Instrumentation and Detection Flashcards
The function of a cascade impactor is to seperate
Particles according to mass
Purpose of an ‘annular kinetic impactor head’ is to trap
Airborne particles. Does not collect radon and thoron
What scintillator material has a high efficiency for alpha measurement?
ZnS. NaI is used for gamma spec.
Approximate efficiency for radioiodines on an activated charcoal filter at the appropriate flow rate
95%
Disadvantage of a long sample counting time for optimization of counting statistics?
Background may change
Approximate eifficiency for the detection of Xenon-133 on activated charcoal
1%. It is a noble gas, so it doesn’t want to interact with anytihng
Primary function of proportional counters is to
Measure and distinguish alpha from beta
A low energy alpha detector is usually effective if the detector is _____ distance from the source
1/4 inch. Low energy alphas can only travel less than 1/2 inch in air
Proportional counters have gas amplification that are
greater than 1.
Ion chambers have gas amplification factors of:
equal to 1
A proportional counter has enough voltage to cause
Secondary electrons with short range avalanche
What radiation instrument operates on the principal of discharging of a capacitor to estimate radiation dose?
Pocket ion chamber
An ionization chamber has voltage great enough to cause
Ions before recombination; no secondary electrons
A geiger counter has enough voltage to cause
Secondary electrons with a long range avalanche
A scintillation counter takes advantage of properties of materials that _____, after being exposed to energy from radioactive sources
Fluoresce
Semi-conductor detectors are used primarily for their:
High energy resolution
Cost effective means available to test for radon gas in a building
Alpha track detector, Charcoal cannister
Very thin windowed GM detectors can detect what types of ionizing radiation
Alpha, Beta, Gamma
Full Width Half Maximum (FWHM) of the photopeak is a measurement of
Resolution
What is the effective half life in the human body for Co-60, if the biological half-life is eight days?
7.97 days. The effective half-life is never greater than the biological half-life. Teff= (Tb*Tr)/(Tb+Tr)
Absolute efficiency
Number of pulses recorded/number of radiation quanta emitted by source
Intrinsic efficiency
Number of pulses recorded/number of radiation quanta incident on the detector
Relationship between E-absolute and E-intrinsic
E-Intrinsic = E-absolute * (4(pi)/Solid angle of detector)
% resolution formula for photo-peaks
% resolution = (FWHM/Energy Peak)*100
Constituents of fill gas P-10
90% argon 10% methane. Used in gas-filled detectors in the proportional and G-M regions. The argon serves as the ionization gas, the methane is an organic quench gas.
Formula for standard deviation in counting
1 Sigma = (Sample Count Rate/Sample Count Time)^1/2
Quality factor is a function of the
Collision stopping power of the radiation in water
Why is sulfur used in criticality accident dosimetry?
The S-32(n,P)P-32 reaction produces measurable beta activity.
Resolving time/Dead time correction
True count rate = recorded count rate/(1-Recorded rate*dead time)
Isokinetic sampling
Constant velocity sampling. V1(D2)^2=V2(D1)^2
95% confidence level
2 sigma= 1.96[(bkg rate /time)+(sample rate /sample time))]^.5
What is the ionizing region
The region of voltage over which the saturation current is produced. Saturation current is when one electron reaches the detector electrode for each electron released by ionization in the detector.
What instrument has a good sensitivity to neutrons while providing the best discrimination against gammas?
BF-3 proportional counter in polyethylene moderator. The proportional region of the detector is set at an alpha voltage to detect the B-10(n,alpha)Li-7 reaction.
The most energetic Compton electron results from a 180 degree backscatter of the incident photon. This electron is called
Compton edge.
Compton edge will occure at _____ of the photopeak energy
Photopeak energy - 250 keV
To avoid excessive dead time on multi-channel gamma analyzers you should keep samples below:
10mR/hr at 2 inches.
A semi-conductor is distinguished from an insulator or conductor by its:
Band Gap. Insulator gaps are large, 5 eV or more. In a semi-conductor the gap is in the range of 0.7-2.1 eV
Dead time for proportional counters is approximately:
0.5 microseconds. GM counter dead times can exceed 100 microseconds.
A sample with mixed alpha and beta emitting nuclides is counted with a gas flow proportional counter at the alpha voltage (A) and at the beta voltage (B). The alpha and beta count rates, respectively, may be estimated from these count rates as:
A and B-A
Usual FWHM for the Cs-137 photopeak is
50 keV
The term “tissue Equivalent” when used to describer ionization chamber construction means that the
Rev. 1
Chemical composition of the wall material and gas filling the ion chamber are selected such that the response of the ion chamber will be a measure of the absorbed dose in tissue
A polyethylene-walled ion chamber filled with ethylene gas will
Rev. 1
Measure the absorbed dose from photons and will give an over-estimate of the absorbed dose from neutrons
Initial recombination of ion pairs produced in an ionization chamber gas is
Rev. 1
Independent of dose rate. Recomb depends on LET
Volume recombination (recombination during drift towards electrodes) in the ion chamber gas is
Rev. 1
Dependent on dose rate
Initial recombination in a high pressure tissue equivalent ionization chamber can be used to:
Rev. 1
Determine the average quality factor in a mixed neutron and photon field. (think LET dependence)
The number of ion pairs collected in an air-filled aluminum walled ionization chamber depends primarily on:
Rev. 1
Absorbed dose rate
Ion pair collection in ionization chambers used to monitor pulsed radiation depends on:
Rev. 1
Ion pairs produced during the pulse
An aluminum ion chamber filled with CO2 will best measure:
Rev. 1
the high energy neutron fluence in mixed radiation fields.
The efficiency for collecting ion pairs produced in an ionization chamber is greater using:
Rev. 1
Parallel plate electrodes. (electrical field is stronger compared to cylindrical or spherical electrodes)
