Physics 4 Answers
1
E.
Exposure is given by the charge liberated in air by photons per unit mass and expressed in C/kg (energy deposited per unit mass is absorbed dose; dose x QF is dose equivalent; and photons per unit area is fluence).
2
C.
Exposure measured in roentgen (R) is the charge liberated per unit mass of air.
3
E.
The linear energy transfer for alpha particles is high and equal to about 100 keV/μm.
4
B.
The LET for x-rays is in the range of 0.3 to 3 keV/μm, is much lower than the LET for alpha particles, and has no direct relationship with measurement of x-ray exposure.
5
B.
Dose equivalent (measured in sievert or rem) is the product of the absorbed dose (measured in gray or rad) and the quality factor (Note that in radiology the quality factor is generally 1.0; thus the dose equivalent and absorbed dose are numerically equal.)
6
A.
The f-factor converts exposure (roentgen) into absorbed dose (gray or rad). At diagnostic x-ray energies, f is approximately 1 for low Z materials like soft tissue and air increases with atomic number Z, and is generally in the range of 3 to 4 for bone.
7
D.
Bone will result in the highest dose since it has the highest f-factor (dose in bone will be about 4 rad since f for bone at 80 kVp will be about 4).
8.
A-iv;
B-ii
C-iii;
D-i.
9
E.
The sensitized grains, containing about 106 to 107 atoms, are reduced to specks of metallic silver which is black in appearance.
10
B.
The fixer removes unexposed silver halide.
11
D.
Film density, D, is given by log10(Io/It) where Io is the incident light intensity, and It is the transmitted light intensity.
12
C. Logarithm10(100) = 2.0.
13
B.
0.8. One advantage of the logarithmic scale is that film optical densities are additive.
14
E.
All of the film characteristics listed may be obtained directly from the characteristic curve.
15
D.
A gradient of 3 is a high contrast film and is likely to result in high image contrast (used in mammography).
16
B.
Increasing the developer temperature will generally increase the observed film fog level
17
C.
Portable chest x-rays are done at low kVps, which increases contrast (high exposures in the lung region and low exposures in the mediastinum) and would most likely benefit from wide latitude films.
18
C.
Charged couple devices (CCDs are used to detect light (not x-rays) as in video camcorders. To use CCDs in radiology a scintillator must be placed in front of the CCD to capture the x-rays and convert a fraction of the absorbed energy into light.
19.
A-False; ionization chambers are used to measure the total output (mR) of an x-ray tube;
B-False; G-M detectors give the same response irrespective of the energy of an absorbed photon;
C-True; as used by gamma cameras by nuclear medicine;
D-False; TLDs measure the total energy absorption, not the energy of single photons.
20.
A-ii; physicists use ionization chambers to measure x-ray tube outputs in mR or mR/mm;
B-iii; G-M detectors are very sensitive and ideal for detecting low-level contamination in a nuclear medicine department;
C-iv; NaI crystals coupled to a photomultiplier tube result in a signal that is proportional to the energy absorbed in the crystal, which is used for performing pulse height analysis to identify unknown radioisotopes;
D-i; TLDs are read out by measuring the emitted light during heating.
21
B.
Photocathodes absorb light photons and emit electrons.
22.
A-True;
B-True; photomultiplier tubes are used to detect the light emitted by thermoluminescent dosimeters as they are heated;
C-False; ionization chambers or diodes are normally used;
D-False;
E-True; photomultiplier tubes are normally used to detect the light emitted by any x-ray detector.
23
E.
Quenching gases are used in Geiger-Muller detectors to minimize the production of secondary discharges,
24
B.
Charged ions liberated by the radiation,
25.
A-ii;
B-i;
C-iii;