Instrumentation and Detection

Question 1

The function of a cascade impactor is to seperate

Answer 1

Particles according to mass

Question 2

Purpose of an ‘annular kinetic impactor head’ is to trap

Answer 2

Airborne particles. Does not collect radon and thoron

Question 3

What scintillator material has a high efficiency for alpha measurement?

Answer 3

ZnS. NaI is used for gamma spec.

Question 4

Approximate efficiency for radioiodines on an activated charcoal filter at the appropriate flow rate

Answer 4

95%

Question 5

Disadvantage of a long sample counting time for optimization of counting statistics?

Answer 5

Background may change

Question 6

Approximate eifficiency for the detection of Xenon-133 on activated charcoal

Answer 6

1%. It is a noble gas, so it doesn’t want to interact with anytihng

Question 7

Primary function of proportional counters is to

Answer 7

Measure and distinguish alpha from beta

Question 8

A low energy alpha detector is usually effective if the detector is _____ distance from the source

Answer 8

1/4 inch. Low energy alphas can only travel less than 1/2 inch in air

Question 9

Proportional counters have gas amplification that are

Answer 9

greater than 1.

Question 10

Ion chambers have gas amplification factors of:

Answer 10

equal to 1

Question 11

A proportional counter has enough voltage to cause

Answer 11

Secondary electrons with short range avalanche

Question 12

What radiation instrument operates on the principal of discharging of a capacitor to estimate radiation dose?

Answer 12

Pocket ion chamber

Question 13

An ionization chamber has voltage great enough to cause

Answer 13

Ions before recombination; no secondary electrons

Question 14

A geiger counter has enough voltage to cause

Answer 14

Secondary electrons with a long range avalanche

Question 15

A scintillation counter takes advantage of properties of materials that _____, after being exposed to energy from radioactive sources

Answer 15

Fluoresce

Question 16

Semi-conductor detectors are used primarily for their:

Answer 16

High energy resolution

Question 17

Cost effective means available to test for radon gas in a building

Answer 17

Alpha track detector, Charcoal cannister

Question 18

Very thin windowed GM detectors can detect what types of ionizing radiation

Answer 18

Alpha, Beta, Gamma

Question 19

Full Width Half Maximum (FWHM) of the photopeak is a measurement of

Answer 19

Resolution

Question 20

What is the effective half life in the human body for Co-60, if the biological half-life is eight days?

Answer 20

7.97 days. The effective half-life is never greater than the biological half-life. Teff= (Tb*Tr)/(Tb+Tr)

Question 21

Absolute efficiency

Answer 21

Number of pulses recorded/number of radiation quanta emitted by source

Question 22

Intrinsic efficiency

Answer 22

Number of pulses recorded/number of radiation quanta incident on the detector

Question 23

Relationship between E-absolute and E-intrinsic

Answer 23

E-Intrinsic = E-absolute * (4(pi)/Solid angle of detector)

Question 24

% resolution formula for photo-peaks

Answer 24

% resolution = (FWHM/Energy Peak)*100

Question 25

Constituents of fill gas P-10

Answer 25

90% argon 10% methane. Used in gas-filled detectors in the proportional and G-M regions. The argon serves as the ionization gas, the methane is an organic quench gas.

Question 26

Formula for standard deviation in counting

Answer 26

1 Sigma = (Sample Count Rate/Sample Count Time)^1/2

Question 27

Quality factor is a function of the

Answer 27

Collision stopping power of the radiation in water

Question 28

Why is sulfur used in criticality accident dosimetry?

Answer 28

The S-32(n,P)P-32 reaction produces measurable beta activity.

Question 29

Resolving time/Dead time correction

Answer 29

True count rate = recorded count rate/(1-Recorded rate*dead time)

Question 30

Isokinetic sampling

Answer 30

Constant velocity sampling. V1(D2)^2=V2(D1)^2

Question 31

95% confidence level

Answer 31

2 sigma= 1.96[(bkg rate /time)+(sample rate /sample time))]^.5

Question 32

What is the ionizing region

Answer 32

The region of voltage over which the saturation current is produced. Saturation current is when one electron reaches the detector electrode for each electron released by ionization in the detector.

Question 33

What instrument has a good sensitivity to neutrons while providing the best discrimination against gammas?

Answer 33

BF-3 proportional counter in polyethylene moderator. The proportional region of the detector is set at an alpha voltage to detect the B-10(n,alpha)Li-7 reaction.

Question 34

The most energetic Compton electron results from a 180 degree backscatter of the incident photon. This electron is called

Answer 34

Compton edge.

Question 35

Compton edge will occure at _____ of the photopeak energy

Answer 35

Photopeak energy - 250 keV

Question 36

To avoid excessive dead time on multi-channel gamma analyzers you should keep samples below:

Answer 36

10mR/hr at 2 inches.

Question 37

A semi-conductor is distinguished from an insulator or conductor by its:

Answer 37

Band Gap. Insulator gaps are large, 5 eV or more. In a semi-conductor the gap is in the range of 0.7-2.1 eV

Question 38

Dead time for proportional counters is approximately:

Answer 38

0.5 microseconds. GM counter dead times can exceed 100 microseconds.

Question 39

A sample with mixed alpha and beta emitting nuclides is counted with a gas flow proportional counter at the alpha voltage (A) and at the beta voltage (B). The alpha and beta count rates, respectively, may be estimated from these count rates as:

Answer 39

A and B-A

Question 40

Usual FWHM for the Cs-137 photopeak is

Answer 40

50 keV

Question 41

The term “tissue Equivalent” when used to describer ionization chamber construction means that the

Rev. 1

Answer 41

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

Question 42

A polyethylene-walled ion chamber filled with ethylene gas will

Rev. 1

Answer 42

Measure the absorbed dose from photons and will give an over-estimate of the absorbed dose from neutrons

Question 43

Initial recombination of ion pairs produced in an ionization chamber gas is

Rev. 1

Answer 43

Independent of dose rate. Recomb depends on LET

Question 44

Volume recombination (recombination during drift towards electrodes) in the ion chamber gas is

Rev. 1

Answer 44

Dependent on dose rate

Question 45

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

Rev. 1

Answer 45

Determine the average quality factor in a mixed neutron and photon field. (think LET dependence)

Question 46

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

Rev. 1

Answer 46

Absorbed dose rate

Question 47

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

Rev. 1

Answer 47

Ion pairs produced during the pulse

Question 48

An aluminum ion chamber filled with CO2 will best measure:

Rev. 1

Answer 48

the high energy neutron fluence in mixed radiation fields.

Question 49

The efficiency for collecting ion pairs produced in an ionization chamber is greater using:

Rev. 1

Answer 49

Parallel plate electrodes. (electrical field is stronger compared to cylindrical or spherical electrodes)