Radiation Dosimeters

General requirements for dosimeters
#Dosimeter is a device that measures directly
or indirectly
• Exposure
• Kerma
• Absorbed dose
• Equivalent dose
• Or other related quantities.
#The dosimeter along with its reader is
referred to as a
dosimetry system.

A useful dosimeter exhibits the following
properties:
• High accuracy and precision
• Linearity of signal with dose over a wide
range
• Small dose and dose rate dependence
• Flat Energy response
• Small directional dependence
• High spatial resolution
• Large dynamic range

 Accuracy specifies the proximity of the mean
value of a measurement to the true value.
 Precision specifies the degree of
reproducibility of a measurement.
Note:
High precision
is equivalent to small standard deviation.

Examples for use of precision and accuracy:
High precision High precision Low precision Low precision
and and and and
High accuracy Low accuracy High accuracy Low accuracy

Note: The accuracy and precision associated
with a measurement is often expressed in
terms of its uncertainty.

New Concept by the International Organization
for Standardization (ISO) "Guide to the
expression of uncertainty in measurement“
 This new guide serves as a clear procedure
for characterizing the quality of a
measurement.
 It is easily understood and generally
accepted.
 It defines uncertainty as a quantifiable
attribute.

 Standard uncertainty: is the uncertainty of a
result expressed as standard deviation.
 Type A standard uncertainty is evaluated by
statistical analysis of a series of observations.
 Type B standard uncertainty is evaluated by
means other than statistical analysis. This
classification is for convenience of discussion
only.
 It is not meant to indicate that there is a
difference in the nature of
 the uncertainty such as random or systematic.

 Combined uncertainties:
The determination of the final result is normally
based on several components.
Linearity:
 The dosimeter reading should be linearly
proportional to the dosimetric quantity.
 Beyond a certain range, usually a non-linearity sets
in.
 This effect depends on the type of dosimeter.

Two possible cases
Case A:
• linearity
• supralinearity
• saturation
Case B:
• linearity
• saturation

Dose rate dependence :
 M/D may be called the response of a dosimeter
system
 When an integrated response is measured, the
dosimetric quantity should be independent of the
dose rate dD/dt of the quantity.
 Other formulation:
The response M/D should be constant for different
dose rates (dD/dt)1 and (dD/dt)
2. M = 􀀁 (M / D)(dD / dt)dt
M = (M / D)􀀁 (dD / dt)dt

 Energy: The response of a dosimetric system
is
generally a function of the radiation energy.
 The term "radiation quality" is often used to
express a specific distribution of the energy
of radiation.
 Therefore, a dependence on energy can also
be called a dependence on radiation quality.
 Since calibration is done at a specified beam
quality, a reading should generally be
corrected if the user's beam quality is not
identical to the calibration beam quality.

A small radiation monitoring device worn by persons
entering environments that may contain radiation .
# Desirable characteristics
 Should be lightweight, durable, and reliable
 Should be inexpensive

 Healthcare or laboratory workers in non-
emergency environments that may contain
radiation
 Examples: radiology, nuclear medicine, and
radiation oncology department staff
 Workers in emergency environments that
may contain radiation
 Examples: first responders and first receivers
 Workers in industrial environments where
radiation is used
 Examples: nuclear power plant workers or
employees at radiation sterilizing facilities

 Flat badges are usually worn on the torso, at the
collar or chest level, but can be worn on the
belt, or forearm
 Ring shaped badges can be worn on the finger
when dose to the finger may exceed dose to the
badge worn elsewhere on the body
 First responders and first receivers
 Wear water-resistant personal dosimeters on the
outer layer of personal protective equipment (PPE).
 Should be able to easily see and hear a dosimeter
alarm while wearing PPE
 May wear a personal dosimeter underneath
waterproof outerwear

 Film badge
 Pocket ionization chambers
 Thermo luminescent dosimeters (TLD)
 Optically stimulated luminescence (OSL)
 Solid State

 Most widely used and most economical
 Consists of three parts:
 Plastic film holder
 Metal filters
 Film packet
 Can read x, gamma, and beta radiation
 Accurate from 10mrem - 500rem
 Developed and read by densitometer
 A certain density value equals a certain level of radiation
 Read with a control badge
 Results generally sent as a printout

 Lightweight, durable,
portable
 Cost efficient
 Permanent legal record
 Can differentiate between
scatter and primary beam
 Can discriminate between
x, gamma, and beta
radiation
 Can indicate direction from
where radiation came from
 Control badge can indicate
if exposed in transit
 Only records exposure
where it’s worn
 Not effective if not worn
 Can be affected by heat
and humidity
 Sensitivity is decreased
above and below 50 keV
 Exposure cannot be
determined on day of
exposure
 Accuracy limited to + or -
20%

 The most sensitive personnel dosimeter
 Two types
 Self-reading
 Non self-reading
 Can only be read once
 Detects gamma or x-radiation

 Small, compact,
easy to use
 Reasonably
accurate and
sensitive
 Provides
immediate reading
 Expensive
 Readings can be
lost
 Must be read each
day
 No permanent
record
 Susceptible to
false readout if
dropped or jarred

 Looks like a film badge
 Contains a lithium fluoride crystal
 Responds to radiation similarly to skin
 Measured by a TLD analyzer
 Crystal will luminescence if exposed to
radiation, then heated
 More accurate than a film badge

 Crystals contained
in TLD interact
with ionizing
radiation as tissue
does
 Determines dose
more accurately
 The initial cost is
greater than that
of a film badge
 Can only be read
once
 Records exposure
only where worn

• “Captures” information in an Aluminum
Oxide matrix
• Releases information by laser stimulation
• Can be reread after processing
• Durable
• Landauer Only

 Provides instantaneous information
regarding dose accumulation
 Simple to use
 Not a “legal” record
 Dose range device dependent

Radiation Dosimeters

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