Measurement and Instrumentation

Question 1

The term “tissue equivalent” when used to describe ionization chamber construction means that that the:

Answer 1

Chemical composition of the wall material and gas filling the ionization chamber are selected such that the response of the ionization chamber will be a measure of the absorbed dose in tissue

Question 2

A polyethylene-walled ionization chamber filled with ethylene gas measuring a field of photons and neutrons will

Answer 2

measure the absorbed dose from the photons and will give an over-estimate oft he absorbed dose from neutrons. (the ratio for the hydrogen atom density in poly to tissue is ~1.4, thus producing an over-response)

Question 3

The absorbed dose rate measured by a polyethylene-walled, ethylene gas filled ionizaiton chamber in a 5 rem/h fast neutron field (QF=5) would most likely be

Answer 3

1.5 rad/hr (5 rem/hr / QF) 1.4 hydrogen atom density ratio) = 1.4

Question 4

Initial (aka columnar) recombination of ion pairs produced in an ionization chamber gas is

Answer 4

independent of dose rate initial recombination occurs along tracks of the charged particles so it mostly depends on LET

Question 5

Volume recombination in the ionization chamber gas is

Answer 5

dependent on dose rate Volume recombination occurs outside of the initial ionization tracks, so average density of the ions in a volume is important. More dose = more ions.

Question 6

Initial recombination in a high pressure tissue equivalent ionization chamber can be used to

Answer 6

determine the average quality factor in a mixed neutron and photon field

Question 7

The number of ion pairs collected in an air-filled aluminum walled ionization chamber depends primarily on

Answer 7

absorbed dose rate

Question 8

Ion pair collection in ionization chambers used to monitor pulsed radiation depends primarily on

Answer 8

the ion pairs produced during the pulse

Question 9

An aluminum ion chamber filled with C02 will best

Answer 9

Measure the high energy neutron fluence in mixed radiation fields (n, alpha) and (n,p) reactions on Al-27

Question 10

Which is more efficient at collecting ion pairs produced in ionization chambers: Parallel plate electrodes Cylindrical-shaped collecting electrodes Spherical-shaped collecting electrodes

Answer 10

Parallel plate PP = X CE = X/r SE = X/(r^2)

Question 11

Best type of dosimeter to measure high-energy neutrons

Answer 11

Proton recoil badge

Question 12

Best type of dosimeter material to measure lower-energy neutrons

Answer 12

Li-6

Question 13

For TLDs, one reason LiF is widely used is because

Answer 13

LiF-based materials are a close match between the atomic number of its constituents and those of soft tissue. Energy deposited in LiF is closely correlated with the gamma-ray exposure over a wide range of gamma-range energy

Question 14

Good TLD material for a wide range of gamma-ray energies

Answer 14

Li-7

Question 15

Good TLD dosimeter for high energy photons

Answer 15

Calcium Sulfate, dysprosium-activated TLD

Question 16

For atmospheric air ionization chambers, describe a potential source of error with measuring gamma and beta dose rates while moving in and out of a noble gas environment

Answer 16

the “memory effect” is present. The noble gas seeps inside the ion chamber and will remain the chamber as the meter moves out of the cloud. This will yield a false-positive reading

Question 17

List two environmental conditions which could have an adverse effect on the accuracy of the open air ionization chamber response while in an area

Answer 17

Temperature, pressure. Both affect the density of the air within the chamber and will therefore directly affect the number of ion pairs produced

Question 18

Most significant source of error associated with measuring true beta and gamma surface dose rates from contact measurements of small sources (open air ionization chamber)

Answer 18

Geometry factor between the source and the receptor. Instrument is tied to the number of ions collected at the center of the detector, not the dose rate at the surface of the source. The smaller the source, the smaller the volume irradiated.

Question 19

For open air ionization chambers, explain a source of error associated with measuring beta dose rates from large area sources, with each source compromised of a different radionuclide

Answer 19

Beta energy for emitted particles. Higher-energy beta particles respond closer to the true beta dose rate. Lower-energy beta particles under respond. This is because beta particles are attenuated across the chamber depending on their energy.

Question 20

What is the purpose of a build-up cap on an ionization chamber?

Answer 20

Ensure charged particle equilibrium in the chamber. Specifically, it ensures equilibrium distribution of charge is present at the detector wall or at the interface between the wall and the chamber gas.

Question 21

Optimized sample counting time for reducing errors within a set period of time

Answer 21