detection Flashcards
which two radiation sources will have continuous energy spectra?
beta particles and bremsstrahlung
relate energy and wavelength.
E = hc/lambda
What are the differences between fast electrons and heavy charged particles?
Fast electrons lose energy at a slower rate. They may be lost by radiative processes as well as coulomb interactions.
What energy is typical for bremssahlung photons?
It’s generally very low and is absorbed close to point of origin.
What does the transmission curve for beta particles look like?
It is near exponential.
What is backscattering and when does it usually happen?
When an electron entering an absorber is deflected enough to go back the way it came. Most pronounced with electrons that have low incident energy and absorbers with high atomic number.
What are common statistical models used in detection experiments?
Binomial distribution, poisson distribution, gaussian or normal distribution.
What is the difference between the binomial, poisson, and gaussian distributions?
Gaussian distribution is a special case of poisson which is a special case of the binomial distribution. All must have constant p. In poisson p is also small, in gaussian we have a relatively large number of successes (>20 or 30)
what is rise time? what is it dependent on?
the time it takes to recognize charge, dependent on reactor
what is the sequence of machines from source to computer?
source - detector - preamp - linear amplifier - pulse shaper - multichannel analyzer - computer
what does the detector do?
Turns radiation into electric charge
how much energy does it take to ionize one atom in a gas detector?
order of 30 ev
how much energy does liberated charge usually have?
pico coulombs
what does the preamp do?
converts charge into current or voltage
what is the gain for a preamp?
G = V_out/Q_in
What does the linear amplifier do?
makes signal bigger so it is easier to see for MCA. produces gaussian to optimize signal to noise ratio.
what does the pulse shaper do?
turns curved signal into square pulse (easier for computer to read)
what does MCA do?
digitizes signal
What are the four general types of atomic or nuclear radiation?
Fast electrons, Heavy Charged Particles, Electromagnetic Radiation, Neutrons
What is a fast electron?
Beta particles (positive or negative) emitted in nuclear decay, as well as energetic electrons produced by other processes.
What are Heavy Charged Particles?
All energetic ions with mass of 1 amu or greater. ie alpha particles, protons, fission products.
What type of electromagnetic radiation is of interest?
X rays emitted in the rearrangement of electron shells of atoms, gamma rays.
At what energy is radiation ionizing?
10ev
Arrange typical forms of radiation in terms of “hardness” from soft to hard .
Alpha/low energy xrays; beta particles; gammas and neutrons
specific activity
activity/mass = (lambda N_A)/M
auger electrons
like internal conversion electrons, but excitation energy originates in the atom rather than in the nucleus
how is alpha particle parent half life and energy correlated?
highest energy, shortest half life. beyond 6.5 MeV, the half life is less than a few days.
annihilation radiation
result of beta + decay, positron combines with electrons to create photons.
shape of a typical fission spectrum
dN/dE = E^{1/2}e^{-E/T}
linear stopping power
S = - dE/dx differential energy loss within material divided by differential path length. also called specific energy loss
mean range
absorber thickness that reduces alpha particle count to exactly one-half its value in the absence of the absorber
photoelectric absorption
photon undergoes an interaction with an absorber atom in which the photon disappears, in its place a photoelectron is ejected
compton scattering
between incident gamma ray photon and an electron in absorbing material. incident photon yields recoil electron and scattered photon
pair production
energetically possible when gamma ray energy exceeds twice the rest mass energy of an electron (1.02 MeV), but generally confined to higher energy gamma rays. gamma ray photon disappears and is replaced by electron positron pair.
activity
A = lambda N
momentum
p = sqrt(2mE)
count rate
CR = C/T (counts per minute)
count rate uncertainty if time is certain
sqrt(counts)/ T
charge
Vpulse tpulse/ resistance
voltage
charge/capacitance
current
charge / collection time
fano factor
measures dispersion of a probability distribution. think of it as a noise to signal ratio. variance squared over mean. (sigma^2/ mu)
why does the pulse height for a gieger tube continue to increase with applied voltage even after a full geiger discharge is obtained?
in geiger tubes avalanche process proceeds until there is enough space charge to reduce the electric field so that no additional multiplication happens. the amount of space charge produced is independent of the number of initial ion pairs created in the interaction and independent of incident particle energy. so the amount of space charge needed depends on initial electric field strength before discharge. for constant field strength space charge and output pulse height are constant. as voltage is increased, field strength is also increased and more space charge is necessary and output pulse height increases.
can dead time behavior of geiger tube better be described by paralyzable or non paralyzable model? why?
Depends on how detector is operated. if counts are only recorded if they result in full charge, then any events that happen after the first event has fully discharged and the system has recovered won’t be counted. these events delay recovery time, so paralyzable is better. if partial discharges are recorded then you could use nonparalyzable.
dead time
time after each event when detector is not able to record another event
what is the difference between paralyzable and non-paralyzable?
in paralyzable systems not only are events happening during deadtime lost, they restart the deadtime. in non-paralyzable systems the event is just lost.
what is the difference between a single and multi channel analyzer?
for a SCA a voltage window is defined and only pulses in that window are counted. MCA has many bins and gives a spectrum.
absolute efficiency
detected/emitted = CR/(A_0 e^{-lambda t})
geometric efficiency
incident/emitted
intrinsic efficiency
detected/incident
what is one of the assumptions of the klein nishina formula?
electron is at rest
imagine an infinitely small detector and perfect collimation. what would we expect the compton scattering to look like from the klein nishina formula? what actually happens? why?
the formula predicts a sharp peak , but there is actually some width due to motion of the electron (doppler broadening)
what does intensity generally refer to in radiation detection?
count rates
what can we see from the graph of the klein nishina formula?
highest energy gamma rays scatter the most straight.
what are two things that limit time resolution?
walk and jitter
suppose you have a beta source that has been blocked from emitting beta particles, but your detector still detects something beyond background. what could it be?
interactions with the aluminum causing bremsstrahlung and emitting xrays
whats a good way to measure deadtime?
two source method. with two equal sources you will get different count rates due to dead time.
what does the bragg curve for alpha and other heavy charged particles look like? for gamma rays?
for alphas it starts midway flat, goes up, and then drops down dramatically. for gamma rays it goes up a little and then down exponentially.
what is happening at the bragg peak?
up until then the particle is picking up electrons because its ionized. at the peak it becomes neutral
what does the graph of intensity as a function of distance for alpha particles look like?
straight and then drops off exponentially. straggling at the end.
what is range straggling?
fluctuations in penetration distance
which has better energy resolution, germanium or sodium iodide (scintillator) detectors?
ge because high z, high number of charge carriers and high density
which is more efficient overall, sodium iodide or germanium detectors?
sodium iodide because you can grow larger crystals
which is more efficient per unit volume, germanium or sodium iodide detectors?
theyre about the same
what is a 50% germanium detector?
efficiency relative to sodium iodide detector
what is a charge carrier?
a particle that is free to move carrying an electric charge. electrons ions and holes
how do semiconductor detectors work?
ionizing radiation produces free electrons and holes. the number is proportional to the energy of the radiation. electrons and holes travel to electrodes where they result in a pulse that can be measured. the energy required to create an electron hole pair is known so radiation can be determined.
what are the benefits of semiconductor detectors?
- better energy resolution (less energy is required to produced electron hole pairs than producing ions in a gas detector, so statistical variation of pulse height is smaller)
- good time resolution (since electrons travel fast)
- high density which allows for smaller detectors
what are n type and p type detectors?
refer to n+ and p+ ohmic contact
electronic charge of an electronic charge of an electron hole pair
1.6 x 10^(-19)
energy required to liberate e-h pair in silicon
3.62 ev
calculate energy resolution
delta E/E = FWHM / E = sqrt(2.35^2 FEW + noise^2)
what are the features (in order) of gamma spectrum?
x-rays, backscatter peak, annihilation peak, double escape peak, compton edge, photo peak
what is the energy of the compton edge?
E_source - E’_gamma
what is the energy of the backscatter peak?
E_source - E_comptonedge
what energy is the single escape peak?
E_source - 0.511 MeV
Where is the double escape peak?
E_source - 1.022 MeV
what does a smaller energy resolution percentage mean?
better energy resolution.
in which ways are inorganic scintillators better than organic ones?
inorganic have better 1. light output 2. linearity of light output with deposited energy 3. detection efficiency for higher gamma rays.
how can you reduce the compton continuum?
anti compton shield by surrounding detector w/more detectors that detect scattered photons w/ high efficiency & can reject events.
how can you reduce single and double escape peaks?
increase detection volume so the 511 kev annihilation photons have a longer distance to travel, increasing chance of photoelectric absorption.
Advantages of CZT compared to HPGe
- room temperature 2. simpler processing 3. higher z 4. higher density
disadvantages of CZT compared to HPGe
- larger band gap and energy per e/h pair 2. lower electron and especially hole mobility 3. shorter charge carrier lifetimes 4. crystals must be smaller with higher applied fields
what factors have a major influence on detector intrinsic peak efficiency?
- density 2. average atomic # 3. geometry of source detector system
what factors have major influence on energy resolution
- energy required to create scintillation photon 2. gain of PMT 3. quantum efficiency 4. light collection efficiency.
why are several different types of detectors required to monitor the neutron flux in a pressurized water reactor?
because there is a large range of neutron fluxes and neutron to gamma ray background ratio that exist in starting and operating PWRs. In startup mode there are small neutron fluxes and relatively large gamma ray background so BF3s are used and operated in pulse mode to enable discrimination between neutrons and gamma rays. In the intermediate power rage the neutron flux is high enough that pulse mode is no longer possible so compensated ion chambers are used to discriminate neutrons and gamma rays. in full power full neutron flux dominates so uncompensated ion chambers or fission chamber are used.
explain how tree rings are used to calibrate radiocarbon dates
The number of tree rings provides and independent and accurate measure of the age of the tree which is used for calibration. Without some type of calibration radiocarbon dating is not necessarily accurate since C14/C_total ratio is not known a priori.
what are the similarities between neutron activation analysis and xray flourescence analysis
radioactivity is induced by external radiation and then used to identify species in the sample. Often, photons are observed to determine amount and type of specific isotopes.
what are the differences between neutron activation analysis and xray flourescence
NAA is based on neutron induced nuclear reactions and XRF is based on xray reactions in the atomic shell. In XRF xrays are observed and used as fingerprints. In NAA it’s mainly gamma rays.
how is the signal in a gas filled ionization detector generated?
by the motion of the charges in an electric field.
accurate measurement of the number of radiations emitted by a source requires…
an accurate efficiency calibration
what is a charge sensitive counting system?
a system that produces output voltage proportional to input charge
what are the advantages of a p-n junction semiconductor over a gas filled ionization chamber?
better energy resolution, higher density, better time resolution
what does it mean for a detector to have high intrinsic efficiency?
it records a large fraction of the radiations incident on the detector
what is the purpose of x-ray flourescence?
to identify and assay elements
for which type of detector is the height of the output pulse independent of the magnitude of the applied bias voltage?
a gas filled ionization chamber
how could one detect fast neutrons using neutron detectors available in your lab class?
the fission chamber and BF3 detector can only detect slow neutrons so the neutrons need to be moderated with paraffin or polyethylene. the 3He detector can detect both fast and slow neutrons, but has higher efficiency for slow neutrons due to high n capture cross section at lower energies.
what are the advantages of using a fission counter?
large neutron induced fission signal due to large energy of the fission fragments, therefore you can distinguish between neutrons and background radiation such as gamma rays and alpha particles
what are the disadvantages of using a fission chamber?
since u-235 primarily emits alpha particles, the disadvantage is constant background due to alpha particles, but since the neutron signal is sufficiency larger than the alpha signal, you can discriminate.
imagine you are using a 3He counter to detect thermal neutrons. what happens to the pulse height spectrum if the pressure were raised by a factor of 2.
the range of the produced triton and proton is reduced which reduces the wall effect. (more reaction products are fully absorbed in the 3He gas)
in which ways are organic scintillators better than inorganic ones?
organic scintillators have better 1. speed of response 2. cost
what are wall effects?
when a detection reaction occurs close the the detector wall, one of the products is absorbed in the wall and the other deposits its kinetic energy in the stop gas. then the pulse height spectrum looks like two peaks and an additional plateau region and the proton and neutron peaks merged together and can’t be resolved.
what are the advantages of a SiLi detector?
- operated at room temperature 2. good for charged particles 3. cheaper, no need for expensive coolants
what are the disadvantages of a SiLi detector?
- suffer degradation over time due to radiation 2. not great at detecting gamma rays.
what is pulse pileup?
its when pulses arrive closer in time than the pulse resolution time for the system
what is the effect of pulse pileup on the gamma ray spectrum?
it will create peaks past the photopeak
what is the effect of the lead bricks on the gamma ray spectrum?
the x-rays at the beginning on the spectrum.
activity in an activation foil.
A(t) = R(1-e^(-lambda t))
what are two standard models of dose? which one is used for federal standards?
Linear No Threshold Model and Radiation Hormesis. LNT is used for Federal regulations.
what are the problems with the models used
these were created by extrapolating data from survivors of nuclear weapons in japan. at low exposure the signal to noise ratio is bad and its possible that radiation is even beneficial at low dose.
what does the energy efficiency curve for germanium look like?
upward slope at beginning, notch at 11 keV (k edge of germanium), and downward slope at the end (around 100 keV)
Why should you not use the 511 keV peak of Na-22 for determining efficiency calibration?
it didn’t necessarily come from the source, it could have come from the surrounding area
Why should you not use the 511 keV peak of Na-22 for determining peak shape (energy resolution)?
theres a residual momentum of positron and electron that means its broadening, but we don’t know how much.
Why should you not use the 511 keV peak of Na-22 for determining energy calibration?
when a positron finds an electron they form positronium before decaying into 511 kev gamma rays. the electron has extra binding energy, so it’s not actually 511.
bethe relationship
-dE/dx is proportional to kMz^2/E
