{"id":431,"date":"2026-07-05T23:41:08","date_gmt":"2026-07-05T23:41:08","guid":{"rendered":"https:\/\/physicsfundamentalsinfo.com\/blog\/?p=431"},"modified":"2026-07-05T23:41:09","modified_gmt":"2026-07-05T23:41:09","slug":"types-of-radiation-physics","status":"publish","type":"post","link":"https:\/\/physicsfundamentalsinfo.com\/blog\/modern-physics\/types-of-radiation-physics\/","title":{"rendered":"Radioactivity: Alpha, Beta & Gamma"},"content":{"rendered":"\n
Definition<\/div>

\n

The three main types of radiation physics identifies are alpha (\u03b1), beta (\u03b2) and gamma (\u03b3), all emitted by unstable atomic nuclei. Alpha particles are helium nuclei stopped by paper; beta particles are fast electrons stopped by a few millimetres of aluminium; gamma rays are high-energy electromagnetic waves that even thick lead only reduces.<\/p>\n<\/p><\/div>\n\n

Look up for a moment. If there is a smoke alarm on your ceiling, a genuinely radioactive source \u2014 a speck of americium-241 smaller than a grain of salt \u2014 is probably sitting a few metres above your head right now, quietly firing out alpha particles. It has been doing so for years, and it is completely safe.<\/p>\n\n

That is the strange charm of radioactivity. It is everywhere, yet the three kinds of radiation behave nothing like one another: one cannot get past your skin, one needs a sheet of metal, and one will sail through a brick wall. Knowing which is which is the whole game.<\/p>\n\n

What Are the Types of Radiation in Physics?<\/h2>\n\n

The story starts in 1896, when Henri Becquerel found that uranium salts fogged photographic plates straight through their light-proof wrapping. Something invisible was streaming out of the atoms themselves \u2014 Marie and Pierre Curie soon named the phenomenon radioactivity.<\/p>\n\n

Three years later, Ernest Rutherford sorted that mysterious output into two components by how easily each was absorbed, labelling them alpha and beta after the first letters of the Greek alphabet. In 1900 Paul Villard spotted a third, far more penetrating component, which Rutherford later christened gamma.<\/p>\n\n

\n\n \"Becquerel's\n\n
Becquerel’s 1896 plate: uranium salts fogged the film through opaque paper.<\/figcaption>\n\n<\/figure>\n\n

All three are forms of ionising radiation<\/a>: they carry enough energy to knock electrons clean off the atoms they strike. That is precisely what makes them useful in medicine and industry \u2014 and hazardous to living tissue.<\/p>\n\n

One caution about the vocabulary. Despite the historical name “rays”, only gamma is a true ray; alpha and beta are particles \u2014 physical lumps of matter flung out of a nucleus. (A fourth type, neutron radiation, appears in reactors and cosmic-ray showers, but natural decay overwhelmingly produces the classic trio.)<\/p>\n\n

Alpha vs Beta vs Gamma: What Is the Difference?<\/h2>\n\n

Here is the pattern worth tattooing onto your revision notes: ionising power and penetrating power run in opposite directions. A particle that ionises furiously spends its energy budget within a short distance, so it cannot travel far; a weak ioniser spends slowly and keeps going.<\/p>\n\n

Alpha sits at one extreme \u2014 a heavy, doubly charged bruiser that tears up everything nearby and exhausts itself within centimetres. Gamma sits at the other: an uncharged electromagnetic wave travelling at the speed of light<\/a> and interacting so rarely that most of it passes straight through you. Beta lands in between.<\/p>\n\n

\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Property<\/th>\nAlpha (\u03b1)<\/th>\nBeta (\u03b2\u207b)<\/th>\nGamma (\u03b3)<\/th>\n<\/tr>\n<\/thead>\n
What it is<\/strong><\/td>\nHelium-4 nucleus: 2 protons + 2 neutrons<\/td>\nA fast electron ejected from the nucleus<\/td>\nHigh-energy electromagnetic wave (photon)<\/td>\n<\/tr>\n
Charge<\/strong><\/td>\n+2e<\/td>\n\u22121e<\/td>\n0<\/td>\n<\/tr>\n
Relative mass<\/strong><\/td>\n\u2248 4 u (heaviest)<\/td>\n\u2248 1\/1836 u (about 7,300\u00d7 lighter than \u03b1)<\/td>\nMassless<\/td>\n<\/tr>\n
Typical speed<\/strong><\/td>\n\u2248 5% of the speed of light<\/td>\nUp to \u2248 99% of the speed of light<\/td>\nThe speed of light<\/td>\n<\/tr>\n
Ionising power<\/strong><\/td>\nVery high<\/td>\nModerate<\/td>\nLow<\/td>\n<\/tr>\n
Penetrating power<\/strong><\/td>\nVery low<\/td>\nModerate<\/td>\nVery high<\/td>\n<\/tr>\n
Stopped by<\/strong><\/td>\nA sheet of paper, skin, or a few cm of air<\/td>\n\u2248 5 mm of aluminium<\/td>\nReduced (never fully stopped) by several cm of lead or \u2248 1 m of concrete<\/td>\n<\/tr>\n
Range in air<\/strong><\/td>\n\u2248 3\u20135 cm<\/td>\nUp to a few metres<\/td>\nVery long \u2014 weakens with distance and shielding<\/td>\n<\/tr>\n
Deflection in electric\/magnetic fields<\/strong><\/td>\nSmall (heavy, charge +2)<\/td>\nLarge, opposite direction (light, charge \u22121)<\/td>\nNone (uncharged)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n\n\n <\/rect>\n What stops each type of radiation?<\/text>\n <\/rect>\n <\/circle>\n Radioactive<\/text>\n source<\/text>\n <\/rect>\n Paper<\/text>\n <\/rect>\n Aluminium<\/text>\n <\/rect>\n Lead<\/text>\n \u03b1<\/text>\n <\/line>\n <\/circle>\n <\/circle>\n <\/circle>\n stopped by paper<\/text>\n \u03b2<\/text>\n <\/line>\n <\/circle>\n <\/circle>\n <\/circle>\n <\/circle>\n stopped by ~5 mm of aluminium<\/text>\n \u03b3<\/text>\n <\/path>\n <\/path>\n thick lead only reduces it<\/text>\n \u03b1 = helium nucleus (+2) \u00b7 \u03b2 = fast electron (\u22121) \u00b7 \u03b3 = electromagnetic wave (0)<\/text>\n<\/svg>\n

Figure 1 \u2014 The classic absorber test: paper stops alpha, a few millimetres of aluminium stop beta, and thick lead only thins gamma out.<\/p>\n\n

Electric and magnetic fields tell the same story from another angle. Alphas curve gently towards a negative plate, betas swing hard the opposite way because they are nearly 7,300 times lighter, and gamma rays plough straight on \u2014 no charge, no deflection.<\/p>\n\n

The Radioactive Decay Formula<\/h2>\n\n

Which particular nucleus decays next? Nobody can say \u2014 not with any instrument, not even in principle. Yet a large sample is perfectly predictable, the way a stadium crowd is predictable even though each individual fan is not.<\/p>\n\n

That statistical regularity is captured by the radioactive decay law:<\/p>\n\n

N = N0<\/sub> e\u2212\u03bbt<\/sup><\/div>\n\n