Low Background Gas Flow Beta Counter

This system uses gas flow type main and guard counters both operating in the G.M region. The counting medium is argon gas mixed with isopropyl alcohol vapour. The counter assembly is kept in a 3” lead shield. In the main counter aluminized mylar film of ~0.8mg/sq.cm. is acting as window cum cathode. Window area of the counter is 16 sq.cm. A tungsten wire of 50micron diameter stretched from one end of the sensitive volume of the counter to the other acts as anode. A drawer assembly has been provided to change the sample quickly. The guard counter is a large area (400sq.cm) gas flow counter used for background reduction. For the guard counter aluminium plates are used as cathode and a tungsten wire of 50micron dia stretched from one end of the active volume of the counter to the other end in 8 segments acts as anode. The counting system operates on the principle of anticoincidence for reducing the background of the system.

COUNTER PERFORMANCE PARAMETERS
a)	Main Counter
Plateau Threshold voltage		~ 1100 V
Plateau length		100 to 150 V
Plateau slope		5% for 100 V
Operating Voltage		Plateau threshold voltage + 50 V
Background (Inside the shield)		8 –12 cpm
b)	Guard Counter
Plateau Threshold voltage		~ 1300 V
Plateau length		100 to 150 V
Plateau slope		8% for 100 V
Operating Voltage		Plateau threshold voltage + 50 V
Background (Inside the shield)		200 to 400 cpm
c)	System Performance in Anticoincidence Mode
Background		1 to 1.5 cpm
Efficiency (for 1.3MeV betas)		40%
Efficiency (for 0.15MeV betas)		13%

This system is very useful in estimating gross beta activity in environmental samples and samples of low specific beta activity. The twin advantage of low background and high efficiency offers excellent sensitivity for this counting system.