RADIOACTIVITY
radioactive decay
= disintegration of unstable nuclei to stable prodcuts by emission of alpha / beta-particle or gamma-radn
nuclear decay unaffected by external factors (pressure, temp, E/B-field)
nuclear decay; spontaneous & random (fluctuations in count rate- sometimes fast / slow count during specific time intervals)
detection of radn: photographic film, Wilson's cloud chamber, gold leaf electroscope, GM tube/counter
alpha decay
alpha particle; -2 protons, 2 neutrons, -indentical to helium-4 nucleus
⁴₂ / ⁴₂He
beta decay
beta particle; electron emitted from neutron when neutron changes into proton and electron (proton remains in nucleus)
⁰_-1e /
gamma decay
gamma ray photon emitted when excited nucleus returns to ground state (nucleus becomes excited when alpha/beta-particle emitted)
existence of diff E levels explain 'line-type' E spectrum of gamma-rays

particle	nature	charge	speed (typical)	E (typical)	ionizing effect (ion pairs per mm in air)	penetration (typical)
alpha	2 neutrons + 2 protons	2e	0.1c	10MeV	~105	stopped by 0.5mm paper / 50mm air
beta	electron	-1e	up to 0.9c	0.03 to 3MeV	~103	stopped by 5mm Al
gamma	v.short wavelength em	no charge	c (= 3×108m/s)	1MeV	~1	intensity halved by 100mm Pb

alpha, beta, gamma radn all cause ionisation
-removes electron from atoms > form ion-pair (made of removed electron & +ve charge atom) which conducts electricity
-alpha particles interact most w/ air > least penetrative
beta particles emitted at various speeds unlike alpha particles
gamma: intensity ∝ 1/(dist from source)², -not stopped by absorber (intensity reduced)
alpha, beta particles: deflected by E/B-field
background radn
=low-level radn due to rocks, soil, cosmic rays
-needs to be taken into account in radioactivity exp
activity, A
= # of disintegrations per unit time
(A: activity, N: number of atoms present) (unit: becquerel, Bq, 1Bq = 1s^-1)
each disintegration produces alpha/beta particle (and gamma radn sometimes)
gamma radn emitted as photon; photon & particles cause pulses in detector
activity unaffected by chem changes / physical conditions
decay constant,
on average: activity ∝ # of undecayed nuclei (more disintegration likely w/ more nuclei present)
A = -dN/dt = N (: radioactive decay const)
decay law
dN/dt = -N

[ln N]^N_No = -t => ln(N/N₀) = -t => N = N₀e^-t

t = 0, N = N₀, A = A₀ = N₀
A = N = N₀e^-t = A₀e^-t
count rate, c; c ∝ A => C = C₀e^-t
Half-life, t_½/
= av time for activity / # of undecayed nuclei to halve

n: N = N₀(½)ⁿ
t = N = N₀/2
N = N₀/2 = N₀e^- => ½ = e^- => ln2 =
= ln2/

Hazards of ionizing radiation
-radiation burns (mainly due to & radn)
-sterility
-genetic mutations
-leukaemia
-blindness, cataracts
Precautions
-use tons to handle radioactive source
-wear gloves + protective clothes
-radioactive material kept in thick lead containers
-don't pt radioactive source at pp;
-use robotic arms behind lead imbued glass barrier (v.highly radioactive subs)
-wear badge containing photographic film (measures radn dose)
-proper disposal of waste materials
Uses of radioisotopes
Tracers
-used to trace movements (human- digestive system, plants- track intake of materials by using trace, Magnesium-28, in soil water, -to detect leaks in pipes)
-use small, safe amounts
Penetrating properties of materials
- rays: photograph solid objects- detect cracks in engines
- particles: regulate thickness of materials by having constant count rate
Radiotherapy
- rays: penetrate deep into skin > kill living cells, Cobolt-60 used to kill cancer cells
Power source
-Uranium-235 in nuclear power stations
Carbon dating


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