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Nuclear Physics overview

Topic 2 of 10

Alpha, beta and gamma

Radiation type determines what is emitted and how it interacts. Penetration and ionisation comparisons also depend on its energy and the material through which it travels.

The nature and characteristic interactions of three nuclear emissions
EmissionNature and chargeTypical comparison in a stated absorber
Alpha, αA helium-4 nucleus: two protons and two neutrons, charge +2e, mass approximately 4 u.Dense ionisation over a short range; ordinary alpha radiation can be absorbed by paper or a thin barrier.
Beta-minus, β-An electron produced in a nuclear process, charge -e, electron rest mass.Intermediate penetration and ionisation in the comparison; a suitable thin metal or plastic absorber can greatly reduce transmission.
Gamma, γAn electromagnetic photon with no charge or rest mass.Generally much more penetrating, with less direct ionisation along the path. Thick dense shielding attenuates it.

A beta electron is not a pre-existing orbital electron simply leaving the atom. Gamma emission reduces nuclear energy without changing its nucleon or proton count. Zero photon rest mass does not mean zero photon energy or momentum.

Compare transmission with a stated absorber

Compare transmission with a stated absorberThree separate schematic rows share the same source and detector positions. The alpha row places a thin paper barrier between them and has no transmitted alpha arrow. The beta-minus row uses a suitable plastic or metal absorber and retains a transmitted arrow labelled reduced count. The gamma row uses thick dense shielding and retains a transmitted arrow labelled reduced count: shielding attenuates gamma rather than making transmission identically zero. Barrier width and radiation arrow width are qualitative, with no supplied material thickness, energy, count efficiency or universal stopping range.α: alphaThin paperNo alpha throughβ-: beta-minusSuitable plastic / metalReduced countγ: gammaThick dense shieldingReduced countSourceDetector

Absorption and attenuation depend on the emitted energy and the material. These ordinary school-source comparisons do not assign a universal range. A reduced transmitted count is not a statement that every surviving photon has lost the same fraction of its energy.

The sign of charge determines the initial bending direction

The sign of charge determines the initial bending directionRadiations initially travel right into a magnetic field directed into the page, shown by crosses. A positive alpha trajectory curves upwards; a beta-minus electron trajectory curves downwards; the gamma ray remains straight. The two charged curves use equal illustrative radii only to show opposite signs, not to compare real radii. The field does not exert an electric-charge magnetic force on gamma photons. Momentum, charge and field strength, not charge alone, set a charged particle path radius.B into the pageEntryα (+)β- (-)γNo radius comparison is implied.

For the same rightward entry and field into the page, positive charge is forced up and negative charge down. The curves are schematic: their radii do not compare the particles' momenta. A cross here denotes magnetic field into the page, not current.

The absorber comparison keeps the geometry fixed and shows attenuation, not an absolute universal stopping thickness. The separate field view has entry to the right and B into the page: alpha bends up, beta-minus down, and gamma is not magnetically deflected. Curvatures are qualitative; charge alone does not determine their relative radii.

Charged alpha and beta particles also experience electric forces in opposite directions in a given electric field. Gamma has no electric charge force. Charged-particle speeds depend on their kinetic energies and are below c; gamma travels at c in vacuum.

A penetration statement must name the radiation energy and absorber context. Gamma is not completely stopped by one universal thickness, and short alpha range does not imply weak ionisation. For living tissue, an internal alpha-emitting material can deposit energy densely close to its location.

Optional check Why does the short range of ordinary alpha radiation not make an alpha-emitting substance inside the body harmless?
Why does the short range of ordinary alpha radiation not make an alpha-emitting substance inside the body harmless?