radiation detection & gamma surveys lec 7&8

Question 1

specific activity

Answer 1

amount of activity to a gram of material
Radioactive material with long half-lives have low specific activity.

Question 2

why are neutrons hard to detect

Answer 2

no charge
can cause damage to the lattice

Question 3

more charge=

Answer 3

more friction so shorter range

Question 4

attenuations

Answer 4

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)

Question 5

Shielding

Answer 5

neutrons need something hydrous
gamma (concrete or lead)

Question 6

higher atomic number

Answer 6

larger u in general

Question 7

Photoelectric effect (gamma interaction)

Answer 7

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

Question 8

Compton scattering (gamma interaction)

Answer 8

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

Question 9

pair production (gamma interaction)

Answer 9

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.

Question 10

types of detectors -gas detectors

Answer 10

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)

Question 11

types of detectors- solid state detectors
scintillator

Answer 11

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

Question 12

scintillation detection 6 steps

Answer 12

• 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

Question 13

solid state detectors- semiconductors

Answer 13

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

Question 14

gamma surveying

Answer 14

geophysics technique
gamma spectrometers
solid state and scintillate for surveys
aerial- can be unmaned

Question 15

gamma spectrometers

Answer 15

can identify specific radionuclides
measurement are usually slow and expensive
typically laboratory based

Question 16

radiometric survey measures the spatail distribution of three radioactive elements

Answer 16

potassium, thorium, uranium
use thing further down the decay chain- daughter emits gamma
energy of gamma rays characteristic of element

Question 17

gamma rays stopped by

Answer 17

water and other molecules -hence rad map only detects near surface sources
wet soil hinders survey

Question 18

gamma ray spectra U

Answer 18

gammas lower down in the chain
have to assume its in secular equilibrium- takes the longest half life to refill all the decay chain

Question 19

secular equilibrium

Answer 19

activity of any of the isotopes present is the same as that of the initial parent
1:1 ratio

Question 20

survey methodology
flight line direction

Answer 20

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

Question 21

survey methodology
flight line spacing

Answer 21

determined by budget
1km line spacing typical
wide line spacing first- then closer line spacing in contaminated areas

Question 22

calibrations and processing
equipment dead time

Answer 22

time it takes detector to process
if 10% of time not collecting, times it by 1.1 to bring back the dead time

Question 23

calibrations and processing
cosmic background rad

Answer 23

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

Question 24

calibrations and processing
radon background

Answer 24

sticks to everything

Question 25

calibrations and processing
survey altitude

Answer 25

radiation dispersal
higher lose radiation -drops of exponentially

Question 26

calibrations and processing
radioelement concentrations

Answer 26

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

Question 27

gamma surveying all the corrections

Answer 27

dead time, background, stripping, attenuation correction
all standardized one meter above surface

Question 28

Non-collimated mapping is useful for

Answer 28

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.

Question 29

background dose rad for Bristol

Answer 29

0.2-0.3 uSv/hr

Question 30

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?

Answer 30

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.

Question 31

attenuation length -length to stop radiation

Answer 31

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

Question 32

Thermoluminescent dosimeter / film badge good for

Answer 32

Monitoring exposure of personnel during routine work in a nuclear power station

Question 33

pulse mode- semiconductors

Question 34

current mode- semi conductors

Answer 34

No saturation, no preamplifier and no upper count rate

Question 35

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?

Answer 35

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.