radiation detection & gamma surveys lec 7&8 Flashcards
specific activity
amount of activity to a gram of material
Radioactive material with long half-lives have low specific activity.
why are neutrons hard to detect
no charge
can cause damage to the lattice
more charge=
more friction so shorter range
attenuations
When an energetic charged particle passes through matter it will rapidly slow down and lose its energy by interacting with the atoms of the material (detector or body)
Shielding
neutrons need something hydrous
gamma (concrete or lead)
higher atomic number
larger u in general
Photoelectric effect (gamma interaction)
electron comes in, kicks out a electron, complete absorbion, no scattering, has to be higher than the binding energy so enough energy to kick it out
emits charteristic energy
0.03MeV
Compton scattering (gamma interaction)
interaction happening in the electron cloud
electron has bit more energy, nocks electron and scatters, gamma energy continues and can hit another one, lots of low energy event
annoying for shielding or counting
pair production (gamma interaction)
positron annihilation
high energy comes along (<1.02MeV), interacts with nucleus creating electron/positron pair
Both the electron and the positron lose energy via ionization until an
annihilation event takes place yielding two photons of 0.51 MeV moving in
opposite directions.
types of detectors -gas detectors
geiger counters -alpha beta gamma
just tells you something is there (clicks)
cheap, easy, sensitive, no specific
uses light gas (he), need very thin window for alpha (diff windows-diff rad)
rad comes in causes collisions /ionizations and clicks (chain reaction)
types of detectors- solid state detectors
scintillator
flash of light when interacts with something (electromagnetic rad)
will detect alpha gamma and beta (w a thin window)
bigger flash of light-more rad
500 pounds
scintillation detection 6 steps
- Inside a scintillator:
1. Excitation due to absorption of radiation
2. Emission of light photons from de-excitation
3. Transit of light to photocathode inside photomultiplier tube - Inside a photomultiplier tube:
4. Production of photoelectrons in photocathode
5. Multiplication of photoelectrons - At the back-end of the scintillator and photomultiplier tubes:
6. Conversion of electronic detector output to useful information
solid state detectors- semiconductors
moving electrons to the hole
little fluctuation in output for a given energy of rad, fast, higher spectral resolution, sensitive to vibration
expensive, need to be cooled
gamma surveying
geophysics technique
gamma spectrometers
solid state and scintillate for surveys
aerial- can be unmaned
gamma spectrometers
can identify specific radionuclides
measurement are usually slow and expensive
typically laboratory based
radiometric survey measures the spatail distribution of three radioactive elements
potassium, thorium, uranium
use thing further down the decay chain- daughter emits gamma
energy of gamma rays characteristic of element
gamma rays stopped by
water and other molecules -hence rad map only detects near surface sources
wet soil hinders survey
gamma ray spectra U
gammas lower down in the chain
have to assume its in secular equilibrium- takes the longest half life to refill all the decay chain
secular equilibrium
activity of any of the isotopes present is the same as that of the initial parent
1:1 ratio
survey methodology
flight line direction
flight direction should be perpendicular to geological structure of interest
in mountain terrain -follow contours of the ground
fallout monitoring- line should be perpendicular to the winds which were blowing at the time
survey methodology
flight line spacing
determined by budget
1km line spacing typical
wide line spacing first- then closer line spacing in contaminated areas
calibrations and processing
equipment dead time
time it takes detector to process
if 10% of time not collecting, times it by 1.1 to bring back the dead time
calibrations and processing
cosmic background rad
the higher the survey the more background
need to take it away from the values measured
no terrestrial gamma rays have energies above 3 MeV so we bin all of them
flights carried out over the sea when on-shore breeze so radon contribution to all channels in negligible
calibrations and processing
radon background
sticks to everything
calibrations and processing
survey altitude
radiation dispersal
higher lose radiation -drops of exponentially
calibrations and processing
radioelement concentrations
fully corrected count rate data can be used to estimate the concentrations in
the ground of each of the three radioelements, K, U and Th
Concentration of element=count rate/source sensitivity
gamma surveying all the corrections
dead time, background, stripping, attenuation correction
all standardized one meter above surface
Non-collimated mapping is useful for
improved coverage. With a collimated mapper you will be more sensitive to a smaller area of ground and might suffer large physical gaps in your data
and lower count rates.
background dose rad for Bristol
0.2-0.3 uSv/hr
Explain why it is inappropriate to model the Fukushima fallout zone as a point source of radiation for the purposes of mapping. Does the inverse-square law still apply?
Blanketed with radioactive material so acts as a wide planar (rather than point) source. A planar source can be thought of as many point sources contributing simultaneously. Inverse square law may provide a rough estimation but no longer really valid – as the detector moves away it samples contributions from more sources.
attenuation length -length to stop radiation
bigger AL =more shielding and takes longer to stop
pb = high mass, long linear attenuation length (good blocker)
polyethylenes = low mass, short linear attenuation length
Thermoluminescent dosimeter / film badge good for
Monitoring exposure of personnel during routine work in a nuclear power station
pulse mode- semiconductors
current mode- semi conductors
No saturation, no preamplifier and no upper count rate
Measurements for radon gas are often carried out during house surveys. Why might this isotope be
present in areas containing granite? Which areas of a house might you expect to find it accumulate?
Granites contain uranium, decaying to radon. Volatile radon gas can escape the rock into basements which generally show the highest levels. Radon denser than air so accumulates in depressions.
