Biomedical Engineering Reference
In-Depth Information
Table 12.5
Mean Quality Factors
Q
and Fluence Rates for
Monoenergetic Neutrons that Give a Maximum
Dose-Equivalent Rate of 1 mSv in 40 h
Fluence Rate (cm
-2
s
-1
)
Neutron Energy (eV)
Q
0.025 (thermal)
2
680
0.1
2
680
1.0
2
560
10.0
2
560
10
2
2
580
10
3
2
680
10
4
2.5
700
10
5
7.5
115
10
5
5
×
11
27
10
6
11
19
10
6
5
×
8
16
10
7
6.5
17
10
7
1.4
×
7.5
12
10
7
6
×
5.5
11
10
8
4
14
10
8
4
×
3.5
10
Source
:From
Protection Against Neutron Radiation
,NCRP
Report No. 38, National Council on Radiation Protection and
Measurements, Washington, D.C. (1971). In its 1987 Report
No
.
91, the NCRP recommends multiplying the above values
of
Q
by two (and reducing the above fluence rates by this
factor).
12.7
Dose Measurements for Charged-Particle Beams
For radiotherapy and for radiobiological experiments one needs to measure the
dose or dose rate in a beam of charged particles. This is often accomplished by
measuring the current from a thin-walled ionization chamber placed at different
depths in a water target exposed to the beam, as illustrated in Fig. 12.8. The dose
rate is proportional to the current. For monoenergetic particles of a given kind
(e.g., protons) the resulting “depth-dose” curve has the reversed shape of the mass
stopping-power curves in Fig. 5.6. The dose rate is a maximum in the region of
the Bragg peak near the end of the particles' range. In therapeutic applications,
absorbers or adjustments in beam energy are employed so that the beam stops at
the location of a tumor or other tissue to be irradiated. In this way, the dose there (as
well as LET) is largest, while the intervening tissue is relatively spared. To further
spare healthy tissue, a tumor can be irradiated from several directions.
If the charged particles are relatively low-energy protons (
400 MeV), then es-
sentially all of their energy loss is due to electronic collisions. The curve in Fig. 12.8
will then be similar in shape to that for the mass stopping power. Higher-energy
