Appendix A: Radiation—Risks, Dose Assessment, and Shielding A.2.1 Alpha Decay A.1 Introduction In alpha decay, the nucleus of an element with mass number “Radiation” and “nuclear” are words feared by people. Even A and atomic number Z emits an alpha particle. Alpha in technologically developed countries, the public has little 1 1 particles are made of two protons and two neutrons, that is, or no knowledge of radiation, and when they do, they they are helium nuclei. Nucleus 1 (the “parent”) is replaced associate them with weapons, accidents, fallout, and cancer. by a new nucleus 2 (the “daughter”) whose mass number A Only a few know that about half of the Earth heating is due 2 is equal to A1—4 and whose atomic number Z2 is Z1—2, and to spontaneous fission of radioactive elements present in the 222 an alpha particle is emitted. For instance, Rn, ARn = 222, crust and mantle. Without this steady radioactive decay, the 218 ZRn = 86, decays into Po (Polonium 218), meaning that Earth would be a far colder place (Gando et al. 2011). The 222 the nucleus of Rn emits an alpha particle (Aa =4,Za = 2), radiation flux emerging from the crust is responsible for the leaving behind a nucleus whose mass number is (222 − 4) = so-called background radiation permeating our environment 218 and atomic number (86 − 2) = 84, that is, 218Po. since Earth was formed. Also, for the same amount of power The mass (energy) of the parent nucleus must exceed the generation, coal combustion spreads far more ash radioac- sum of the masses (energies) of the daughter nucleus and of tivity than nuclear waste (McBride et al. 1978; Hvistendahl the alpha particle emitted. This decay constraint can be 2007). Because only specialists (and not necessarily physi- expressed as follows (Mukhin 1987): cians) know about natural background exposure or medical use of radiation, proposals to use nuclear energy, in partic- ð ; Þ [ ð À ; À Þþ ð 4Þð: Þ M A Z M A 4 Z 2 M He A 1 ular on or for rockets, have always encountered strong resistance from policymakers and the public. The purpose of this appendix is to inform the non-specialist about what radiation and its dose are, about A.2.2 Beta Decay effects of radiation on humans, and about sources of radia- tion, including estimates of the dose from nuclear propulsion Beta decay is the spontaneous transformation of an unstable systems, its shielding, and impact on interplanetary travel. nucleus into a new nucleus with charge differing by DZ ¼ Æ1 due to the emission of an electron (b− decay) or a positron (b+ decay) or due to the capture of an electron A.2 Radioactivity (e-capture). In the b− decay, one of the neutrons of the nucleus Radioactivity is the process undergone by unstable nuclei becomes a proton after emitting an electron. The mass (radionuclides), as well as nuclei in excited states, causing number A does not change, while the new nucleus has an spontaneous changes, or transformations, in composition atomic number higher by 1. 3 and/or internal energy of the nucleus. This means that Tritium ( H, often symbolized by the symbol T), AT =3 − 3 radioactivity may change a chemical element into another, and ZT =1,b decays into He, AHe = 3 and ZHe =2, releasing or absorbing energy in the process. The most meaning that one of the two neutrons of the tritium nucleus common transformations are as follows: alpha decay, beta emits an electron and becomes a proton. The mass number decay, and gamma decay. A material that spontaneously does not change, i.e., AT = AHe, while the positive charge of emits such radiation due to decay is said radioactive. the new nucleus increases by 1 unit: © Springer-Verlag GmbH Germany 2018 381 P.A. Czysz et al., Future Spacecraft Propulsion Systems and Integration, Springer Praxis Books, DOI 10.1007/978-3-662-54,744-1 382 Appendix A: Radiation—Risks, Dose Assessment, and Shielding ZHe ¼ ZT þ 1 ðA:2Þ nucleus goes directly from an excited to the ground (stable) state following the emission of a single c quantum, or there fi The energy constraint speci es that the mass (energy) of may be multiple transitions, i.e., a cascade, bringing the the parent nucleus must be larger than the sum of the masses nucleus to the ground state and involving multiple emissions (energies) of the daughter nucleus and of the electron, as of c quanta. The energy of the c quantum emitted is deter- expressed by the following (Mukhin 1987): mined by the energy difference between the two energy levels of the transition. MðA; ZÞ [ MðA; Z þ 1Þþme ðA:3Þ Many mechanisms can excite nuclei and lead to gamma In the b+ decay, the unstable nucleus emits a positron (a radiation. Quite commonly, alpha and beta decays can leave positive electron). The b+ decay can be treated as the the nucleus in an excited state. An alpha decay is usually transformation of a proton into a neutron, because also in followed by the emission of low-energy (<0.5 MeV) c this case, the parent and the daughter nuclei have the same quanta, while after a beta decay, the c quanta emitted may mass number A, while the atomic number Z of the daughter have energy up to 2.0–2.5 MeV (Mukhin 1987). is lower by 1. The proton mass is lower than the neutron mass (energy). The transformation of the proton into a neutron is possible since the proton is bonded to a nucleus A.3 Radiation, Dose Quantities and the excess energy to become a neutron is extracted from and Units the nucleus itself. The energy constraint can be expressed in − analogy with the b case as follows (Mukhin 1987): An ad hoc set of quantities and related units required to describe radiation decay and its effects have been developed MðA; ZÞ [ MðA; Z À 1Þþm ðA:4Þ e since the effects of nuclear radiation were discovered and 11 + 11 For instance, C , AC = 11 and ZC =6,b decays into B , gradually understood (Klein 1988; US Nuclear Regulatory AB = 11 and ZB = 5, and the missing charge of Boron 11 is Commission 2008; Petrangeli 2006). A list of them follows. that of the positron emitted. The third type of beta decay is electron capture. It consists of the capture of an electron by a nucleus from its own electron A.3.1 Activity (Bq) shell. For heavy nuclei, where the K shell is close to the nucleus, this phenomenon, also called K-capture, is quite Given any radiation decay (a, b, c, etc.), the activity of an common. Captures from L shell (L-capture), M shell element is the rate at which any and all transitions (i.e., (M-capture), etc. have also been observed. After the capture, emissions of a, b, and c rays) occur. A radionuclide has an the nucleus has the same mass number A, but its atomic activity of 1 becquerel (Bq), when it undergoes one transi- number Z decreases by 1. The electron captured and one of the tion per second. An older unit is the curie (Ci), equivalent to protons of the nucleus become a neutron in the daughter 3.7 Á 1010 transitions per second. Mme Curie defined it as “… 7 nucleus. For instance, Be , ABe =7,ZBe = 4, after capturing an la quantité d’émanation en équilibre avec un gramme de 7 electron from its K shell, becomes Li , ALi =7,ZLi = 3. The radium …,” that is that quantity of radon-222 in equilibrium mass number does not change: ABe = ALi = 7, while the atomic with 1 g of its parent radium-226 (Anon 2016a). It is worth number Z of the lithium is lower by 1. The mass (energy) noting here that both SI units and old ones, partly deriving constraint is that the sum of the masses (energies) of the from the c.g.s. (cm, gram, second) system, are currently used captured electron and the parent nucleus must be larger than to define not only activity, but also most other radiation the mass (energy) of the daughter nucleus (Mukhin 1987): units. Units of activity and symbols are MðA; ZÞ [ MðA; Z À 1Þþm ðA:5Þ transition e 1 Á Bq ¼ 1 ðA:6aÞ second Because of the vacancy created in the electron shell, the transition of one of the shell electrons to that vacancy is 1 Á Ci ¼ 3:7 Á 1010Bq ðA:6bÞ accompanied by the emission of X-rays. Activity is not a synonym of power or energy; thus, it has nothing to do with the effects of radiation on matter, living or A.2.3 Gamma Rays not. “ ” Unstable nuclei going from a higher ( excited ) energy state A.3.2 Half-Life, T1/2 (s) down to a less energetic, and eventually stable, state can − emit energy quanta in the c-ray wavelength (10 8 k The half-life is the time over which half the nuclei of a given − 2.0 Á 10 11 cm). There may be single transitions, where the radionuclide decay. Depending on the radionuclide Appendix A: Radiation—Risks, Dose Assessment, and Shielding 383 considered, the half-life varies from billions of years (i.e., U238 has a half-life of 4.468 Á 109 years) down to small fractions of seconds (i.e., that of Po214 is 164 ms). As an example, Pb214 has a half-life of 26.8 min; this means that N nuclei of Pb214 after 26.8 min become N/2 nuclei, the other N/2 having become Bi214 because of beta decay; and after 53.6 min, only N/4 nuclei of Pb214 will exist, since 3/4 N have become Bi214, and so on.