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Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

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182 CHAPTER 5<br />

TABLE 5-4. Values of the Mass Energy–Absorption Coefficients<br />

MASS-ENERGY-ABSORPTION COEFFICIENT (μen/ρ)cm2 PHOTON<br />

/g<br />

ENERGY Compact<br />

(MeV) Water Air Bone Muscle<br />

0.010 4.89 4.66 19.0 4.96<br />

0.015 1.32 1.29 5.89 1.36<br />

0.020 0.523 0.516 2.51 0.544<br />

0.030 0.147 0.147 0.743 0.154<br />

0.040 0.0647 0.0640 0.0305 0.0677<br />

0.050 0.0394 0.0384 0.158 0.0409<br />

0.060 0.0304 0.0292 0.0979 0.0312<br />

0.080 0.0253 0.0236 0.0520 0.0255<br />

0.10 0.0252 0.0231 0.0386 0.0252<br />

0.15 0.0278 0.0251 0.0304 0.0276<br />

0.20 0.0300 0.0268 0.0302 0.0297<br />

0.30 0.0320 0.0288 0.0311 0.0317<br />

0.40 0.0329 0.0296 0.0316 0.0325<br />

0.50 0.0330 0.0297 0.0316 0.0327<br />

0.60 0.0329 0.0296 0.0315 0.0326<br />

0.80 0.0321 0.0289 0.0306 0.0318<br />

1.0 0.0311 0.0280 0.0297 0.0308<br />

1.5 0.0283 0.0255 0.0270 0.0281<br />

2.0 0.0260 0.0234 0.0248 0.0257<br />

3.0 0.0227 0.0205 0.0219 0.0225<br />

4.0 0.0205 0.0186 0.0199 0.0203<br />

5.0 0.0190 0.0173 0.0186 0.0188<br />

6.0 0.0180 0.0163 0.0178 0.0178<br />

8.0 0.0165 0.0150 0.0165 0.0163<br />

10.0 0.0155 0.0144 0.0159 0.0154<br />

Source: FromPhysical Aspects of Irradiation. Washing<strong>to</strong>n, DC: US Government Printing Office; March 1964. NBS<br />

Handbook 85.<br />

neutrons. These fission neutrons have a wide range of energies, as shown in Figure<br />

5-21. The distribution peaks at 0.7 MeV and has a mean value of 2 MeV.<br />

Except for several fission fragments of very short half-life, there are no radionuclides<br />

that decay by emitting neutrons. Californium-252, however, an alpha emitter,<br />

undergoes spontaneous nuclear fission at an average rate of 10 fissions for every<br />

313 alpha transformations. Since the half-life of 252 Cf due <strong>to</strong> alpha emission is<br />

2.73 years, its effective half-life, including spontaneous nuclear fission, is 2.65 years.<br />

Californium-252 thus simulates a neutron-emitting radionuclide. The neutron emission<br />

rate has been found <strong>to</strong> be 2.31 × 10 6 neutrons per second per μg 252 Cf. The<br />

emitted neutrons span a wide range of energies. The most probable energy is about<br />

1 MeV, while the average value of the energy distribution is about 2.3 MeV.<br />

All other neutron sources depend on nuclear reactions for the emission of neutrons.<br />

Copious neutron beams may be produced in accelera<strong>to</strong>rs by many different<br />

reactions. For example, bombardment of beryllium by high-energy deuterons in a<br />

cyclotron produces neutrons according <strong>to</strong> the reaction<br />

9 2<br />

4Be + 1D → 11 5B ∗ 10<br />

→ 5B + 1 0n. (5.48)

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