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| Part
Three: Detectors and Meters. |
NEU
Abbreviated
Study Guide
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- (MDC) Minimum detectable counts is
the smallest number of sample counts that are statistically
greater than the background counts and are a result of radioactivity
in the sample.
- (MDA) Minimum detectable activity
is the minimum activity present in a sample that may be detected
by a particular instrument.
- The purpose of the chi-squared test
is to ensure that the instrument's results lie within a normal
Gaussian Distribution.
- Pulse height determines which source
it is. Alpha pulses are the highest.
- You don't check the desiccant for a initial check on instruments
prior to use, or the beta shield calibration (duh).
- The following checks are made on
a survey instrument prior to use: check the instrument for
date calibrated (calibration expiration date), proper battery
condition, and a response check.
- The principal that explains the operation
of most radiation detection instruments is the radiation causes
ionization in the detector which is converted into a meter
reading.
- Radiation is not measured directly,
but is measured instead by its effect on the detector walls
or the material within the detector.
- In a gas filled detector, radiation
ionizes the fill gas releasing electrons that are attracted
to the positively charged anode and measured by the electrical
circuitry.
- The ratio of intensity measured by
the detector (intensity of the primary beam and scatter radiation)
to the calculated actual intensity is the buildup factor.
- The type of gas used would affect the sensitivity of an
ion chamber detector because some gases ionize more easily
than others.
- An ion chamber produces output based on the ions produced
in the wall of the chamber.
- In the ionization chamber, the detector output is approximately
equal to the amount of primary ionization produced in the
gas.
- An ion chamber survey instrument is used to ascertain
the amount of exposure personnel would receive as it accounts
for all the ionization events occurring.
- The instrument used for the best tissue equivalent
dose rate is the ion chamber.
- The Dead Time for GM detectors is
the period in which a second pulse will not be detected.
- Unless some type of quenching gas
is used, a Geiger-Mueller detector will re-trigger its avalanche
because of electrons released while the positive ions interact
with the outer wall.
- When compared to ion chambers, proportional
counters and
fission chambers, the Geiger-Mueller detectors are the
most sensitive to low level gamma fields, but not more accurate.
- In the Geiger-Mueller detector the
output pulse is approximately independent of the type and
energy of the radiation entering the detector.
- Proportional counters operate at higher voltages than
ionization chambers. (RIPLGC)
- A proportional counter system can tell the difference
between alpha and beta radiation because the size of the pulse
caused by the alpha interaction will be much higher than the
pulse caused by the beta interaction.
- The main advantage of using a proportional counter
is it is able to differentiate between different types of
radiation.
- The function of the Photo Cathode in a photo-multiplier
(PM) tube is to release electrons when it is struck by light
emitted from scintillation.
- In a scintillation detector, the photomultiplier tube
converts and amplifies the electrons freed by ionization in
the scintillation crystal to a larger light output.
- In general, scintillation detectors are more sensitive
then gas-filled detectors for gamma radiation because they
contain material with a higher density for gamma's to interact.
They are often used for counting measurements in laboratories.
- Surface medium for scintillators, alpha is zinc sulfide
(ZnS), beta is anthracene, and gamma is sodium iodine (NaI).
- The GeLi is a Semi Conductor Detection System. The
middle section is composed of germanium and lithium mixture.
Pure germanium is on one side of the mixture and pure lithium
on the other side. When a gamma enters the center region of
the crystal it undergoes an interaction that frees an electron
from one of the atoms in the crystal. When this happens, a
vacancy, or hole is left in the inner electron shell around
the nucleus. The center region is the part of the semiconductor
crystal that is sensitive to ionizing radiation.
- When a gamma ray is detected by a semi-conductor crystal
a flow of free electrons is initiated toward the positive
end of the crystal.
- For a semiconductor material to detect radiation, the
voltage applied to the detector must be large enough to separate
the positive charges from the negative charges to form a depletion
region.
- In Germanium crystal detectors, the flow of free electrons
towards the positive electrode and the apparent movement of
holes toward the negative electrode create a current flow
in the circuit.
- A whole body counter is for measuring internal gamma emitters.
- In-vivo or whole body counting, determines the amount of
a radionuclide deposited in an individual by liquid or solid
crystal scintillation detectors.
- It is important to add moderating materials to shield
for a fast neutron source since the absorption cross section
for most elements increases as the velocity or energy of the
neutron decreases.
- A good thermal neutron absorber used in the portable detectors
is cadmium.
- The neutron interaction used in portable survey instrumentation
is neutron capture with the emission of a charged particle.
- Neutron detectors are easy to design because there are
a large number of materials with large cross sections for
absorption.
- Paraffin is used to attenuate fast neutrons.
- Individual dosimeter placement could be described as
an error associated with relating a dosimeter reading to an
actual dose.
- Beta and gamma exposure is determined in a Beta-Gamma Radiation
field using film/TLD dosimetry by subtracting the closed window
(shielded) film reading from the open window (unshielded)
film/TLD reading.
- Finger rings don't have a need for neutron sensitivity.
- Minute by minute update on exposure is not an error or
problem in relating TLD readings to the actual dose received
by the body.
- A TLD's disadvantage is that it is expensive to use.
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