DETECTOR EFFICIENCY

BY:AMRITA SHRIVASTAVA

0812EI071004

Definition: The efficiency of a detector is defined as the

percentage of ionizing radiation hitting the detector that is measured.

General formula:

Efficiency = Observed Counts Expected Counts

Factors Affecting Detector Efficiency:

Some radiations go directly from the radioactive material into the detector….(1)

Some radiation will backscatter off the surface into the detector….(2)

Some radiation are absorbed by the detector covering….(3)

Some radiation travels away from the detector…. (4)

The distance between the detector and the material will determine how much radiation will strike the detector….(5)

Detection Efficiency: The detection efficiency of any device is defined

as the probability that a quantum of radiation incident on the detector will produce a signal.

Two types of Detection Efficiencies:Total efficiency & Peak efficiency

If the counter produces signal for n out of N events, then

is an unbiased estimator of efficiency, with variance

Dependence of Detector Efficiency: Type of detector Size and shape of the detector Distance between the detector and the

radioactive material Radioisotope and type of radiation measured Backscatter of radiation towards the detector Absorption of radiation before it reaches the

detector

Efficiencies of Liquid Scintillation Counter (LSC) and Geiger Muller Counter (GM)

Minimal Detectable Count: Minimal Detectable Count is defined as the

number of counts that can be easily detected by the detector.

Its unit is counts per minute (cpm) or counts per second (cps).

MDC = 2.71 + 4.65 (B/T)^1/2where B is total counts for blankand T is the total counting time.

MDC = 38.7 cpm or 0.646 cps forB= 60 counts and T = 1 minute

Minimal Detectable Activity: Minimal Detectable Activity is MDC modified

by detector efficiency and size. Its unit is Bacquerel (Bq).

MDA = MDC/ Efficiency Example:

Efficiency of GM survey meter for C-14 = 10%

Detector size = 12.5 cm^2

Modification by detector efficiency: MDA would be increased by a factor of 10 over MDC.

MDA = 0.646 cps/(10/100)MDA = 6.46 Bq

Modification by size:MDA would be increased by a factor of 12.5

MDA = 6.46 Bq X 12.5 cm^2MDA = 80.72 Bq cm^2

References:

http://medicine.utah.edu/radiology/Medical_Physics/training/NucMedTech/Detector%20Efficiency.pdf

http://rd11.web.cern.ch/RD11/rkb/PH14pp/node44.html

THANKYOU!!