Rad Dosimeters II

environmental conditions-unsealed ion chamber

affected by temp and pressure so the air density within chamber also varies

mass of air in the chamber increases when pressure increases and temp decreases

when does chamber reading for fixed exposure increase

as temp decreases or pressure increases

calibration factors at standard reference conditions

temp: 22 celcius

pressure: 760 mm Hg

t/f a good dosimeter has high energy dependence

false

t/f linearity means response is proportional to dose

true

t/f ion recombination affects dose rate dependence

true

t/f environmental conditions affect ion chamber readings

true

what do integrate dosimeters measure and in what units

total rad dose accumulated over a period

SV, Gy, Rem

examples of integrated dosimeters

TLD, OSL, badges

how do integrated dosimeters work

passive devices store energy from radiation as trapped electrons in crystal, the stored signal is later analyzed to determine dose

integrated dosimeter primary use

routine occupational monitoring, regularoty compliance, permanent dose records

what do rate dosimeters measure and what unit

instantaneous rate that radiation received

units; dose per time

how do rate dosimeters work and give examples

active battery powered devices give real time readings and trigger alarms with exceeded limits

GM counter, ion chamber, semiconductor detectors (area of personal monitor)

rate dosimeter primary use

real time safety monitoring in dynamic or high radiation environments

modern devices

electronic personal dosimeters can combine both functions:

dose rate reading alarms, store cum dose

t/f absolute dosimeters do not require calibration against another device

true

t/f tlds are primary standard instruments

false

t/f secondary dosimeters require calibration

true

t/f integrated dosimeters measure dose OT

true

t/f rate dosimeters provide real time reading

true

thimble chamber-guard electrode

minmizes leakage and allows electric field uniformity

what is required of the thimble chamber

effective atomic number of the thimble and central electrode must be chosen so that the system acts like a free air ion chamber

thimble farmer type dependence and used for

radial beam direct independent

electron, photon, proton, ion

extrapolation chambers are employed in

absolute dosimetry with tissue equivalent phantoms

volume vs mass of air in chamber

volume of air is fixed but mass changes with temp and pressure

t/f ion chambers measure continuous current

true

t/f ion chamber output is proportional to radiation dose

trie

t/f ion chambers are very sensitive at low dose rates

false

t/f ion chambers have slow response time

true

t/f parallel plate chambers are used for electron dosimetry

true

t/f the electric field between the chamber wall and central electrode is created by a parallel plate

false by electrometer that applies a bias voltage

film radiography-film sensitivty depends on

emulsion grain size and radiation type

how is optical density of films measured

log10 (Io/It) measured with a densitometer

film is ideal for

electron beam dosimetry (minimal energy dependence)

shows energy dependence in photon beams

how is film used for photon beams

for QA and portal imaging only bc of energy dependence

how does radiochromic film work

self developing film that requires no chem processing

polymer dye turns blue with rad exposure

dose is measured with an optical densitometer

t/f film density depends on emulsion grain size

true

t/f film has no energy dependence in photon beams

false

t/f radichromic film requires chem processing

false

t/f radiochormic film changes color when irradiated

true

fluroescene time

10-10 to 10-8

phosphorescence time

greater than 10^-8 seconds

t/f luminescence is light emission after radiation exposure

tru

t/f fluroescence occurs over longer times than phosphorescence

false

t/f thermoluminescence is timulated by heat

true

t/f OSL is stimulated by light

true

t/f TLDs are used for persinnel monitoring

true

what are semiconductor dosimeters

solid state detectors with sensitivity much higher than gas filled detectors

what are silicon diodes and what are they used for

semiconductor dosimeter used for electron dosimetry, output checks, in vivo dosimetry

silicon diodes vs ion chambers

silicon diodes have high sensitivity, real time response, small size, and durability compared to ion chambers

unlike ion chamber, diodes do not require high voltage bias

silicon diodes limitation

photon energy dependence, directional sensitivty, temp effects, radiation damage

measuring principle for MOSFET

based on threshold voltage, which is a linear function of absorbed dose

integrated dose can be measured during or after irradiation

what do mosfets equire

connection to a bias voltage during irradiation