Instrumentation Flashcards
Gas filled detectors
Ionization chambers (dose calibrators)
G-M detectors
Ionization chambers/dose calibrators are in range where the voltage applied is enough that for every radiation event entering the gas, all the created ion pairs get collected.
The energy of incident radiation is what determines the number of ion pairs formed.
Meaning: EVERY time a given energy radiation enters the gas in the chamber, the exact same number of ions are collected, i.e. the same size pulse output occurs.
By measuring the number of ion pairs in known volume of air, it measures exposure rate (mRem/HR).
It then concerts exposure rate to Activity for a given type of isotope gamma rays.
G-M:
Survey meter.
Hi voltage potential applied so max number of iins created even for low amount of incident radiation energy.
Avalanche (bc hi kinetic energy of initial electrons formed allows these to form more ion pairs, and so on).
It’s very sensitive to detect energy but can’t decipher specific amount of energy present (i.e. the activity).
Just tells you “counts per minute.”
K
Amplitude (size) of each pulse from the PMT is proportional to the amount of energy hitting it in the form of light photons from the crystal, which in turn is proportional to the amount of energy deposited in the crystal by radiation.
The number of pulses is related to the amount of radioisotope present, which is measured by activity.
K
Pulse height analysis
Pulse height (amplitude) is proportional to the incident photon energy
K
FWHM is smallest in our scintillation cameras for photons around 150 keV;
meaning that the cameras perform best at accurately detecting Tc photon energies and converting them to accurate count statistics.
vs thallium photons–the detector intrinsic error (poorer energy resolution) with 72 keV photons;
The system just does not pick these up as well in the final result of scintillation–>PMT–>dynodes–>voltage pulse–>pulse height analysis.
*this is the MAIN REASON for poorer contrast in Thall images than Tc.
Get wider photopeak curve (larger FWHM)
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