1Interactions of radiation with Matter

2Interaction with beta

Electrons excited or kicked off. ionization

Energy dissipated as heat.

As Z of material increases, so does bremsstrahlung.

Note that range is different from path.

3Interaction with gamma

Photon travels until it hits something, either an electron or a nucleus.

Several types of interactions have been observed.

4Interaction of gamma with matter

• Photoelectric effect– Photon hits electron, all of energy is transmitted,

electron is ejected. – Most likely with low energy photons, high Z material

http://www.faqs.org/docs/qp/images/peeffect.gif

5Gamma interaction-2

• Compton scattering– Not all energy transmitted to electron.– Electron ejected, secondary photon emitted– With low energy photons, independent of Z

http://www.phys.jyu.fi/research/gamma/publications/akthesis/img220.png

6Raleigh scattering and nuclear magnetic resonance

• Both involve impact of gamma on nucleus– Raleigh: gamma is deflected (elastic collision),

keeps going.– occurs when particles are very small compared to

the wavelength of the radiation. (10-15 vs 10-10)– NMR: absorbed, emitted in a new direction

hosting.soonet.ca/.../scattering.gif

7Pair production and annihilation

Two gamma collide, convert to a positron and a negatron. Complete energy to matter conversionThese two betas collide, converting to 2 gammas with equal energy of 511 kev. Complete matter to energy conversion.

www.mhhe.com/.../fix/ student/images/26f14.jpg

8Summary of interactions

• Alpha– Penetrates short distance into matter, giving up its

energy by ionizing matter and releasing heat.

• Beta– Bounces around, giving up energy by ionizing

matter and dissipating kinetic energy as heat.

• Gamma– Penetrates, colliding with electrons

• Photoelectric effect, Compton scattering– Collides with nuclei (Raleigh scattering, NMR)– Collides with another gamma

9About interactions

• Radiation is moving energy – All types have kinetic energy– Alpha and beta particles have charge

• Energy cannot be created or destroyed– Energy is transferred

• Dose is a measure of how much energy is deposited in an “absorber”– Absorber could be inanimate or could be flesh– Energy left as heat, electrical potential, etc.

10Bragg Effect

• As particles (alpha, beta) slow down, ionizations increase near the end of their paths.– Proton anti-cancer therapy relies on this.

11About Dose

• Linear Energy Transfer– Average energy deposited in absorber per unit

distance traveled by charged particle.

• RAD: radiation absorbed dose– The amount of energy absorbed per unit of

absorbing material. (new units: Gray)

• RBE: Relative Biological Effectiveness– Depends directly on the LET, a quality factor “Q”

used in determining the effect of LET on the absorbed dose, i.e. how much damage.

12More on dose

• REM: roentgen equivalent man– Effective dose resulting from the RAD and the RBE– REM = Q x dose (in RAD)– Q is a measure of RBE as determined from LET.– New unit is sieverts

– Slowly moving, greatly ionizing alpha particles have a much higher LET, so Q will be >1, and the energy absorbed will have a bigger biological effect (if absorbed by living tissue)

13More on calculating REM

LET (keV per µm) Q example

3.5 and less 1 X-rays,β,

7 2 neutrons

23 5

53 10

175 and over 20 alpha

14Comparing old, SI units

Old SI

Radioactive material curies becquerels

Deposited energy Rads Grays

Dose to humans Rems Sieverts

Units of energy in air Roentgens none

Rad = 100 ergs/gram; Rem = rad x Q;

1 Gray = 100 Rads, 1 j/kg; 1 Sievert = 100 rem;

15Radiation Safety Rules of Thumb

1. Alpha particles up to 7.5 MeV are stopped in the dead layer of normal skin.

2. Beta particles will penetrate about 4 meters in air per MeV of energy.

3. Beta particles will penetrate about 0.5 cm in soft tissue per MeV of energy.

4. Beta particles up to 70 KeV are stopped in the dead layer of normal human skin.