Detectors

Question 1

Gas filled detectors include these 3 types

Answer 1

ionization chamber

proportional counter

geiger müller

Question 2

In detectors, recombination occurs when __________ voltages are use

Answer 2

low

Question 3

Ionization chambers operate in the __________ region

Answer 3

saturation

Question 4

When the voltage becomes sufficient to cause complete collection of all of the charges produced, the curve enters a plateau called the

Answer 4

saturation region

Question 5

The saturation region voltage ranges from

Answer 5

~ 50 - 300 V

Question 6

The amount of energy to produce an ionization in air is

Answer 6

34 eV

Question 7

Ionization chambers generally contain a cap which can be removed in order to

Answer 7

measure non-penetrating radiation such as beta particles, low energy photons (<10 keV) and alpha particles

Question 8

A typical survey meter can measure exposure rates down to approximately

Answer 8

1 mR/hr

Question 9

Unlike other types of ionization chambers dose calibrators employ sealed and pressurized chambers filled with ______.

Answer 9

argon gas

Question 10

A device that records total charge collected over time is a

Answer 10

pocket dosimeter

Question 11

Percentage of photons actually interacting in an ionization chamber

Answer 11

<1%

Question 12

Unsealed ioniation chambers must be corrected for the effects of

Answer 12

temperature and pressure

Question 13

In an unsealed ionization chamber pressure and density are

Answer 13

proportional

Question 14

In an unsealed ionization chamber, density and temperature are

Answer 14

inversely related

Question 15

The pressure and temperature correction factor for an ionization chamber

Answer 15

(P_ref x T) / (P x T_ref)

T is in Kelvin (273 + °C)

Question 16

In the proportional region, gas amplification can be as high as

Answer 16

10⁶

Question 17

In a GM counter, the center wire has this voltage sign

Answer 17

+

(anode)

Question 18

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

Answer 18

visible or ultraviolet wavelengths

Question 19

The role of quenching gas in an ionization chamber

Answer 19

Neutralize positive ions by donating electrons

Absorb UV radiation from the positive ions to prevent a second avalanche

Question 20

In a GM system, gas amplification can be as high as

Answer 20

10¹⁰

Question 21

The curve for three main regions of ionization chambers as a function of voltage

Answer 21

Question 22

Occurs with voltages above the GM range

Answer 22

spontaneous discharge

Question 23

The wire screen on the detector is meant to

Answer 23

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

Question 24

GM meters are about _________ times more sensitive than ionization chamber survey meters

Answer 24

10

Question 25

The removeable cover on GM meter is meant to

Answer 25

help distinguish between penetrating and non-penetrating radiations

Question 26

GM counters designed for counting only relatively penetrating radiations, such as γ rays and high-energy βparticles, have thicker, sturdier windows made of

Answer 26

0.1-mm-thick aluminum or stainless steel

Question 27

Because the solid detector materials used in semiconductor detectors are ___________ more dense than gases

Answer 27

2000 to 5000

Question 28

The most commonly used semiconductor detector materials are

Answer 28

silicon (Si) and germanium (Ge)

Question 29

Newer semicondutor detecting materials include

Answer 29

cadmium telluride (CdTe) or cadmium zinc telluride (CZT)

Question 30

In semiconductor detectors, the amount of energy to produce an ionization is

Answer 30

3 to 4 eV

(roughly 1/10 of an ionization chamber)

Question 31

For the same absorbed energy, a semiconductor detector can produce a signal ______ times large than an ionization chamber

Answer 31

10

Question 32

One problem with Si and Ge (especially Ge) detectors is

Answer 32

production of thermal noise

Question 33

Impurities in Si and Ge detectors can

Answer 33

create electron traps, absorbing electrons from ionization events

Question 34

The thickness of Si Ge detectors is limited by

Answer 34

impurities which create electron traps and greatly reduce the signal

Question 35

Deliberately doping or introducing Li impurities into Si or Ge detectors can compensate for electron traps by

Answer 35

donating electrons

Question 36

Because Li ions tend to “condense” at room temperatue Si(Li) and Ge(Li) detectors must be

Answer 36

super-cooled with liquid nitrogen

Question 37

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:

Answer 37

(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.

Question 38

CdTe or CZT detection elements are usually small because of

Answer 38

difficulty and expense of growing large pieces with the required purity

Question 39

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.

Answer 39

inorganic substances in the form of solid crystals and organic substances dissolved in liquid solution

Question 40

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

Answer 40

proportional to the energy deposited by the incident radiation in the scintillator

Question 41

For a single scintillator γ-ray interaction within the energy range of interest for nuclear medicine imaging, the number of photons produced is

Answer 41

few hundred to a few thousand

Question 42

electronic tubes that produce a pulse of electrical current when stimulated by very weak light signals

Answer 42

photo multiplier tubes (PMTs)

Question 43

This is an illustration of

Answer 43

Photo-multiplier Tube (PMT)

Question 44

In photocathodes, the conversion efficiency for visible light to electrons, also known as the quantum efficiency, is typically

Answer 44

1 to 3 photoelectrons per 10 visible light photons striking the photocathode

Question 45

A common photo emissive substance coating the photomultiplier tube

Answer 45

Cesium antimony (CsSb)

Question 46

The dynode is maintained at a positive voltage, typically __________ relative to the photocathode

Answer 46

200-400 V

Question 47

Successive dynodes are maintained at voltages _________ higher than the previous dynode

Answer 47

50-150 V

Question 48

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

Answer 48

Mu-metal

Question 49

PMTs used in nuclear medicine may range in size from

Answer 49

1 to 7.5 cm in diameter

Question 50

An alternative to a PMT is

Answer 50

light-sensitive semiconductor detector, such as a Si photodiode

Question 51

While Si photodiodes have high quantum detection efficiency (60-80%) they have _________ gain

Answer 51

unity

requiring very low-noise electronics for readout

Question 52

Si avalanche photodiode (APD) uses a very high internal electric field which enables it to

Answer 52

amplify the signal

Question 53

Si avalanche photodiode (APD) detectors can reach gains of ____________but still require low-noise electronics for successful operation

Answer 53

10² to 10³

Question 54

APDs also can be operated at higher bias voltages in

Answer 54

geiger mode

Question 55

APDs are often found in these systems

Answer 55

small animal scanners

Question 56

Inorganic scintillators are crystalline solids that scintillate because of characteristics of their

Answer 56

crystal structure

Question 57

Order from lowest to highest density for LSO, BGO, NaI

Answer 57

NaI

BGO

LSO

Question 58

Peak emission wavelength of NaI scintillator

Answer 58

415 nm

Question 59

The most commonly used scintillator for detectors in nuclear medicine

Answer 59

NaI(Tl)

Question 60

Dimensions of crystals for gamma cameras that use NaI(Tl) crystals that are typically

Answer 60

30-50 cm in diameter by 1-cm thick

Question 61

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

Answer 61

30%

Question 62

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

Answer 62

0.1 - 0.4

Question 63

5 reasons to use NaI(Tl) scintillators in NM

Answer 63

1. 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.
2. It is a relatively efficient scintillator, yielding one visible light photon per approximately 30 eV of radiation energy absorbed.
3. 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.
4. It can be grown relatively inexpensively in large plates, which is advantageous for imaging detectors.
5. The scintillation light is well-matched in wavelength to the peak response of the PM tube photocathode

7.

Question 64

Important inorganic scintillator characteristics

Answer 64

efficiency

index of refraction

emission spectrum

Question 65

Maximum event rate of a scintillator to prevent event “pileup”

Answer 65

1 / 2τ where τ is the decay time for the scintillator

Question 66

Liquid scintillation solutions consist of

Answer 66

Organ solvent

primary solute (or primary fluor)

sceondary solute or waveshifter

additives to improve performance

Question 67

All liquid scintillation counters suffer from

Answer 67

quenching - any mechanism that reduces the amount of light output from the sample

Question 68

Three types of quenching in liquid scintillation systems

Answer 68

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)

Question 69

The iodine escape peak appears at

Answer 69

30 keV below the photopeak

Question 70

Photopeak

Answer 70

peak in spectra for energy of the emission

Question 71

Can be set to allow only certain photon energy levels to be counted, decreasing Compton scatter and pulse pile-up in the image

Answer 71

Pulse Height Analyzer (PHA)

Question 72

As count rate increases in NM spectroscopy

Answer 72

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

Question 73

As x-ray energy increases it becomes __________ to separate the photopeak from the scatter

Answer 73

easier

(less contribution from lower energies in the photopeak)

Question 74

Refers to the proportionality between output pulse amplitude and energy absorbed in the detector

Answer 74

Energy linearity

Most NaI(Tl) systems are quite linear for energies between .2 and 2 MeV

Question 75

Energy resolution in NM spectroscopy is decreased due to

Answer 75

random statistical variations:

1. number of scintillation light photons per keV
2. number of photoelectrons released from photocathode
3. variation in electron multiplication factors from dynodes
4. nonuniform sensitivity across scintillator from PMTs
5. nonuniform light collection efficiency
6. nonlinear response of scintillator
7. electronic noise

Question 76

FWHM(%) is

Answer 76

(ΔE at FWHM / E_γ) * 100%

Question 77

A reasonable energy resolution in NM gamma cameras with Tc-99m and Cs-137 is

Answer 77

10% for Tc-99m at 140 keV

6% for Cs-137 at 662 keV

Question 78

For a gaussian shaped curve, the FWHM is approximately

Answer 78

2.35 x standard deviation

Question 79

typical integration time for a NM camera

Answer 79

approximately 1 usec

Question 80

The best energy resolution is obtained with

Answer 80

semiconductor detectors

due to approximately 1 charge per 3 eV of radiation vs. 10 photoelectrons per keV for NaI

Question 81

Number of photoelectrons for keV in NaI(Tl) PMT system

Answer 81

10 per keV

Question 82

The larger number of charges generated in a semiconductor detector decreases

Answer 82

statistical variation and increases energy resolution

Question 83

A Ge(Li) detector resolution at 140 keV for Tc-99m

Answer 83

.42% versus 10 %

Question 84

For a CZT detector spectrum of Tc-99m, what is the bump at around 20 keV?

Answer 84

K x-rays of Tc-99m emitted after internal conversion

Question 85

Liquid scintillators provide __________ energy resolution relative to NaI(Tl)

Answer 85

poor

Question 86

geometric efficiency

Answer 86

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

Question 87

intrinsic efficiency

Answer 87

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.

Question 88

To combine efficiencies you

Answer 88

multiply them

geometric * intrinsic * …

Question 89

Surface area of sphere

Answer 89

4πr²

Question 90

Volume of sphere

Answer 90

4/3 π r³

Question 91

geometric efficiency is approximately

Answer 91

.5 (1-cosθ)

Question 92

Dead time losses occur due to

Answer 92

overlapped pulses falling outside the selected analyzer window

Question 93

typical dead times for

GM tube

NaI(Tl ) and semiconductors

liquid scintillation and gas proportional counters

Answer 93

50-200 usec

.5 - 5 usec

.1 - 1 usec

Question 94

non paralyzeable system

Answer 94

if an event occurs within the dead time, τ, of a preceding event, it is not counted

Question 95

paralyzable system

Answer 95

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

Question 96

Most radiation detectors are

paralyzable or nonparalyzable?

Answer 96

paralyzable

Question 97

For a nonparalyzable system the observed counting rate, R_o, is

Answer 97

Question 98

For a paralyzable system the observed count rate is

Answer 98

Question 99

Another word for the “resolving time”

Answer 99

dead time

Question 100

In a paralyzable system, at very high true counting rates the observed counting rate

Answer 100

approaches zero

Question 101

For a paralyzable system, the observed counting rate rises to a maximum value given by

Answer 101

e = 2.718… the base of natural logarithms

Question 102

For small values of true count rate as dead time (<.1) the percentage loss is approximately

Answer 102

True count rate * dead time * 100%

Question 103

When measuring dead time with a system that has a pulse height analyzer, you must account for the

Answer 103

window fraction

the relationship between the true dead time and apparent dead time is

τ_a = τ / w_f

_{(does not apply when doing 2 source method)}