Compact Alpha Contamination Monitor

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International Journal on Advanced Electrical and Computer Engineering (IJAECE)

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ISSN(Online): 2349-9338, ISSN(Print): 2349-932X Volume -2, Issue -1, 2015 23

Compact Alpha Contamination Monitor

1H.Anu, ²R. Amudhu Ramesh Kumar

1Lecturer, PSB College, Chennai, India, ²IGCAR, Kalpakkam, Chennai, India Email : ¹[email protected]

Abstract - Alpha contamination monitors are essential for the detecting the contamination in contamination prone areas like Nuclear Reactors, Fuel fabrication plant, Reprocessing Plants, Active analytical labs and nuclear waste management plants. Designing a compact Alpha Contamination monitors with self diagnostic feature is the industrial requirement. This paper shows the different type of meter with specifications. This paper describes the design, development and calibration of such monitor.

Key word: Display, Sensor, Battery, detector

I. INTRODUCTION:

Alpha particle is highly ionizing and harmful to human.

However, previous studies have shown that the detection of such particle in the contaminated area or on contaminated human is achieved by bulky contamination system. Scintillated based techniques are famous for their simple gross counting of alpha particles using Photo Multiplier Tubes (PMT) and associated electronic circuits. The design of the monitor incorporate the features of automatic detection hand or material closer to detection area, HV fail indication, Saturation detection due to light ,immediate alarm on contamination and portable design.

1.1 Radioactive contamination Definition:

Radioactive contamination, also called radiological contamination, was the deposition of, or presence of radioactive substances on surfaces or within solids, liquids or gases (including the human body), where their presence was unintended or undesirable.

Such contamination presents a hazard because of the radioactive decay of the contaminants, which emit harmful ionising radiation such as alpha particles or beta particles, gamma rays or neutrons. The degree of hazard is determined by the concentration of the contaminants, the energy of the radiation being emitted, the type of radiation, and the proximity of the contamination to organs of the body. It is important to be clear that the contamination gives rise to the radiation hazard, and the terms "radiation" and "contamination" are not interchangeable.

1.2 Sources of contamination

Radioactive contamination is typically the result of a spill or accident during the production or use of, radionuclide has unstable nuclei which are subject to radioactive decay.

1.3 Contamination monitoring

Depends entirely upon the correct and appropriate deployment and utilization of radiation monitoring instruments.

1.4 Surface contamination

Surface contamination may either be fixed or "free". In the case of fixed contamination, the radioactive material cannot by definition be spread, but its radiation was still measurable.

1.5 Airborne contamination

The air can be contaminated with radioactive isotopes in particulate form.

Airborne contamination was measured by specialist radiological instruments that continuously pump the sampled air through a filter. Airborne particles accumulate on the filter and can be measured in a number of ways:

1. The filter paper was periodically manually removed to an instrument.

2. The filter paper is static and was measured in situ by a radiation detector.

3. The filter was a slowly moving strip and is measured by a radiation detector. These are commonly called

"moving filter" devices and automatically advance the filter to present a clean area for accumulation, and thereby allow a plot of airborne concentration over time.

II. TYPES OF METER

2.1 Model CM 250SP:

It allows an easy and accurate control of alpha hand contamination for the personnel who work in a controlled area. This unit was presented as a compact, self contained instrument including the detector, a processing hardware, a LCD display and visual and audible alarms. Figure 1 shows the CM 250SP model contamination meter.

Figure 1 CM 250SP

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The case can be used on a desktop or wall mounted. A four-button keypad allows parameter definition and operation control through a user friendly software. The measurement start is triggered by an infrared beam cutoff. The result of the measurement is displayed as a digital and analog value on a large LCD graphic back- light screen. Visual and audible alarms were triggered when an adjustable threshold was exceeded. Light indicators and messages are displayed on the screen to indicate the operation status of the unit.

2.2 Model LB 147

Radionuclide was widely used in many fields of research, nuclear medicine and in life science. Wherever unsealed radioactive materials were used, radiation protection regulations require contamination monitoring for hands, feet and clothing.

Figure 2 LB 147 contamination meter

The hand-foot monitor LB 147 is based on innovative scintillation detection technology for the measurement of radioactive contamination due to that it is possible to measure alpha and beta/gamma contaminations simultaneously. The results can be represented in freely selectable units Bq/cm² or in cps. Highlights of this meter was S(Ag)-scintillation detectors, High detector efficiencies, Removable hand probe for frisker measurement, Space-saving design, Simple operation with graphic display and touch panel - USB, Ethernet, RS 232 and RS 485 interface, Permanent data memory for 1750 measuring data.

2.2.1 Applications

Used in Radionuclide Laboratories - Nuclear Facilities - Environmental Measurement, Homeland Security - Nuclear Medicine.

2.3 Bench type model CM 712 2.3.1 Detector assembly

The instrument can be supplied with three detector probes, two detectors for Beta gamma and one for alpha, with the specifications given below.

2.3.2 Beta gamma detector

Side window probe with Halogen quenched GM detector, thin walled (25 / 30) mg/sq. cm. in a protective

housing with a rotatable shutter for cutting off beta particles. This will be used for measuring beta as well as gamma contamination.

2.3.3 Alpha Detector

ZnS (Ag) scintillator, approximately 20 mg /cm² on perspex; covered by 1 mg/cm² aluminised , pin-hole free, light-resistant mylar film, protected by a suitable thin metallic grill.

2. Principle and Operation:

Alpha particle detection is achieved by scintillation detection, Thin mylar is used to block the light particle and to allow alpha particle to interact with ZnS(Ag).

Light in visible spectrum generated though alpha interaction with Zn(Ag) is couple to the Photo multiplier tube, which amplifies the electron based on light input and contributes a current , proportional to the input light.

The current output form PMT is converted into pulse using RC circuits. These pulses have been amplified and discriminated from the noises using discriminator (comparator). The pulses with are converted to uniform size using a mono stable multivibrator. The pulses are counted in the Micro Controller (MC) and the alarms are generated based on alarm set limits.

Figure 1 : Block diagram

Prompt alarm generated when count exceeds the alarm limit and not waiting for the count time to complete. HV was bleeded to a suitable low voltage, which is monitored by MC and HV fail alarm is generated promptly. Saturation current of PMT is monitored continuously to generate PMT saturation. Background subtraction of the monitor is an option provided to detect the background periodically and subtract it from gross count such that the net count can be detected. Four button keyboard has been used as Input to the MC to set the HV fail limit, Alarm limit, Background subtraction, counting time, Background counting time, CPM/CPS mode, test mode ,date and time.

III. TESTING AND CALIBRATION:

The designed system has been tested for the intended targets, status indication, fail indication, mode for as its functional testing. The system has been tested with Natural Uranium Pu 239 sources of know activity. The system has been calibrated hand monitor source (nat U) for a counting time of 60 seconds and 30 iterations and

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the efficiency was observed around 27%, which is better than the conventional systems.

3.1 Detector efficiency depends upon:

i. Type of detector (GM, Nal Scintillation, Plastic Scintillation, Proportional)

ii. Detector size and shape

iii. Distance from the detector to the radioactive material

iv. Radioisotope and type of radiation measured (alpha, beta and gamma radiations and their energies)

v. Backscatter of radiation toward the detector vi. Absorbtion of radiation before it reaches the

detector.

3.2 Method of Measurement 3.2.1 Radioactive contamination

may be measured directly or indirectly. Direct measurement means the use of portable radiation detection instruments to detect both fixed and removable contamination.

3.2.2 Instrument Selection

The ability of various radiation detection instruments to detect radioisotopes of interest will vary with the instrument and manufacturer.

3.2.3 Locations of Measurement

The locations that are to be monitored should be numbered on a plan of the radioisotope work area.

3.2.4 Instrument Checks and Calibration

Non-portable instruments used for counting wipes, such as liquid scintillation counters, well-crystal type gamma counters, gas-flow proportional counters, semiconductor

gamma spectrometers and gamma cameras, should be routinely serviced according to the manufacturer's instructions. Keep a record of the service information and dates.

IV. RESULTS AND CONCLUSION:

The designed system has been tested with standard source and real-time plant conditions and the results are satisfactory. The system has non contact contamination monitoring which avoids the cross contaminations and ease of use. The prompt alarm makes the user comfort not to count for entire count time and go for decontamination operation quickly. The system is simple, compact and portable, makes it stationary and mobile applications.

V. ACKNOWLEDGEMENT

Thank God and His almighty power to finish His research work by using me, my friends and my students for His ultimate work.

REFERENCES:

[1] Radiation Detection and Measurement by Glenn F. Knoll John Wiley & Sons; 4th Edition edition, 2010.

[2] Programming Embedded systems using C by Michel J.Pont, University of Leicester, 2012.

[3] W. J. Price, Nuclear Radiation Detection, 2nd Ed., Chapter 3, McGraw-Hill, 1994. International Atomic Energy Agency.

[4] IAEA Safety Glossary: Terminology Used in Nuclear Safety and Radiation Protection.

Vienna: IAEA. ISBN 92-0- 100707-8.

[5] International Atomic Energy Agency (2010).

Programmes and Systems for Source and Environmental Radiation Monitoring. Safety Reports Series No. 64.

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