5 is connected to the triggering input pin 2 of IC 10. The IC 10 is connected for monostable operation. The duration of high state can be calculated by the following equation .
TH = 1.1 x R39 x C9 (in seconds) (3)
According to design requirements, the output pulse width of the designed monostable multivibrator is TH = 12ms = 12 x 10-3s. Therefore, if C9 = 10µF then. R39 = 1.09 KΩ
The output pin 3 of IC 10 is directly connected to the base of Q6. The emitter terminal of Q6 is connected to ground via parallel combination of astable multivibrator circuit and LED driver circuit. When the background radiation present, the output pin 3 of IC 10 goes high, this turns on the transistor Q6. As a result the LED8 gives light and the astable circuit start to oscillation. The astable multivibrator circuit consists of R41, R42, C11, C12 and ICI 1. It generates square waves of frequency 2.5 Hz. The output pin 3 of IC 11 is connected to the base of Q7 via R43. The collector of Q7 is connected to supply voltage via buzzer2. When the output pin 3 of IC11 is at high state, Q7 turns on and the buzzer2 gives audible sound (i.e. click), this indicates the presence of background radiation.
Conclusion
Bangladesh has to import area radiation monitor from abroad at the expense of hard earned foreign exchange. With a view to reduce the dependence on others and to save foreign currency, this particular endeavour of design and development was undertaken which ultimately ended in success. The designed area radiation monitor is very simple in construction, highly efficient and gives very satisfactory results by measuring the radiation in and around the nuclear research reactor. This device is reliable in operation and its costs approximately U$ 1,000 for its fabrication, whereas the price of the similar instrument in the international market is not less then U$ 4,000. The high voltage power supply unit of this area radiation monitor was put under a series of test and its performance was very satisfactory. And its output was compared with the standard power supply Euro card module (NIM). The output of the designed high voltage power supply was ripple less and stable. This instrument (i.e. ARM) is calibrated from Health Physics and Radioactive Waste management and Monitoring unit, Institute of nuclear science and technology.
