18.4.4 Properties of Radiation

When radioactivity was first discovered around 1896, physicists were able to identify three distinct types of radiation due to their different properties. At that time, nuclei, electrons and photons were still unknown. So these three radiations were given the names a, b, g (the first three letters of the greek alphabets).[1]

 alpha particlesbeta particlesgamma ray
Ionizing Powervery strongstrongweak
Penetrating Powerlowmediumhigh
Stopped bypaper~mm aluminum~cm lead
Range in Air~cm~m
Deflection in E-fieldyesyesno
Deflection in B-fieldyesyesno

Now we know the exact composition of each radiation. Alpha and beta turn out to be energetic particles, while gamma ray is actually a very high frequency electromagnetic wave (or very energetic photons, if you prefer).

 alpha particlesbeta particlesgamma ray
Symbol\displaystyle {}_{2}^{4}He\displaystyle {}_{{-1}}^{0}e\displaystyle {}_{Z}^{A}X^{*}\to {}_{Z}^{A}X+\gamma
Mass~4u~u/18000
Charge+2ee0
Speed~0.05c~cc

Deflection in E Fields

Since alpha and beta particles are oppositely charged, they are deflected in opposite directions in electric fields. For the same electric field strength, the deflection for alpha particles is smaller. This is because of their larger mass (~8000 times) despite their larger charge (4 times).

\displaystyle a=\frac{{qE}}{m}

On the other hand, gamma rays are uncharged, so electric fields have totally no effect on them.

Deflection in B Fields

In magnetic fields, alpha and beta particles will trace out circular paths in opposite directions. Again, the deflection for the alpha particles is much smaller thanks to their larger momentum.

\displaystyle \begin{aligned}Bqv&=\frac{{m{{v}^{2}}}}{r}\\r&=\frac{{mv}}{{Bq}}\end{aligned}

Gamma rays, being uncharged, do not experience any magnetic forces.

Ionizing Power

Alpha particles cause ionization by pulling orbital electrons out of the atoms they encounter in their track. Beta particles cause ionization by knocking out orbital electrons. It may be counter-intuitive, but the slower speed of the alpha particles is the main reason for their higher ionizing power (up to 100,000 ions per mm) compared to beta particles. The beta particles’ interaction with each atom is so fleeting the rate of ionization is much lower.

Gamma rays, being uncharged, do not cause ionization directly. However, because of the large amount of energy packed in each gamma photon, they can bring about very energetic electrons when they knock out electrons from atoms they encounter, effectively turning those electrons into beta particles. These secondary beta particles can go on to produce indirect ionization. Even then, since this mechanism occurs at very low rate, gamma rays are only weakly ionizing.

Penetrating Power

It may be counter-intuitive, but the higher ionizing power of the alpha particles is the reason for their shorter range compared to beta particles. Every time an alpha particle produces an ion, it loses a fraction of its kinetic energy. Since it produces so many ions per unit distance, it loses all its energy after a short distance (upon which it will absorb two electrons and become a neutral helium atom).

Gamma rays, being weakly ionizing, is highly penetrative. A thick layer of lead (lead is chosen because of its high density), however, can attenuates a gamma beam effectively.

Demonstration

Penetration through Different Materials (QuantumBoffin)

Cloud Chamber (Harvard Natural Sciences)

Concept Test

3620

Interesting Stuff

Radiation vs Radioactive Atoms (Veritasium)


[1] If you’re interested to know why certain nuclides undergo alpha decay whereas other nuclides undergo beta decay, you can read Appendix D: Segre chart.

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