Exam 2 - Detection and Measurements

-gas-filled
-Scintillation
-Solid-state, Semiconductor
-others:
–Cloud chambers
– Bubble Chambers
– Superheated Drop
– Cryogenic
– AMANDA and IceCube

• Detection Mode
• Operation Mode
• Dead Time / Interaction Rate
• Paralyzable / Non-Paralyzable
• Efficiency

– Counters – detect number of interaction events
– Spectrometers – detect number of events as a
function of energy
– Dosimeters – detect accumulated energy by all
interactions

Pulse – detects (and generally counts) individual interactions
Current –individual interactions averaged to produce current

Maximum rate at which data can be recorded

n = m/(1-(m*tau))

m: recorded ramp rate
n: true count rate

Duty cycle of slowest component determines
dead time

Geiger-Müller (GM) counting
• In multichannel analyzer systems - analog-to-digital converter

10-100 ms

<3-8 ms

• In pulse mode, events must be separated by
more than the dead time to be detected
• A second interaction in this interval will not be
detected
• A second interaction very close to the first
interaction may distort the signal from the first
interaction

• Systems in which dead-time events extend
dead time are paralyzable. Otherwise,
systems are nonparalyzable.
• At very high interaction rates, paralyzable
systems will not detect any interactions
after the first, causing the detector to
indicate a count rate of zero

pulse mode

• Amplitude of each pulse is proportional to the
energy deposited in the detector by the
interaction causing that pulse
• The energy deposited by an interaction is not
always the total energy of the incident particle or
photon
• Pulse height spectrum - graph of the number of
interactions depositing a particular amount of
energy as a function of energy

In current mode, all information regarding
individual interactions is lost, but these systems
can be designed with no dead time
• If the electrical charge collected from each
interaction is proportional to the energy
deposited by that interaction, then the net
current is proportional to the dose rate in the
detector material

current mode operation

• The efficiency (sensitivity) of a detector is
a measure of its ability to detect radiation
• Efficiency of a detection system operated
in pulse mode is defined as the probability
that a particle or photon emitted by a
source will be detected

#detected/#emitted AND geometric efficiency*intrinsic efficienct

#reaching detector/#emitted

#detected/#reaching detector

• Often called the quantum detection efficiency or
QDE
• Determined by the energy of the photons and
the atomic number, density, and thickness of the
detector

intrinsic efficiency = 1-e^(-mu*x)

• A gas-filled detector comprises gas between two
oppositely charged electrodes
• Ionizing radiation produces ion pairs in the gas
• Positive ions (cations) migrate to negative
electrode (cathode); electrons or anions migrate
to positive electrode (anode)

anode is wire inside the container
cathode is the wall of the container that holds the gas

– Ionization chambers
– Proportional counters
– Geiger-Müller (GM) counters

The voltage applied between the 2 electrodes

wider range of physical shape (parallel plates, concentric cylinders, etc.)

Proportional counters and GM counters (look up answer in the book)

When the gas is air and the walls of the chamber are a material whose effective atomic number is similar to air

-Portable survey meters
-performing QA testing of diagnostic and therapeutic x-ray machines
-detectors in most x-ray phototimers

Low density of gases and low atomic numbers of most gases

•Must contain a gas with specific properties
• Commonly used in standards laboratories,
health physics laboratories, and for
physics research
• Seldom used in medical centers

• GM counters also must contain gases with
specific properties
• Gas amplification produces billions of ion pairs
after an interaction – signal from detector
requires little amplification
• Often used for inexpensive survey meters
• In general, GM survey meters are inefficient
detectors of x-rays and gamma rays
• Over-response to low energy x-rays – partially
corrected by placing a thin layer of higher atomic
number material around the detector
• GM detectors suffer from extremely long dead
times – seldom used when accurate
measurements are required of count rates
greater than a few hundred counts per second
• Portable GM survey meter may become
paralyzed in a very high radiation field – should
always use ionization chamber instruments for
measuring such fields

• Scintillators are used in conventional film-screen
radiography, many digital radiographic
receptors, fluoroscopy, scintillation cameras,
most CT scanners, and PET scanners
• Scintillation detectors consist of a scintillator and
a device, such as a PMT, that converts the light
into an electrical signal

• Amount of light emitted after an interaction
increases with energy deposited by the
interaction
• May be operated in pulse mode as
spectrometers
• High conversion efficiency produces
superior energy resolution

– Depends on crystalline structure and hence must be crystalline
or poly-crystalline
– Involve high Z elements (compared to organics), especially as
activator/dopants, and are therefore effective gamma absorbers
– Generally hydroscopic and fragile

– Depend on molecular rather than crystalline structure, and hence
more easily fabricated.
– Less sensitive generally and especially to gamma radiation
– Low Z-numbers produce less backscatter
– Good for electron and beta detection and for fast neutron
detection (fast neutrons produce fast H and C that ionize as they
slow).

– High conversion efficiency
– Decay times of excited states should be short
– Material transparent to its own emissions
– Color of emitted light should match spectral sensitivity
of the light receptor
– For x-ray and gamma-ray detectors, µ should be large
– high detection efficiencies
– Rugged, unaffected by moisture, and inexpensive to
manufacture

• Sodium iodide activated with thallium
[NaI(Tl)], coupled to PMTs and operated in
pulse mode, is used for most nuclear
medicine applications
– Fragile and hygroscopic
• Bismuth germanate (BGO) is coupled to
PMTs and used in pulse mode as
detectors in most PET scanners

• An incident photon can deposit its full energy by:
– A photoelectric interaction (A)
– One or more Compton scatters followed by a
photoelectric interaction (B)
• A photon will deposit only a fraction of its energy
if it interacts by Compton scattering and the
scattered photon escapes the detector (C)
– Energy deposited depends on scattering angle, with
larger angle scatters depositing larger energies

• Detectors normally shielded to reduce effects of
natural background radiation and nearby
radiation sources
• An x-ray or gamma-ray may interact in the shield
of the detector and deposit energy in the
detector:
– Compton scatter in the shield, with the scattered
photon striking the detector (E)
– A characteristic x-ray from the shield may interact with
the detector (F)

• Cs-137 decays by beta particle emission to Ba137m, leaving the Ba-137m nucleus in an
excited state
• The Ba-137m nucleus attains its ground state by
the emission of a 662-keV gamma ray 90% of
the time
• In 10% of decays, a conversion electron is
emitted instead, followed by a ~32-keV K-shell
characteristic x-ray