1 Nuclear Stability The larger the atom, the greater the proportion of the nucleus that must be neutrons. –The A/Z ratio is greater than 2 (or the N to P ratio is increasingly greater than 1) Atoms in which the A/Z ratio is outside a certain range undergo radioactive decay –A neutron or proton breaks down, releasing radiation, and restoring the A/Z ratio to a stable number. Slides:

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3 Types of radioactive decay/radiation

4 Alpha particles Two protons and 2 neutrons –Same as a Helium atom nucleus –A stable combination, apparently Very large –Short range, but with high energy –Projectile analogy: like a boulder Can’t throw very far, but packs a punch. Narrow energy band, typically around 5 MeV Change in A of -4, change in Z of -2 –Am-241 decays to Np-237 (Z: 95 to 93)

5 Types of radioactive decay/radiation-2

6 Beta particles Beta particles are similar to electrons –Have the same mass, often the same charge –Origin is the nucleus, however. Beta emission accompanied by an antineutrino –Antineutrino carries a variable amount of energy –Beta therefore can have a range of energy levels Have a maximum and average energy Depends on isotope Common in biological research –H-3, C-14, S-35, P-32 all beta emitting isotopes

7 Beta particle energy distribution Alpha particle energy distribution

8 More on Beta particles Wide range of energy levels –Tritium (H-3) only MeV Can’t travel through air to reach your skin –P-32 is toasty (1.71 MeV) Can travel across the room to get you Positively charged beta is a positron –Positron emission: when A/Z (or N/Z) ratio is too low, i.e. not enough neutrons for # of protons –Neutrino emission accompanies decay

9 Beta decay Negatron emission –Excess Neutron turns into Proton –Neutron sheds negative charge, gains + charge –A stays the same, Z goes up +1, element changes –Example: C-14 becomes N-14 Positron emission –Excess proton turns into neutron –Proton loses positive charge, becomes neutral –A stays the same, Z goes -1, element changes –Competes with electron capture

10 Types of radioactive decay/radiation-3

11 Gamma rays Photons –Not particles, but packets of energy –Discrete energy range like alphas No pure gamma emitting isotopes –Following some other decay, nucleus is in excited state, excess energy then emitted as a gamma ray –Gamma emission not always immediate “metastable” state: e.g. Tc-99m used in medicine –Also formed from annihilation reaction Projectile analogy: small, high velocity, like BB

12 Gamma rays have a discrete energy level Gamma spectrum of a 60Co radioactive source taken with: (a)scintillation NaI(Tl) detector, and (b) semiconductor GeLi detector. Bratislava/Detectors.htm

13 Neutrons neutron_capture.jpg  Neutrons also radiation when moving  Associated with nuclear reactors and with industrial applications  “High” mass, but uncharged, more difficult to detect/measure.  Depending on energy level: a) bounce off nuclei, scatter b) get absorbed, create isotopes c) get absorbed, initiate fission reactions.

14 X-rays Photons –High energy, not particles –Longer wavelength than gamma rays, lower energy Distinguished from gamma rays by source –Gamma rays originate in the nucleus –X-rays come from outside nucleus, from electron shells or from interaction with a particle passing by

15 training/nuc_med/intro2.html Graphic_new_550.jpg

16 Generation of X-rays Electrons emitted by the cathode are attracted to the anode. When they reach it and stop moving, their kinetic energy is converted to x-rays.

17 Bremsstrahlung bremsstrahlung.htm “Braking energy” Electron is attracted to, slowed by nucleus; kinetic energy is converted to x-ray.

18 Shielding of P-32: Plexiglas P-32 emits energetic β particles (same size and charge as electrons); Atoms with large nuclei (high Z) like lead block β particles but allow production of X-rays! Low Z materials like plastic are much safer shielding materials. training/nuc_med/intro2.html

19 Practical summary of the penetrating power of the main types of ionizing radiation www-2.cs.cmu.edu/~dst/ Cowen/essays/radiation.html