In 1896, the French scientist Henri Becquerel discovered that crystals of a Uranium compound would darken photographic plates (like our modern photographic film), even through black paper that kept out all light.He thought that the compound must have been giving off some other form of radiation, which could pass through the paper. Marie Curie later named this "radioactivity".

He shared the Nobel Prize for Physics with Marie and her husband, Pierre, in 1903.Becquerel was born in Paris, and studied at the Ecole Polytechnique. In 1892, he became professor of physics at the Museum of Natural History, and in 1895 also at the Ecole Polytechnique. He was elected president of the French Academy of Sciences in 1908.

We now name a unit after him:

one Becquerel (Bq) is one radioactive disintegration per second.

The turning point in the battle between theoretical physicists and empirical geologists and biologists occurred in 1896. In the course of an experiment designed to study x-rays discovered the previous year by Wilhelm Röntgen, Henri Becquerel stored some uranium-covered plates in a desk drawer next to photographic plates wrapped in dark paper. Because it was cloudy in Paris for a couple of days, Becquerel was not able to "energize'' his photographic plates by exposing them to sunlight as he had intended. On developing the photographic plates, he found to his surprise strong images of his uranium crystals. He had discovered natural radioactivity, due to nuclear transformations of uranium. The significance of Becquerel's discovery became apparent in 1903, when Pierre Curie and his young assistant, Albert Laborde, announced that radium salts constantly release heat. The most extraordinary aspect of this new discovery was that radium radiated heat without cooling down to the temperature of its surroundings. The radiation from radium revealed a previously unknown source of energy. William Wilson and George Darwin almost immediately proposed that radioactivity might be the source of the sun's radiated energy.

Radioactivity is natural

photo from Newgrange Visitor centre.

for more details on how carbon dating actually works follow this link to ABC

The Cloud Chamber.

Using a device called a cloud chamber, 3 different types of radiation have been discovered.

Each of these 3 types of Radiation have there own Unique Properties.

The cloud chamber can allow us to view some of those properties.

Experimental evidence for three kinds of radiation: by deflection in electric or magnetic fields or ionisation or penetration.

To explain, a cloud chamber is used for detecting particles of ionizing radiation. In its most basic form, a cloud chamber is a sealed environment containing a supercooled, supersaturated water or alcohol vapor. When an alpha particle or beta particle interacts with the mixture, it ionizes it. The resulting ions act as condensation nuclei, around which a mist will form (because the mixture is on the point of condensation). The high energies of alpha and beta particles mean that a trail is left, due to many ions being produced along the path of the charged particle. These tracks have distinctive shapes (for example, an alpha particle's track is broad and straight, while an electron's is thinner and shows more evidence of deflection by collisions).

One in action,

watch the video on this page on the right

watch it twice!

Diagram: Side view of the Cloud Chamber in use.

The Cloud Chamber shows us at least 2 features of the different particles.

The Cloud Chamber from above

Firstly when placed in a magnetic or an electric field the particles can be distinguished by the direction of their paths.

The Alpha particle is drawn toward a negative electric pole and shows a positive charge.

The Beta Particle is attracted towards the positive pole, thus making the particle negative.

The Gamma particle does not change its path under the influence of either electric or magnetic fields.

Second, on viewing the chamber from above we can also notice how quickly they ionise the particles around them,

Alpha particles very readily Ionise

Beta particles moderately Ionise

Gamma Particles rarely Ionise.

Following on from the Ionisation readiness we see the same facts from the other side, the ability for radiation to pass through materials.

Alpha particle react so readily they do not travel far about 20cm in air, but not through a sheet of card

Beta Particles, travel a moderate distance in air before they interact with other matter. A thin sheet of Aluminium will stop the vast majority of beta particles.

Gamma particles see almost all matter as transparent.

Good Descriptive Diagram here

Build your own Cloud Chamber

Nature and properties of alpha, beta and gamma emissions.


Alpha (α) decay is a method of decay in large nuclei.

An alpha particle (helium nucleus, He2+), consisting of 2 neutrons and 2 protons, is emitted.

Because of the particle's relatively high charge, it is heavily ionizing and will cause severe damage if ingested. However, due to the high mass of the particle, it has little energy and a low range; typically alpha particles can be stopped with a sheet of paper (or skin).


The Alpha Particle is essentially a Helium Nucleus, having 2 protons and 2 neutrons. Therefore when an atom emits alpha radiation its mass number decreases by 4 (2p + 2n) and its Atomic number decreases by 2!


An alpha particle (helium nucleus, He2+), consisting of 2 neutrons and 2 protons


Alpha particles have a large charge(+2), so they easily ionise other atoms that they pass. Ionising atoms requires energy, so alpha particles lose energy rapidly as they travel. Thus they have a range of only a few centimetres in air.


Very Ionising


very short due to the ionisation ability

Most Hazardous when


Examples of emitters

Alpha-decay occurs in very heavy elements, for example, Uranium and Radium.

Alpha Decay

_{95}^{241}\textrm{Am}\rightarrow _{93}^{237}\textrm{Np}+_{2}^{4}\alpha

an α-emitter, e.g.

241Am, used in smoke detectors

U 238,

Pa 238

Po 210



Beta - (β-) is when a neutron decays to a proton, an electron and antineutrino

Beta-minus (β-) radiation consists of an energetic electron. It is less ionizing than alpha radiation, but more than gamma. The electrons can often be stopped with a few centimeters of metal. It occurs when a neutron decays into a proton in a nucleus, releasing the beta particle and an antineutrino.

Beta + (β+) is when a proton decays into a neutron, positron and neutrino

Beta-plus (β+) radiation is the emission of positrons. Because these are antimatter particles, they annihilate any matter nearby, releasing gamma photons.


A Beta particle is the same as an electron.

It has a charge of -1, and a mass of around 1/2000th of a proton.

But wait a minute! If a nucleus contains protons and neutrons, what's an electron doing coming out of a nucleus?

To answer this, we need to know more about protons and neutrons:

Protons & neutrons are made of combinations of even smaller particles, called "quarks". Under certain conditions, a neutron can decay, to produce a proton plus an electron. The proton stays in the nucleus, whilst the electron flies off at high speed.


It has a charge of -1


Less ionizing than alpha particles, greater than gamma


Can be stopped by a thin sheet of metal

Most Hazardous when

Examples of emitters

β− decay generally occurs in a neutron rich nucleus. e.g. 14C, Strontium-90 and Iodine-13

Examples of β-emitters

Strontium-90 based radioisotope thermoelectric generators have been used to power remote lighthouses

Carbon-14 is also commonly used as a beta source in research, it is commonly used as a radiotracer in organic compounds. While the energy of the beta particles is higher than those of tritium they are still quite low in energy. For instance the walls of a glass bottle are able to absorb it. Carbon-14 is made by the np reaction of nitrogen-14 with neutrons. It is generated in the atmosphere by the action of cosmic rays on nitrogen. Also a large amount was generated by the neutrons from the air bursts during nuclear weapons testing conducted in the 20th century

A neutron 'turns into' a proton + an electron (with an anti electron neutrino given off also)

therefore the mass number does not change, but the Atomic number increases by 1, therefore the element changes !

A nice piece here on Pauli & the neutrino !

The existence of neutrinos was first proposed by Wolfgang Pauli in a 1930 letter to his physics colleagues as a "desperate way out" of the apparent non-conservation of energy in certain radioactive decays (called beta decays) in which electrons were emitted.

According to Pauli's hypothesis, which he put forward very hesitantly, neutrinos are elusive particles which escape with the missing energy in beta decays. The mathematical theory of beta decay was formulated by Enrico Fermi in 1934 in a paper which was rejected by the journal Nature because "it contained speculations too remote from reality to be of interest to the reader.

At the time, neutrino interaction cross-sections were considered too small for neutrino detection. However, the large neutrino fluxes that later became available with nuclear reactors opened the field of neutrino physics.

Neutrinos from a nuclear reactor were first detected by Clyde Cowan and Fred Reines in 1956.

Pontecorvo and Alvarez (Pontecorvo 1946, Alvarez 1949) suggested reactor experiments using the reaction ν + 37Cl → 37Ar + e– to detect the neutrino.

At the time there was no clear distinction between neutrino and antineutrino. The reaction often goes by the name the Davis-Pontecorvo reaction. The threshold energy is 0.81 MeV and the Argon isotope decays via electron capture with a half-life of 35 days.

Gravity was long believed to be the energy source of the sun. By 1920 it was known that the sun was mainly composed of helium and hydrogen and Eddington proposed that nuclear fusion was the energy source.

However, it took until 1938 before there was a complete theory of the nuclear reactions within the sun (Bethe and Chritchfield 1938).

It was a challenge to prove experimentally that nuclear burning was the energy source of the sun.

Raymond Davis Jr did his first reactor-based chlorine neutrino experiment at Brookhaven in the early 1950s. He used a tank filled with 3,900 liters of CCl4 as a target. Helium was bubbled through the tank to remove the few argon atoms.

The radioactive argon could then be removed from the helium by passing the gas through a charcoal trap with liquid nitrogen (-196 ℃) which adsorbs the argon quantitatively and allow helium to pass.

Neutrinos are very abundant in the Universe. Indeed, the ratio between neutrinos and nucleons (protons or neutrons) in the Universe is about 109 to 1. On the earth, the dominant source of neutrinos is our sun. Every second more than 10 billion (1010) neutrinos pass through every cm2, the majority with low energy (< 0.4 MeV).

Only 0.01% of the solar neutrinos have an energy larger than 5 MeV.

A new and 100 times larger experiment was proposed based on Davis' radiochemical method and on Bahcall's calculated rate of 40 ± 20 SNU (1 SNU = 1 Solar Neutrino Unit, 1 capture per second and per 1,036 target atoms) (Davis 1964, Bahcall 1964). Davis experiment was funded and installed in the Homestake Gold Mine, Lead, South Dakota (depth 1,500 m). The tank contained 615 tonnes of C2Cl4, an agent normally used for dry cleaning. It was ready to start data taking in 1967.

The extraction of argon by helium was done approximately once every two months to match the half-life of 37Ar.

The first results from Davis came in 1968, based on 150 days of data taking (Davis et al. 1968). An upper limit of the solar neutrino flux of 3 SNU was given, much lower than the then calculated rate of 20 SNU (Davis et al. 1968). In this 1968 paper is discussed different possibilities to improve the sensitivity and in particular how to reduce the background.

The experimental challenge, that Davis was so successful in meeting, was to extract an average of only 17 argon atoms among the 2 x 1030 chlorine atoms in the tank every second month.

Davis experiment was running almost continuously from 1970 until 1994. The final results were published in 1998.

During this time it is estimated that a total of 2,200 argon atoms were produced in the tank. Of these 1997 were extracted and 875 counted in the proportional counter. Of the latter, 776 are estimated to be produced by solar neutrinos and 109 by background processes.

The production in the tank was 0.48 ± 0.03 (stat.) ± 0.03 (syst.) argon atoms per day, corresponding to 2.56 ± 0.16 (stat.) ± 0.16 (syst.) SNU. Davis was a true pioneer and his successful mastering of the extraction of a few atoms out of 1030 gave birth to a new field of neutrino physics.


Gamma (γ) radiation consists of photons with a frequency of greater than 1,019 Hz. Gamma radiation occurs to rid the decaying nucleus of excess energy after it has emitted either alpha or beta radiation. gamma rays are given off after an Alpha or Beta particle has been released, this is the excess energy that cannot remain within the nucleus


No particle,


no mass


There is no charge on the gamma radiation and this means they do not pull electrons away from molecules (i.e. ionizes molecules), therefore they do not slow down like Alpha or Beta particles and because they do not Ionize everything they get to go much much further.



Most Hazardous when

Examples of emitters

There is nothing as such as a pure gamma emitter, gamma rays are given off after an Alpha or Beta particle has been released, this is the excess energy that cannot remain within the nucleus. This energy is made and is extra to the after side of the ejection equation. 60 Co.

Useful gamma sources include Technetium-99, which is used as a "tracer" in medicine. This is a combined beta and gamma source, and is chosen because betas are less harmful to the patient than alphas (less ionisation) and because Technetium has a short half-life (just over 6 hours), so it decays away quickly and reduces the dose to the patient.

A very easy way to draw these decays is by using a graph of Atomic Number Vs Mass Number

check this out

The following is a radiation path that an atom of U-238 could follow, notice how it emits particles and thus changes its nature.

Demonstration of ionisation and penetration by the radiations using any suitable method, e.g. electroscope, G-M tube.

Principle of operation of a detector of ionising radiation.

Johannes Wilhelm Geiger was a German physicist who introduced the first reliable detector for alpha particles and other ionising radiation. We still use his basic design today, although more advanced detectors are also in use.

Geiger gained his PhD at the University of Erlangen in 1906, then joined the University of Manchester, becoming one of Ernest Rutherford's most valued colleagues. Here he built his first particle counter and used it in experiments that identified alpha particles as being the same as the nucleus of a Helium atom.

In 1912 he moved to the German National Institute for Science and Technology, where he continued to study atomic structure. Geiger served as an artillery officer during the First World War.

He accepted his first teaching position in 1925 at the University of Kiel, where he worked with Walther Müller to improve the sensitivity and performance of his particle counter. The modern Geiger-Müller tube detects both alpha and beta radiation, along with other photons.

In 1929 Geiger moved to the University of Tübingen, where he investigated cosmic rays, moving on to the Technische Hochschule in Berlin in 1936 to work with nuclear fission and artificial radioactivity, until his death in 1945.

Definition of becquerel (Bq) as one disintegration per second.

Demonstration of G-M tube or solid-state detector.

The radiation comes through the thin Mica window, this will randomly ionize the air particles within the tube. Due to the very large potential difference across the tube (from the core to the shell) these ions are drawn to the the shell. This gives a short pulse of electricity, these short pulses can be counted using a scaler timer. This gives us an indication of the level of radioactive particles in the region in front of the mica window.

build your own GM tube

Interpretation of nuclear reactions.

Activity of a radioactive substance is the number of nuclei that decay per second. This is equal to the number of Becquerels, as one Becquerel (Bq) is one radioactive disintegration per second

Law of radioactive decay.

Its not excellent but certainly worth watching

the rate of decay is proportional to the number of radioactive particles that were contained within a body.

A good simulation of the decay process

Concept of half-life: T1/2.Half-life is a method of classifying the elements that emit radioactive particles and their rate of emission. Half life is a measurement of time and is the time taken for half the particles in the sample (that can) to decay into other matter with the emission of a radioactive particle

Atoms that are radioactive naturally decay, change into (2 or more) smaller atoms. Like leaves falling off trees, we cant say when a particular leaf will fall but I can say that half the leaves will fall off by oct 1. So wrt a sample of rad

ioactive material half the atoms will have decayed in x time. Half life is the time it takes for half the radioactive particles to change. After 1 half life half the particles will have decayed, after 2 halflives we will be left with a quarter the particles and so on ... Like the frog that can jump only half the width of the road .... Trees and leaves ... Hope that helps

Half life!

Is Walter not the best

well maybe he is but here is the alternative look for Atomscope (on this machine or on the i drive) (now in the skool folder)

nonstable isotopes disintegrate or decay in order to determine the half-life of all known isotopes

Concept of decay constant

You can heat the substance up, subject it to high pressure or strong magnetic fields - in fact, do pretty much whatever you like to it - and you won't affect the rate of decay in the slightest.

rate of decay = λ N

Decay Process


T1/2 = ln2 λ

Appropriate calculations (not requiring calculus).

Uses of radioisotopes:

• medical imaging The page below explains in easy to clear and non physicist terms, the entire spectrum of Medical Radioactivity

• medical therapy

• food irradiation

• agriculture

• radiocarbon dating

• smoke detectors

• industrial applications.

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