Practical Communications in Health and Medical Physics: Survey Instruments Overview

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

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

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

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

5. 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

6. 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

7. Observed pulse height versus applied voltage difference

8. 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 1st graph

9. 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

10. 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

11. 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

13. Useful reference • G-M Pancake Detectors: Everything You’ve Wanted to Know (But Were Afraid to Ask) • Paul R. Steinmeyer, Health Physicist • http://www.radpro.com/RSO-10-5-PRS.pdf