Nuclear Physics

Final – Tues 7-10 PMBring ID

Radiation

Is radiation charged?

Detecting radiation (charged)

Fig 44-13a, p.1459

decays – a picture

The Decay Constant

• The rate at which a decay process occurs is proportional to the radioactive nuclei present in the sample

• λ is called the decay constant and determines the rate at which the material will decay

• N is the number of undecayed radioactive nuclei present

• No is the number of undecayed nuclei at time t=0

to

dNN gives N N e

dt

120

0 12

ln22

TNN e T

half-life

Decay Rate

• The decay rate, R, of a sample is defined as the number of decays per second

– Ro = No is the decay rate at t = o– The decay rate is often referred to as the

activity of the sample

t to o

dNR N e R e

dt

Decay Mechanisms

alpha decay

beta decay

The neutrino and the weak interaction

A neutron changesInto a protonand electron and neutrino

14 146 7C N e

gamma decay

Fig 44-17, p.1465

Changing elements“turning lead into gold”

Carbon Dating• Beta decay of C-14 is

used in dating organic materials

• The process depends on the ratio of C-14 to C-12 in the atmosphere which is relatively constant

• When an organism dies, the ratio decreases as a result of the beta decay of the C-14

14 146 7C N e

• A piece of charcoal of mass 25 g is found in the ruins of a city. The sample shows a Carbob-14 acctivity of 250 decays/min. How long has the tree from which the charcoal came been dead.– Half life of carbon-14 is 5730 yr– Fraction of Carbon-14 to Carbon-12 is 1.3x10-12

to

dNN gives N N e

dt

t to o

dNR N e R e

dt

RADS REM and RBE

1 rad=.01 J/kg of absorbed energy by tissue

rem=rad x RBE

Acute Radiation ExposureEffects of Large, Whole-Body Radiation Doses

Effect Dose (rems)

No observable effect 0-25

Slight blood changes 25-100

Significant reduction in blood platelets andwhite blood cells (temporary)

100-200

Severe blood damage, nausea, hair loss, hemorrhage, death in many cases

200-500

Death in less than two months for over 80% >600

How much radiation do we get?

Cancer treatment

30 rads

radiation treatment

linear accelerator

Tracers

Food

Fig 45-23, p.1503

NMR (MRI)

MRI

Fig 44-21, p.1469

Tumor

A nucleus of an element has to have a specific charge, but can vary in the number of

a. protons.

b. neutrons.

c. neutrons plus electrons.

d. electrons.

e. protons plus electrons.