PH 103

Dr. Cecilia VogelLecture 21

Review

Outline

Nuclei properties composition, N, Z, A binding energy

Nuclei decays Radiation damage exponential decay

Conservation and Nuclear Reactions

Charge is conserved in all nuclear reactions

Ex: if a positive particle is emitted

nucleus must become less positive

Number of nucleons is conservedfor example, p can’t turn into a positron alone

Conservation and Nuclear Reactions

Energy is conserved in all nuclear reactions

Remember that mass is a form of energy& may be converted to or from other forms

In a nuclear processif mass is lost, energy is released (exothermic)if mass is gained, energy input

is needed (endothermic)All spontaneous processes are exothermic

such as all nuclear decaysIn all nuclear decays, mass is lost

2|| cmE

Alpha Decay

Occurs in some heavy nucleiParticle emitted is

alpha particle, which is a 4He nucleus

Parent nucleus loses 2 protons and 2 neutronsSo daughter nucleus has Z - 2, A - 4

Alpha DecayParent nucleus loses 2 protons and 2 neutronsSo daughter nucleus has Z - 2, A - 4ex: 212Bi.

Look in Appendix B to see it decays by alpha-decay. Also find Z=83 (in appendix B or periodic table).

Daughter has Z - 2 = 81. Look up -- this is Thallium. A - 4 = 212 - 4 = 208. Daughter is 208Tl

Alpha DecayEnergy is conserved

mass energy is lost, kinetic energy is gained by emitted alpha.

ex: 243Am . Daughter is 239NpUse Appendix B for masses.Initial mass:

mass of 243Am = Final mass:

mass of 239Np = , mass of 4He = total final mass =

Initial mass > final mass!always true in decays

Beta-minus DecayOccurs in neutron-rich nucleiParticles emitted are

e- and antineutrino, (anti)neutrino has zero chargemass very close to zero

Parent nucleus loses a neutronbut gains a proton

So daughter nucleus has Z + 1, same A

Beta-minus DecayParent nucleus loses a neutron

but gains a protonSo daughter nucleus has Z + 1, same Aex: 210Tl.

Find in appendix Bthat it decays by - and that Z = 81.

Daughter has Z + 1=82 Lead. same A=210Daughter is 210Pb

Beta-plus Decay

Occurs in neutron-deficient nucleiParticles emitted are

e+ and neutrino, e+ is a positron, an anti-electron

Parent nucleus loses a protonbut gains a neutron

So daughter nucleus has Z - 1, same A

Beta-plus DecayParent nucleus loses a proton

but gains a neutronSo daughter nucleus has Z - 1, same Aex: 40K.

App B says + decay, andthat Z= 19.

Daughter has Z - 1= 18 Argon. same A=40 Daughter is 40Ar

Gamma DecayOccurs in excited nuclei

nucleus is not in its ground stateParticle emitted is

a photon, a very high energy photonhigh frequency gamma part of EM spectrum

Particle emitted has no charge, no nucleonsonly takes away energy

So daughter nucleus is same isotope in lower energy level

Radiation Damage Visible light

very little damageyellows paper, fades dyes, etc

UVsunburns, some ionization

Ionizing radiation, energetic enough to ionize atoms

and ions are very reactive.Damaging reactions occur in living tissueCells can be damaged, die, or become cancerous

Measure of DamageDamage depends on amount of energy absorbed by the tissue

more energy means more ionization, so more damage

But if the energy is spread out, it is less damaging.So what is important is

energy per unit mass 1 rad = 0.01 J/kg100 rad = 1 J/kg = 1Gy = SI unit, but very big

Measure of DamageDamage depends on amount of energy

1 rad = 0.01 J/kgDamage also depends on type of radiation

Relative biological effectiveness, RBE=WR

= measure of how damaging radiation is

compared to 200-keV X-raysalphas are more damaging than betas, which are more damaging than gammas

RBE>RBE>RBE

Measure of DamageDamage depends on amount of energy

1 rad = 0.01 J/kg1Gy = 1 J/kg

Damage also depends on type of radiation

dose in rem = dose in rad*WR dose in sievert = dose in Gy*WR

For example, consider workers at the Fukushima Daiichi nuclear power plant:

some received doses >100 mSvbut none above Japan's guidance value of 250 mSv for exposure of emergency workers (source: Reuters)

Penetrating RadiationSo then, why are gammas exciting?

Alphas are stopped by cardboard, skinbetas are stopped by sheet metal, rockgammas are only stopped by thick lead!

There are lots of alpha emitters in the rocks

but, the alphas don’t penetrate to vital organsmostly stopped by skin

exception: Radon is an alpha emitterit’s worrisome, because it’s a gas