Test 3 - Radioactivity Detectors Flashcards
C-14
5730 yrs, max E = 156 keV
H-3
12.4 yrs, max E = 19 keV
I-131
8.07 days, max E = 806 keV
I-125
60 days, max E = 35.49 keV
P-32
14.3 days, max E = 1709 keV
S-35
87.5 days, max E = 167 keV
Cl-36
3x10^5 years, max E = 709 keV
Liquid scintillation materials
- Emulsifying and aqueous - safer and organic. Scintillator & emulsifier.
- Organic and non-aqueous - organic aromatic solvent. Scintillator only.
Types of Quenching
Chemical, color, dilution, photon
Quenching
When competing radical-forming chemicals or photon-absorbing materials are present in the solution, or when photon escapes, quenching (incomplete transfer of energy) occurs.
Chemical quenching agents
Nitromethane, chloroform, carbon tetrachloride, acetone
DPM vs CPM
Decay per minute is a 100% efficient Counts per minute. Or, CPM x 100% / DPM = % counting efficiency.
LSC Advantage / Disadvantage
Advantage: small sample size okay, mixture of B distinguishable, short preparation. Disadvantage: expensive, bulky, and calibration required
Liquid scintillation counting
Technique of measuring photoelectron emission from beta source in liquid scintillator with two opposing PMT. Current in PMT corresponds to energy. Gamma source will not work (escapes easily).
Quench curve
Curve of CPM vs energy (so spectrum) with varying degrees of quenching. More quenching = lower counts.
Quench indication parameter
Efficiency of counting can be calculated with this parameter.
tSIE
Transformed spectral index of external standard. Uses Ba-133 to assign a numerical value to quench amount. THE LOWER THE tSIE THE MORE THE SAMPLE IS QUENCHED. Most accurate measure of quench so far.
Gas-filled detectors (ionization chambers)
Ionization chamber, low V. Uses gases (Argon, halogens, organic gas) to monitor ionization, aka exposure. Specific geometry used, like cylinder or sphere. Convert to mR/hr.
Proportional counters
Still ionization chamber, but higher V. Uses special gases, like argon-methane mixture. Response proportional to initial ionization. FOR ALPHA AND BETA COUNTING.
Geiger-Muller detectors
Still ionization chamber, very high V. FOR HIGH-ENERGY BETA/GAMMA (P-32, I-131). Trouble detecting low E beta/gamma, because of over-sensitivity to low E beta/gamma.
Geiger Counter windows
Pancake (large face), end (detector at end of rod), side (detector at side of rod)
Solid Scintillator counters
Portable or stationary. Recommend for gamma emitters like I-125.
H-3 detection is only possible with?
Liquid scintillation counters.
What detectors are best for dose rate calculations?
Ionization chambers or tissue-equivalent organic scintillators. At high dose rate, only ionization chamber is useful. PGM (pancake geiger-muller) useful in an emergency.
What detectors are best for contamination survey?
GM for high-energy gamma and liquid scintillator for H-3.
What detectors are best for ID of source?
Liquid scintillator, well counter, gamma counter (which is energy-resolved!), portable Multi-Channel Analyzer (MCA)
CPM versus DPM on regulations?
USE DPM. CPM is area-dependent, distance-dependent, and yield-dependent value.
Liquid scintillator false positives
Liquid scintillators can give false positives when photoluminescence (lasting ~1 min), chemoluminescence (lasting hours) interfere with radioactivity.
Gamma counter
ENERGY RESOLVED counting of gamma, similar preparation as liquid scintillator (blanks in vials). ONLY good for GAMMAS: I-125, I-131, and 51-Cr. No liquid scintillator required (PMT directly picks up the gamma), and NO false positives.
Multi-Channel Analyzer (MCA)
Cadmium Tellurium (CdTe) detector. Many bins for each energy range.
Personal Monitoring of Raditation for workers
Film, TLD, Electronic, OSL.
