Transcript Med Phys 3A03/3AA1 - McMaster Faculty of Science

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 10th introduction to Survey Instruments
Mon Sept 17th practical
Mon Sept 24th practical/report back
Mon Oct 1st report back
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?
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
st graph
Region
of
limited
proportionality
not
shown
on
1
D–5
E–6
• 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
• http://www.radpro.com/RSO-10-5-PRS.pdf