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Alpha Radiation Ch001.qxd 6/11/07 4:01 PM Page 71 – 1 – Alpha Radiation 1.1 INTRODUCTION The alpha particle, structurally equivalent to the nucleus of a helium atom and denoted by the Greek letter ␣, consists of two protons and two neutrons. It is emitted as a decay prod- uct of many radionuclides predominantly of atomic number greater than 82. For example, the radionuclide americium-241 (241Am) decays by alpha-particle emission to yield the daughter nuclide 237Np according to the following equation: 241 → 237 ϩϩ4 95Am93 Np2 He563. MeV (1.1) The loss of two protons and two neutrons from the americium nucleus results in a mass reduction of four and a charge reduction of two on the nucleus. In nuclear equations such as the preceding one, the subscript denotes the charge on the nucleus (i.e., the number of protons or atomic number, also referred to as the Z number) and the superscript denotes the mass number (i.e., the number of protons plus neutrons, also referred to as the A number). The 5.63 MeV of eq. (1.1) is the decay energy, which is described subsequently. 1.2 DECAY ENERGY The energy liberated during nuclear decay is referred to as decay energy. Many reference books report the precise decay energies of radioisotopes. The value reported by Holden (1997a) in the Table of Isotopes for the decay energy of 241Am illustrated in eq. (1.1) is 5.63 MeV. Energy and mass are conserved in the process; that is, the energy liberated in radioactive decay is equivalent to the loss of mass by the parent radionuclide (e.g., 241Am) or, in other words, the difference in masses between the parent radionuclide and the prod- uct nuclide and particle. We can calculate the energy liberated in the decay of 241Am, as well as for any radioiso- tope decay, by accounting for the mass loss in the decay equation. Using Einstein’s equa- tion for equivalence of mass and energy E ϭ mc2 (1.2) 71 Ch001.qxd 6/11/07 4:01 PM Page 72 72 Chapter 1: Alpha Radiation we can write the expression for the energy equivalence to mass loss in the decay of 241Am as ϭϪϪMM 2 Q ()241Am 237 Np Mc␣ (1.3) where Q is the disintegration energy released in joules, M241Am, M237Np, and M␣ are the masses of 241Am, 237Np, and the alpha particle in kg and c is the speed of light in a vacuum, 3.00 ϫ 108 m/sec). When the nuclide masses are expressed in the more convenient atomic mass units (u) the energy liberated in decay equations can be calculated in units of mega- electron volts according to the equation ϭϪϪMM Q ()(.241Am 237 Np M␣ 931 494 MeV/u) (1.4) The precise atomic mass units obtained from reference tables (Holden, 1997a) can be inserted into eq. (1.4) to obtain Q ϭϪϪ(.241 056822uuuMeV/u 237 . 048166 4 . 00260325 )(. 931 494 ) ϭ (.0000605275uMeV/u)( 931 . 494 ) ϭ 563. MeV The energy liberated is shared between the daughter nucleus and the alpha particle. If the parent nuclide (e.g., 241Am) is at rest when it decays, most of the decay energy will appear as kinetic energy of the liberated less-massive alpha particle and only a small fraction of the kinetic energy remaining with the recoiling massive daughter nuclide (e.g., 237Np). The kinetic energy of the recoiling daughter nuclide is comparable to that of a recoiling canon after a shell is fired; the shell being analogous to that of the alpha particle shooting out of the nucleus (Figure 1.1). Figure 1.2 illustrates the transitions involved in the decay of 241Am. The interpretation of this figure is given in the following paragraph. There are four possible alpha-particle tran- sitions in the decay of 241Am each involving an alpha-particle emission at different energies Figure 1.1 The alpha particle shoots out of a nucleus. In the decay of nuclides by alpha-particle emission the alpha particle will shoot out of the nucleus, and the daughter nucleus will recoil similar to that of a canon that recoils upon firing a shell from its barrel. Ch001.qxd 6/11/07 4:01 PM Page 73 Chapter 1: Alpha Radiation 73 and relative abundances. These are illustrated in Figure 1.2. The decay energy of 5.63 MeV for 241Am calculated above and reported in the literature is slightly higher than any of the alpha-particle energies provided in Figure 1.2. This is because there remains also the recoil energy of the daughter nucleus and any gamma-ray energy that may be emitted by the daughter, when its nucleus remains at an excited state. The recoil energy, Erecoil, of the daughter nucleus is a very small part of the total energy of transition from parent to daughter nuclide, as it is inversely proportional to the mass of the nucleus as described by the following equation derived by Ehman and Vance (1991) ⎛ ⎞ ϭ M␣ Erecoil ⎜ ⎟ E␣ (1.5) ⎝ Mrecoil ⎠ where M␣ is the mass of the alpha particle as defined in eq. (1.3), Mrecoil is the mass of the recoil nucleus and E␣ is the alpha-particle energy. For example, the recoil energy of Figure 1.2 Decay scheme of 241Am. The relative abundances (intensities) of alpha-particle and gamma-ray emissions are expressed in percent beside the radiation energy values in MeV. Ch001.qxd 6/11/07 4:01 PM Page 74 74 Chapter 1: Alpha Radiation the 237Np daughter nucleus for the transition of the 5.545 MeV alpha particle (Figure 1.2) can be calculated according to eq. (1.5) as ⎛ 4. 00260325 u ⎞ E ϭ ⎜ ⎟ 5. 545 MeV recoil ⎝ 237. 0481u ⎠ ϭ (.0 0168851 )(. 5 5445 MeV) ϭ 0. 0936 MeV The transition energy, Etrans, for the above alpha-particle emission is the sum of the alpha particle and recoil nuclear energies or ϭϩ EEEtrans␣ recoil ϭϩ5.. 545MeV 0 0936 MeV (1.6) ϭ 563. MeV In the above case, the transition energy turns out to be equal to the reported and calculated decay energy, because as illustrated in Figure 1.2 the 241Am radionuclides decay directly to the ground state whenever 5.545 MeV alpha particles are emitted. This is not the case when alpha particles of other energies are emitted from 241Am. If we take, for example, the 5.486 MeV alpha-particle transition of Figure 1.2, the decay energy, Edecay, would be the sum of the tran- sition energy plus gamma-ray energy, E␥, emitted from the daughter nucleus or ϭϩ EEEdecay trans ␥ ϭϩ ϩ EE␣␥recoil E ⎛ M ⎞ ϭϩE ␣ EE ϩ ␣ ⎜ ⎟ ␣␥ (1.7) ⎝ Mrecoil ⎠ ϭ 54.886MeVϩϩ(. 0 0168851 )(. 5 486 MeV) 0 . 059 MeV ϭϩ5.. 486MeV 0 0926 MeV ϩ 0 ..059 MeV ϭ 563. MeV The gamma-ray energy emitted from the daughter nucleus for the 5.486 MeV alpha-particle transition in 241Am decay is found in Figure 1.2. Gamma-ray energy values of other radio- nuclides are available from reference tables (Browne et al., 1986; Firestone et al., 1996; L’Annunziata, 2003d). The emission of gamma radiation often accompanies radionuclide decay processes that occur by alpha-particle emission. The properties and interactions of gamma radiation are discussed in Chapter 3. As described in the previous paragraphs alpha particles are emitted with a certain quan- tum of energy as the parent nuclide decays to a lower energy state. The energy emitted from radionuclides as nuclear radiation can be described by a decay scheme such as that given in Figure 1.2. Decay schemes are written such that the energy levels of the nuclides are plateaus along the ordinate, and these energy plateaus are distributed along the abscissa according to atomic number. The alpha particles, as the example shows (Figure 1.2), are emitted with Ch001.qxd 6/11/07 4:01 PM Page 75 Chapter 1: Alpha Radiation 75 certain magnitudes of kinetic energy, which is most often expressed in units of megaelectron volts. The energies of alpha particles from most nuclear decay reactions fall within the range 1–10.5 MeV. Alpha particles are emitted from unstable nuclei with discrete quanta of energy, often leaving the daughter nuclide at an excited energy state. In such cases, when the daughter nuclide occurs at an elevated energy state, it may reach the ground state via the emission of energy in the form of electromagnetic gamma radiation as illustrated in Figure 1.2. The nuclei of daughter atoms of alpha-particle-emitting nuclides are often unstable them- selves and may also decay by further alpha-particle emission. Thus, alpha-particle-emitting nuclides may consist of a mixture of radionuclides, all part of a decay chain, as illustrated in Figure 8.7 of Chapter 8. Additional reading on radionuclide alpha decay is available from Das and Ferbel (1994). 1.3 ALPHA-PARTICLE INTERACTIONS WITH MATTER Now consider what happens to an alpha particle that dissipates its kinetic energy by inter- action with matter. Alpha particles possess a double positive charge due to the two protons present. This permits ionization to occur within a given substance (solid, liquid, or gas) by the formation of ion pairs due to coulombic attraction between a traversing alpha particle and atomic electrons of the atoms within the material the alpha particle travels. The two neutrons of the alpha particle give it additional mass, which further facilitates ionization by coulombic interaction or even direct collision of the alpha particle with atomic electrons. The much greater mass of the alpha particle, 4 u, in comparison with the electron (5 ϫ 10Ϫ4 u) facili- tates the ejection of atomic electrons of atoms through which it passes, either by direct col- lision with the electron or by passing close enough to it to cause its ejection by coulombic attraction.
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