survey meters Flashcards
what fills the meter?
• Either gas filled (e.g., a gas filled cavity with central and outer wall electrodes; wall usually made of tissue equivalent material) or solid state detectors (e.g., scintillators or semiconductor detectors)
different regions of plot of # of ion pairs vs applied voltage
- recombination region 0 -150 V
- ionization region 150- 350 V
- proportional region 350-800 V
- limited proportionality region 800-1000 V
- geiger muller region 1000-1400 V
- continuous discharge 1400-1700 V
in recomb region, 1 MeV beta particles go from 0-100, 100 keV particles fo from 0 to 50
The two lines join as 1 in the middle of the limited proportionality region (around 10^8 ion pairs collected)
Why are GM monitors smaller than ion chambers?
higher voltage = higher sensitivity
what is required to detect beta particles?
a detector with a very thin window for weakly penetrating radiation
o End window GM counters have a removable buildup cap to discriminate beta and gamma rays. For beta measurements, the end cap is removed so that the weakly penetrating beta particles can enter the sensitive volume and be detected while the gamma detection efficiency remains low while buildup cap is off.
what region do neutron survey meters operate in?
proportional region so that the photon background can be easily discriminated against
how are thermal neutrons detected?
o Thermal neutron detectors usually have a coating of boron, or the counter is filled with BF3 gas. A thermal neutron interacts with B-10 to cause an (n,alpha) reaction, and it is the ionizations created by the alpha particles that are detected.
how are fast neutrons detected
same as thermal neutrons (boron coatin to cause alpha reaction), if it surrounded by moderator of hydrogenous material (e.g., polyethylene) which thermalizes the fast neutrons
Low Z materials are ideal for neutron thermalization because energy loss per collision increases with decreasing Z. Therefore, lead is not a good moderator because it will just scatter the neutrons in different directions, while resulting in relatively little energy loss.
how are neutron surveys made to report values that are proportional to the dose equivalent in soft tissue?
o Filter compensation is used to modify the response of the detector so that it follows the ICRP radiation weighting factors. In this case, the output is approximately proportional to the dose equivalent in soft tissue over a wide range of neutron energy spectra.
neutron detecors other than Boron
He-3 function on same principles as boron
charge collected vs voltage appled in ionization region
independent of applied voltage
charge amplification in proportional counters
10^3-10^4X
More sensitive than ion chambers. Amount of charge collected is proportional to the amount of energy deposited in the gas of the counter
GM detectors charge amplification
10^9-10^10X
where should GM counters be used?
-low radiation levels as they are very sensitive
GM monitors are not suitable in areas where radiation rates are high because they have long dead times – ionization chambers should be used in this situation instead
Do GM counters discriminate energy of radiation?
No, pulse height is independent of energy of interacting particles; charge multiplication spreads along the entire length of the anode
describe scintillators (phosphors)
emit light when they absorb radiation
High atomic number phosphors are mostly used for gamma rays whereas plastic scintillators are used for beta particles. A PMT is coupled to the scintillator to convert the light into an electrical signal.
o Scintillation systems are more sensitive than GM counters, and their resolving time is much lower.
