Chapter 6 Radiation Protection Flashcards
explain how solid state dosimeters work
-crystals store energy absorbed during expsure in electron traps
-in TLD, these electrons are released by application of heat and release visible light
-OSLD use laser light to release stored energy
-OSLD can be read out several times whereas TLD can only be read out once
-can measure 0.01 mGy to 10,000 mGy
typical TLD material in radiology
lithium fluoride
reponse independent of x-ray energy
explain how ion chambers work
measure charge liberated when x-ray photons ionize the gas inside the chamber
-anode is used to collect electrons; need large enough voltage
-not very sensitive but very accurate
explain how geiger counters work
-ion chamber with very high voltage across chamber
-incident photon interaction produces small number of free electrons, which are accelrated towards the positive anode, gaining energy
-energetic electrons cause more electrons to be ejected from gas atoms, which are also accelerated, ejecting more electrons (i.e. avalanche)
-any incident beta particle or photon results in similar signal
-very sensitive, used to detect contamination
-each detected photon gives a click
-cannot measure dose rate or tube output, same response regardless of incident photon or particle energy
explain pocket dosimeters
ion chambers that look like large pens
-use a positively charged quartz fiber suspended in an air-filled chamber
-x-rays incident on chamber produce ions that neutralize the charge and cause the fiber to move
-0-2 mGy
-can be recharged
-gives immediate readings
what types of doses are measured for radiation workers?
-skin dose (shallow)
-eye lens dose (can use dosimeter on glasses)
-deep dose
convert mrem to mSv
100 mrem = 1 mSv
for what time period is worker exposure recorded?
current period
current quarter
current year
lifetime
what is NCRP
National committee council on radiological protection and measurements
what does NCRP recommend we take as the effective dose to the worker (from the dosimeter)
0.18 of the dose recorded by dosimeter worn on collar
if worker also wears dosimeter on waist then, 1.5 Hwaist + 0.04 Hcollar
good way to estimate dose to extremities
dosimeter on a finger
regulatory organizations
ICRU (International commission on radiological units and measurements)
UNSCEAR (UN Scientific committee on effects of atomic radiation)
BEIR (National academy of sciences committee on biological effects of ionizing radiation)
ICRP (International commission on radiological protection)
NCRP (National Council on radiation protection)
NRC (Nuclear regulatory commission) - responsible for regulating radioactive materials in US
occupational dose limits
-exclude those from medical exams or background radiation
-100 mSv/5 years with no more than 50 mSv in a given year
-NCRP uses lifetime effective dose limit of 10 times the individual’s age
tyoical dose actually received by radiation workers
- 5 mSv/year unless emergency occupational exposure
eye lens dose limit for radiation worker
-150 mSv/year
-2011 ICRP recommended reducing it to 20 mSv/year but not many countries have adopted this
skin dose limit for radiation worker
500 mSv/year
-skin doses are to be averaged over the most highly exposed 1 cm2
extremity dose limit for radiation worker (ex hands)
500 mSv/year
dose limit for members of public
1 mSv/year
-excludes natural background radiation and medical tests
dose limit for pregnant workers
0.5 mSv/month during pregnancy (5 mSv total)
monitored by dosimeter worn on abdomen
In Canada 4 mSV for duration
US population average dose from diagnostic medical exams
3 mSv
-population doses from diagnostic radiology increased 6 fold in one generation
-CT scans account for 17 % of diagnostic x-ray exams, but half the population medical dose
what is DAM
Don’t order tests that don’t affect management
-patient medical exams have to be justified
ALARA
As low as reasonably achievable
-eliminate unecessary radiation
-eyes, lens, gonads, and breasts may be shielded for some applications
-decrease exposure time, increase distance, use shielding
medical exams on pregnant women
-when x-ray beam does not directly irradiate the fetus or embryo, the corresponding dose will be low
-if a woman has exam and is unaware she is pregnant:
-no medical action for unintended dose up to 100 mGy as risks are low compared to pregnancy
-high doses during organogenesis may warrant consideration of abortion
what is radiation protection in workers designed to do (2 things)
-prevent deterministic effects
-reduce stochastic risks
what is scatter dose level from patients at 1 m?
0.1% of entrance skin dose
benefit of using last minute hold technique during fluoro
patients receive no radiation exposure
ways to reduce exposure during fluoro
-last minute hold technique
-pulsed fluoro rates of 15 frames/s or less
-reduce number of spot images or photospot images where possible
-maximize distance between worker and source
-because of IS law increasing distance is more effective than reducing exposure time
how much Pb do x-ray rooms usually contain in their walls?
1.6 mm (2 m)
what is more effective? shielding, time or distance?
shielding
then distance
then time
60 kV Pb HVL and TVL
0.1 mm and 0.35 mm
80 kV HVL and TVL
0.2 mm and 0.65 mm
130 kV HVL and TVL
0.3 mm and 0.9 mm
sources of workers radiation dose
-dose to 90% of x-ray technologists are below detection limit
-sources of doses include fluoro, interventional radiology, nuclear medicine
fluoro worker yearly dose
1 mSv
nuc med worker yearly dose
2-3 mSv
how thick are Pb aprons
0.25 or 0.5 mm
less effective for higher energies
dose nuclear power workers get yearly
5 mSv
dose air crew workers get yearly
5 mSv
US public exposure from consumer products and activities per year
0.1 mSv
-a third is from tobacco
-a quarter is from radioactivity in building materials
-another quarter is from commercial aviation
medicine and aviation account for what % of the population dose from occupational exposure?
80%
what do occupational doses contribute to US population doses per year?
0.01 mSv
what does nuclear fuel cycle contribute to US population dose per year?
< 0.01 mSv
patient skin dose for lateral skull x-ray
1 mGy
patient skin dose for 1 min fluoro
10 mGy
patient skin dose for CT scan
40 mGy
