Detectors Flashcards
Gas filled detectors include these 3 types
ionization chamber
proportional counter
geiger müller
In detectors, recombination occurs when __________ voltages are use
low
Ionization chambers operate in the __________ region
saturation
When the voltage becomes sufficient to cause complete collection of all of the charges produced, the curve enters a plateau called the
saturation region
The saturation region voltage ranges from
~ 50 - 300 V
The amount of energy to produce an ionization in air is
34 eV
Ionization chambers generally contain a cap which can be removed in order to
measure non-penetrating radiation such as beta particles, low energy photons (<10 keV) and alpha particles
A typical survey meter can measure exposure rates down to approximately
1 mR/hr
Unlike other types of ionization chambers dose calibrators employ sealed and pressurized chambers filled with ______.
argon gas
A device that records total charge collected over time is a
pocket dosimeter
Percentage of photons actually interacting in an ionization chamber
<1%
Unsealed ioniation chambers must be corrected for the effects of
temperature and pressure
In an unsealed ionization chamber pressure and density are
proportional
In an unsealed ionization chamber, density and temperature are
inversely related
The pressure and temperature correction factor for an ionization chamber
(Pref x T) / (P x Tref)
T is in Kelvin (273 + °C)
In the proportional region, gas amplification can be as high as
106
In a GM counter, the center wire has this voltage sign
+
(anode)
Gas amplification occurs in the GM counter as in a proportional counter. In addition to ionizing gas molecules, the accelerating electrons also can cause excitation of gas molecules through collisions. These excited gas molecules quickly (~10−9 sec) return to the ground state through the emission of photons at
visible or ultraviolet wavelengths
The role of quenching gas in an ionization chamber
Neutralize positive ions by donating electrons
Absorb UV radiation from the positive ions to prevent a second avalanche
In a GM system, gas amplification can be as high as
1010
The curve for three main regions of ionization chambers as a function of voltage
Occurs with voltages above the GM range
spontaneous discharge
The wire screen on the detector is meant to
protect the fragile layer of mica (0.01-mm thick) that is sufficiently thin to permit passage of particles and low-energy photons into the counter
GM meters are about _________ times more sensitive than ionization chamber survey meters
10
The removeable cover on GM meter is meant to
help distinguish between penetrating and non-penetrating radiations
GM counters designed for counting only relatively penetrating radiations, such as γ rays and high-energy βparticles, have thicker, sturdier windows made of
0.1-mm-thick aluminum or stainless steel
Because the solid detector materials used in semiconductor detectors are ___________ more dense than gases
2000 to 5000
The most commonly used semiconductor detector materials are
silicon (Si) and germanium (Ge)
Newer semicondutor detecting materials include
cadmium telluride (CdTe) or cadmium zinc telluride (CZT)
In semiconductor detectors, the amount of energy to produce an ionization is
3 to 4 eV
(roughly 1/10 of an ionization chamber)
For the same absorbed energy, a semiconductor detector can produce a signal ______ times large than an ionization chamber
10
One problem with Si and Ge (especially Ge) detectors is
production of thermal noise
Impurities in Si and Ge detectors can
create electron traps, absorbing electrons from ionization events
The thickness of Si Ge detectors is limited by
impurities which create electron traps and greatly reduce the signal
Deliberately doping or introducing Li impurities into Si or Ge detectors can compensate for electron traps by
donating electrons
Because Li ions tend to “condense” at room temperatue Si(Li) and Ge(Li) detectors must be
super-cooled with liquid nitrogen
CdTe and CZT (which has properties very similar to CdTe) are more recently developed semiconductor materials that overcome two of the major disadvantages of Si and Ge:
(1) they can be operated at room temperature without excessive electronic noise, and (2) their high atomic number means that even relatively thin detectors can have good stopping efficiency for detecting γ rays.
CdTe or CZT detection elements are usually small because of
difficulty and expense of growing large pieces with the required purity
The scintillator materials used for detectors in nuclear medicine are of two general types: inorganic substances in the form of solid crystals and organic substances dissolved in liquid solution.
inorganic substances in the form of solid crystals and organic substances dissolved in liquid solution
A characteristic common to all scintillators is that the amount of light produced following the interaction of a single γ ray, β particle, or other ionizing radiation, is
proportional to the energy deposited by the incident radiation in the scintillator
For a single scintillator γ-ray interaction within the energy range of interest for nuclear medicine imaging, the number of photons produced is
few hundred to a few thousand
electronic tubes that produce a pulse of electrical current when stimulated by very weak light signals
photo multiplier tubes (PMTs)
This is an illustration of
Photo-multiplier Tube (PMT)
In photocathodes, the conversion efficiency for visible light to electrons, also known as the quantum efficiency, is typically
1 to 3 photoelectrons per 10 visible light photons striking the photocathode
A common photo emissive substance coating the photomultiplier tube
Cesium antimony (CsSb)
The dynode is maintained at a positive voltage, typically __________ relative to the photocathode
200-400 V
Successive dynodes are maintained at voltages _________ higher than the previous dynode
50-150 V
PM tubes often are wrapped in metal foil for magnetic shielding, called ________ an alloy composed of iron, nickel, and small amounts of copper and chromium
Mu-metal
PMTs used in nuclear medicine may range in size from
1 to 7.5 cm in diameter
An alternative to a PMT is
light-sensitive semiconductor detector, such as a Si photodiode
While Si photodiodes have high quantum detection efficiency (60-80%) they have _________ gain
unity
requiring very low-noise electronics for readout
Si avalanche photodiode (APD) uses a very high internal electric field which enables it to
amplify the signal
Si avalanche photodiode (APD) detectors can reach gains of ____________but still require low-noise electronics for successful operation
102 to 103
APDs also can be operated at higher bias voltages in
geiger mode
APDs are often found in these systems
small animal scanners
Inorganic scintillators are crystalline solids that scintillate because of characteristics of their
crystal structure
Order from lowest to highest density for LSO, BGO, NaI
NaI
BGO
LSO
Peak emission wavelength of NaI scintillator
415 nm
The most commonly used scintillator for detectors in nuclear medicine
NaI(Tl)
Dimensions of crystals for gamma cameras that use NaI(Tl) crystals that are typically
30-50 cm in diameter by 1-cm thick
With efficient optical coupling, good reflective surfaces, and a crystal free of cracks or other opacifying defects, approximately ___________ of the light emitted by the NaI(Tl) crystal actually reaches the cathode of the PM tube
30%
Single crystals of NaI(Tl) for radiation detectors are “grown” from molten sodium iodide to which a small amount of thallium (__________ mole percent) has been added
0.1 - 0.4
5 reasons to use NaI(Tl) scintillators in NM
- It is relatively dense (Δ = 3.67 g/cm3) and contains an element of relatively high atomic number (iodine, Z = 53). Therefore it is a good absorber and a very efficient detector of penetrating radiations, such as x rays and γ rays in the 50- to 250-keV energy range. The predominant mode of interaction in this energy range is by photoelectric absorption.
- It is a relatively efficient scintillator, yielding one visible light photon per approximately 30 eV of radiation energy absorbed.
- It is transparent to its own scintillation emissions. Therefore there is little loss of scintillation light caused by self-absorption, even in NaI(Tl) crystals of relatively large size.
- It can be grown relatively inexpensively in large plates, which is advantageous for imaging detectors.
- The scintillation light is well-matched in wavelength to the peak response of the PM tube photocathode
7.
Important inorganic scintillator characteristics
efficiency
index of refraction
emission spectrum
Maximum event rate of a scintillator to prevent event “pileup”
1 / 2τ where τ is the decay time for the scintillator
Liquid scintillation solutions consist of
Organ solvent
primary solute (or primary fluor)
sceondary solute or waveshifter
additives to improve performance
All liquid scintillation counters suffer from
quenching - any mechanism that reduces the amount of light output from the sample
Three types of quenching in liquid scintillation systems
chemical (compete with primary fluor for energy absorption)
color (substances absorb the primary emissions, e.g., blood)
dilution (large volume of sample added decreasing output efficiency)
The iodine escape peak appears at
30 keV below the photopeak
Photopeak
peak in spectra for energy of the emission
Can be set to allow only certain photon energy levels to be counted, decreasing Compton scatter and pulse pile-up in the image
Pulse Height Analyzer (PHA)
As count rate increases in NM spectroscopy
dead time losses increase
general broadening of the photopeak due to pile-up
shift of the photopeak toward lower energyies beasue of baseline shift in the amplifier
As x-ray energy increases it becomes __________ to separate the photopeak from the scatter
easier
(less contribution from lower energies in the photopeak)
Refers to the proportionality between output pulse amplitude and energy absorbed in the detector
Energy linearity
Most NaI(Tl) systems are quite linear for energies between .2 and 2 MeV
Energy resolution in NM spectroscopy is decreased due to
random statistical variations:
- number of scintillation light photons per keV
- number of photoelectrons released from photocathode
- variation in electron multiplication factors from dynodes
- nonuniform sensitivity across scintillator from PMTs
- nonuniform light collection efficiency
- nonlinear response of scintillator
- electronic noise
FWHM(%) is
(ΔE at FWHM / Eγ) * 100%
A reasonable energy resolution in NM gamma cameras with Tc-99m and Cs-137 is
10% for Tc-99m at 140 keV
6% for Cs-137 at 662 keV
For a gaussian shaped curve, the FWHM is approximately
2.35 x standard deviation
typical integration time for a NM camera
approximately 1 usec
The best energy resolution is obtained with
semiconductor detectors
due to approximately 1 charge per 3 eV of radiation vs. 10 photoelectrons per keV for NaI
Number of photoelectrons for keV in NaI(Tl) PMT system
10 per keV
The larger number of charges generated in a semiconductor detector decreases
statistical variation and increases energy resolution
A Ge(Li) detector resolution at 140 keV for Tc-99m
.42% versus 10 %
For a CZT detector spectrum of Tc-99m, what is the bump at around 20 keV?
K x-rays of Tc-99m emitted after internal conversion
Liquid scintillators provide __________ energy resolution relative to NaI(Tl)
poor
geometric efficiency
efficiency with which the detector intercepts radiation emitted from the source. This is determined mostly by detector size and the distance from the source to the detector
intrinsic efficiency
amount absorbed / amount striking detector
Refers to the efficiency with which the detector absorbs incident radiation events and converts them into potentially usable detector output signals. This is primarily a function of detector thickness and composition and of the type and energy of the radiation to be detected.
To combine efficiencies you
multiply them
geometric * intrinsic * …
Surface area of sphere
4πr2
Volume of sphere
4/3 π r3
geometric efficiency is approximately
.5 (1-cosθ)
Dead time losses occur due to
overlapped pulses falling outside the selected analyzer window
typical dead times for
GM tube
NaI(Tl ) and semiconductors
liquid scintillation and gas proportional counters
50-200 usec
.5 - 5 usec
.1 - 1 usec
non paralyzeable system
if an event occurs within the dead time, τ, of a preceding event, it is not counted
paralyzable system
if an event occurs within the dead time, τ, of a preceding event, it is not counted AND it introduces a dead time τ.
Essentially an “extendable” dead time is created
Most radiation detectors are
paralyzable or nonparalyzable?
paralyzable
For a nonparalyzable system the observed counting rate, Ro, is
For a paralyzable system the observed count rate is
Another word for the “resolving time”
dead time
In a paralyzable system, at very high true counting rates the observed counting rate
approaches zero
For a paralyzable system, the observed counting rate rises to a maximum value given by
e = 2.718… the base of natural logarithms
For small values of true count rate as dead time (<.1) the percentage loss is approximately
True count rate * dead time * 100%
When measuring dead time with a system that has a pulse height analyzer, you must account for the
window fraction
the relationship between the true dead time and apparent dead time is
τa = τ / wf
(does not apply when doing 2 source method)
