Coated large area α-β sources are used for a number of applications. Measuring the absolute rate of emission from such sources is an important requirement for many of the applications. This leaflet gives a brief description of a multiwire proportional counter, for activity calibration of large area coated sources, developed at BARC.

It is a windowless detector based on gas flow arrangement and is useful to determine surface emission rates and absolute activity from the large area coated α/β sources having size up to 10 cm x 15 cm. Its key features are:

1. Detector design is highly reliable, user-friendly and rugged. It needs minimum maintenance.
2. The coated source is mounted within the sensitive volume of the detector chamber thereby losses of radiation due to absorption in the window in the usual design are avoided.
3. Detector design is as per the international regulations (ISO 1988) for calibration of contamination monitor and as per [ISO 8769] recommendations for reference transfer instrument.
4. Detector has adequate sensitive area of multiwire field grid to cover ~2π solid angle of emission.
5. A suitable provision is made for frequent manual loading, unloading of source samples in the source tray. Detector uses an arrangement having an O-ring and toggle clamps, which help isolate the chamber interior from the ambient and ensures that the detector contains only the flowing gas that is supplied by the gas flow arrangement.
6. Anode grid is supplied with positive bias voltage.
7. Detector shows low noise operation and good repeatability.

1. Operating Voltage : ~2000 V
2. Background counts : 4 cpm
3. Plateau length : 400 V
4. Plateau slope : ~0.3% / 100 V

1. For measurement of absolute activity and surface emission rates from a large area coated α/β sources and thus most useful for calibration of sources.
2. The calibrated sources may be further used to calibrate efficiency of contamination monitors.
3. Useful for the absolute activity measurement of weak α/β emitting radioactive solutions with good accuracy.

INTRODUCTION

Fig 1 (a) A completely assembled 2p-proportional counting chamber

Fig 1 (b) Schematic of a chamber with peripherals

2π-proportional counting chamber with multiwire field grid is useful for precise activity measurement of large area α-β coated sources.

1. Detector is gas flow-type, windowless and is based on multiwire geometry. A coated source is mounted within the sensitive volume of the detector, thus there are no counting losses in window.
2. Proportional counting mode ensures good signal to noise ratio and better counting accuracy, thus it is advantageous over GM mode of counting.
3. Sensitive area of the detector covers near 2π solid angle of emission from the source.
4. Detector enables frequent manual loading and unloading of the source samples. Overall dimensions of detector can be scaled for the particular source size.

The proportional counting system is useful for measurement of absolute emission rate from large area α-β coated sources. Detector is based on a multiwire proportional counter with a gas flow arrangement. A coated source is mounted within the sensitive volume of the detector. Design of windowless counter and large sensitive area enables efficient counting of emissions in near 2π solid angle and frequent change of source. PC is hermitically sealed using O-ring and clamps. A suitable provision for frequent manual loading, unloading of source samples is possible.

Detector specifications are as per the international regulations (ISO 1988) for calibration of contamination monitor and as per [ISO 8769] recommendations for reference transfer instrument.

A simple data acquisition system consists of preamplifier, amplifier and counter scalar to record the data. These accurately calibrated sources are further used to calibrate efficiency of hand–foot, floor contamination monitors and are used in checking radioactive spillage at various nuclear facilities.

1. Source size up to 10 cm x 15 cm
2. 100% detection efficiency in 2π direction
3. Source activity range 1 Bq/cm² to 100 Bq/cm²
4. Low background : 4 counts/minute
5. High Voltage plateau length ~ 400 V
6. Anode Bias ~2000 V
7. Count rate capability ~2 x 104 CPS

1. For measurement of absolute activity and surface emission rates from a large area coated α/β sources and thus most useful for calibration of sources.
2. The calibrated sources may be further used to calibrate efficiency of contamination monitors.
3. Useful for measuring the absolute activity of weak α/β emitting radioactive solutions with good accuracy.

SPACE

1. About 400 sq feet (20 x 20 sq. ft) roofed area with proper cleanliness, dust proof with controlled temperature (25±1 °C) and humidity (<55%) with continuous electricity.

EQUIPMENTS

1. PCB etching with accurate multiwire pattern
2. Soldering station and wire mounting assembly with uniform tension
3. Workshop facility for Al machining such as milling machine, lathe machine and turning machine for screw tapping, SS inserts, roller ball sockets
4. Toggle clamps, plumbing lines in SS
5. Vacuum pump (Rotary) and vacuum, pressure gauge
6. P-10 Gas cylinder and gas flow meter
7. Test setup with HV, preamplifier, amplifier, counter, scalar
8. Coated sources and dummy source substrate without coating

Consumables

1. Raw material Al solid blocks
2. Gold plated tungsten wire
3. Process chemicals (trichloroethylene, NaOH solution, alcohol, acetone, silicon grease)

Electrical Power Requirement

1. 220 V, 50 Hz, Single phase, 15 A

MANPOWER

1. One M.Sc. (Physics) or equivalent degree with experience in handling radiation sources and test electronics.
2. Two technicians trained in turning, fitting, vacuum pump handling and thin wire handling/ soldering.

Precautions

1. Test area should be covered by personnel radiation monitoring batches.