Med Phys 3A03/3AA1 Practical Health & Medical Physics Communications D.R. Chettle, with D.F. Moscu TA: Helen Moise

Course is in transition from: Communications in Medical Physics to: Operational Health Physics Laboratory

6 subsidiary objectives, or modules, each taking 4 weeks (so 3 per term). So: Mon Sept 10 th introduction to Survey Instruments Mon Sept 17 th practical Mon Sept 24 th practical/report back Mon Oct 1 st report back

Scheduling It might work better to have: Mon Sept 10 th 13:30 – 14:20 intro to Survey Instruments Mon Sept 17 th 13:30 – 15:20 practical group A Mon Sept 24 th 13:30 – 15:20 practical group B Mon Oct 1 st 13:30 – 14:20 report back Would this be possible?

Intro to survey instruments Get key information with minimum expense/sophistication Need instruments to be robust, not hypersensitive to fine tuning For most applications want hand held Geiger-Mueller counting system fits criteria

Gas filled radiation detector Radiation interacts in gas or in walls, causes ionisation, hence +ve and –ve charges A voltage difference across the gas causes charges to move, e- to anode, +ve charge to cathode As voltage is increased, different behaviours observed

Observed pulse height versus applied voltage difference

Pulse height versus applied voltage difference Regions that correspond are: A – 1 B – 2 C – 3 D – 5 E – 6 Region of limited proportionality not shown on 1 st graph

A – 1 at low voltage, some charge collected at electrodes, some recombines B – 2 sufficient voltage to collect charge, ion chamber C – 3 charge is accelerated sufficiently so that moving charge itself causes secondary ionisation amplifying the signal, making it easier to detect - 4 region of limited proportionality, charge amplification gets so large that some pulses saturate, so no longer get full proportionality between final pulse height and initial amount of ionisation D – 5 G-M region, pulse saturation, so get pulse for every initial ionising event, but no information as to how much ionisation: counter, not spectrometer E – 6 continuous discharge

G-M detectors can be used for alpha, beta or gamma sources Radiation must be able to get into the detection volume, very low energy betas (particularly tritiumwith max beta energy of 18.6 keV) and low energy alphas will not penetrate window and so won’t be detected Photons (gamma, x-ray) are quite likely to pass through window, but may well not deposit energy in detector

We shall be using a “pancake” detector, name comes from physical shape. Using with gamma-ray sources. G-M detector efficiency varies with photon energy. Usually expressed with respect to efficiency for 662 keV gammas from 137Cs

Useful reference G-M Pancake Detectors: Everything You’ve Wanted to Know (But Were Afraid to Ask) Paul R. Steinmeyer, Health Physicist