Radioactivity — Laboratorium

Chart of nuclides

Click a square · drag to pan · after clicking the chart, the arrow keys work

The stable nuclei are the real list. The colors of unstable nuclei come from a simplified model (a formula for nuclear mass), so they are approximate; nuclei marked with a dot have real data. The extent of the chart is approximate.

Selected nuclide

Protons Z

Neutrons N

Decay

Half-life

The uranium series: from uranium-238 to stable lead-206

Protons Z
Neutrons N
Nucleons A
T½

The path of the series grows step by step on the chart of nuclides above (the “U series” view): an α decay is a jump 2 to the left and 2 down, a β⁻ decay — 1 to the left and 1 up. ★ — radium-226 and radon-222, which matter in our everyday surroundings. We show the main branch; a few nuclei also have rare side branches.

Four kinds of decay

What a nucleus is made of

A nucleus is a tightly packed bundle of protons (charge +) and neutrons (no charge). Protons repel each other, but at very short distances the nuclear force pulls them together. Neutrons act like “glue” — that is why heavy nuclei need more of them than protons.

Z — atomic numberthe number of protons; it decides which element it is.

N — neutron numberisotopes of the same element differ in N.

A = Z + Nthe mass number, e.g. uranium-238 has A = 238.

Uranium in the periodic table Where the elements came from

Radon in Polish homes

Radon-222 is a noble gas from the uranium series. It forms in soil and rock and can seep through cracks in floors into basements and ground floors.
Together with its decay products it gives, on average, the biggest share of our yearly dose from natural sources. There tends to be more radon where the ground contains a lot of it, e.g. in parts of the Sudetes mountains.
In Poland the reference level for the average yearly concentration in homes and workplaces is 300 Bq/m³. Measuring it with a detector is simple and inexpensive, and airing rooms and sealing floors usually helps a lot.

Source, shields and counter

Drag a shield from the shelf onto the particle path (or click it). Drag it down to remove it. The drawing is not to scale.

Shields and distance

Source–counter distance 3 cm

Geiger-Müller counter

0CPM

From the source (average): —
Natural background: —
Shield transmission: —

CPM — counts per minute. Natural background is usually a few dozen CPM (it depends on the counter and the place). The reading jumps around because decays are random. An ordinary Geiger tube barely detects neutrons — here we pretend to have a neutron counter (e.g. one with helium-3). Esc mutes the sound.

Material	γ 662 keV: half-value layer	Notes
lead	≈ 0.6 cm	very dense, great against γ
concrete	≈ 4–5 cm	cheap, good against neutrons too
aluminum	≈ 3.4 cm	a few mm stop β
water	≈ 8 cm	hydrogen slows neutrons down

Approximate values for a narrow beam. The half-value layer is the thickness after which half of the γ radiation remains.

Sample

Half-life T½ 5.0 s

Time: 0.0 s
Atoms left: 1,024
Theory N₀·(½)^t/T½: 1,024.0

Graph of N(t)

Start the sample and watch random decays line up into a smooth curve. Click the graph to read off values.

Radiocarbon dating (C-14)

Living things exchange carbon with their surroundings, so they contain a constant, tiny admixture of carbon-14. After death the exchange stops, and C-14 decays with T½ = 5,730 years. All you need is to measure how much is left.

C-14 left (compared with a living organism) 50.0%

—

Puzzle: how old is the sample?

Age in years:

Clocks written in rocks

Potassium–argon (K-40 → Ar-40)T½ ≈ 1.25 billion years. Argon is a gas — it escapes from molten lava and then builds up in the solidified rock. Good for dating volcanic rocks.

Uranium–lead (U-238 → Pb-206)T½ ≈ 4.47 billion years, and alongside it U-235 → Pb-207 (704 million years). Two clocks in one tiny zircon crystal check each other.

The Earth is about 4.54 billion years old — a figure found partly with the lead method applied to meteorites (Clair Patterson, 1956). The oldest known zircon grains, from Australia, are about 4.4 billion years old.

t = T½ · log₂N₀Nage from the fraction that remains; in rocks N₀ = N + the number of daughter nuclei

From microseconds to billions of years

Logarithmic scale. The record holders go even further: bismuth-209 has T½ of about 2·10¹⁹ years — over a billion times longer than the age of the Universe.

Dose ladder (logarithmic scale)

Rough values — the real dose depends on the machine, the flight route, body build and location. Each tick on the scale is 100 times more. Doses at the top of the range (like “1 Sv at once”) only happen in accidents, never in ordinary life.

Smallest first

Rearrange the cards with the arrows so the doses grow from top to bottom. You'll see the values once they're in order.

Gray, sievert, becquerel and curie

Becquerel (Bq) — activity

1 Bq = 1 decay per second. It tells you how “alive” a source is, not how much harm it will do.

The old unit, the curie (Ci), named in honor of the Curies: 1 Ci = 3.7·10¹⁰ Bq — roughly the activity of 1 gram of radium-226.

Gray (Gy) — absorbed dose

1 Gy = 1 joule of energy absorbed by 1 kilogram of the body. This is pure physics: how much energy stayed in the tissue.

In radiotherapy a tumor receives a total of several dozen Gy, but in many small portions and only at the chosen spot.

Sievert (Sv) — effective dose

It takes into account that different kinds of radiation and different organs respond differently. Dose in Gy × weighting factor:

γ photons, X-rays	× 1
electrons and positrons (β)	× 1
protons	× 2
neutrons (depending on energy)	× 2.5–20
α particles	× 20

Your yearly dose — an educational estimate

Radon at home 50 Bq/m³

Altitude above sea level lowland

Hours on planes per year 10 h

Dental X-rays 2

Chest X-rays 0

CT scans 0

— mSv / year

This is not a medical tool. Assumptions (approximate): cosmic radiation about 0.3 mSv in the lowlands, the Earth's crust about 0.5 mSv, potassium-40 and other isotopes in the body about 0.3 mSv, radon about 0.025 mSv a year for every 1 Bq/m³ (according to UNSCEAR; newer ICRP recommendations give more), flying about 5 µSv per hour. Medical scans are done when the benefit outweighs the risk — a doctor decides on them.

Where radiation helps

PET — positron emission tomography

The patient gets glucose labeled with fluorine-18 (T½ ≈ 110 min), which undergoes β⁺ decay. After traveling a fraction of a millimeter, the positron meets an electron and both vanish — they annihilate. Two photons of 511 keV each are produced, flying off in almost exactly opposite directions.

A ring of detectors catches them at the same instant and draws a line. Thousands of lines cross where cells use the most glucose — e.g. in an active tumor.

e⁺ + e⁻ → γ + γeach photon: E = m_ec² ≈ 511 keV

Radiotherapy

Precisely aimed beams destroy the DNA of cancer cells. The dose is split into many portions so healthy tissue has time to repair itself.

The cell and DNA

Technetium-99m

The most widely used isotope in medical imaging: it emits 140 keV γ rays and has T½ of about 6 h. Hospitals “milk” it from a generator containing molybdenum-99.

Blood and circulation

Sterilization

γ radiation (e.g. from cobalt-60) or an electron beam sterilizes syringes and gloves inside sealed packaging. The items do not become radioactive.

Smoke detector

Many detectors contain a tiny amount of americium-241 (about 30–40 kBq). α particles ionize the air in a chamber and a weak current flows; smoke reduces it and sets off the alarm. The casing easily stops α.

Power for space probes

Radioisotope thermoelectric generators (RTGs) turn the heat from the decay of plutonium-238 (T½ ≈ 88 years) into electricity. They have powered the Voyagers since 1977 and the Curiosity rover on Mars.

Solar System Rocket

Energy

Uranium fission in nuclear power plants produces electricity without CO₂ emissions during operation. The fuel lasts a long time and the waste is small in volume, but it must be stored carefully.

Power plant

Marie Skłodowska-Curie (1867–1934)

A physicist and chemist who gave radioactivity its name, discovered two elements and was the first person to win two Nobel Prizes.

1867
On November 7, in Warsaw, on Freta Street, Maria Salomea Skłodowska is born, the youngest of five children in a family of teachers.
1883
She finishes secondary school with a gold medal. At the time women cannot study at a university in the Russian-ruled part of Poland, so she studies at the secret Flying University.
1886
For more than three years she works as a governess, among other places in Szczuki, and helps her sister Bronisława pay for medical studies in Paris.
1891
She moves to Paris and begins studying at the Sorbonne. She earns degrees in physics (1893, ranked first) and in mathematics (1894).
1895
She marries the physicist Pierre Curie.
1896
Henri Becquerel discovers that uranium salts darken a wrapped photographic plate — without any sunlight.
1898
Marie measures “uranium rays” with a sensitive electrometer and notices that pitchblende (uranium ore) is more active than its uranium alone would explain. In July she and Pierre announce polonium — named after Poland, which was not on the map at the time — and in December radium. That same year the word “radioactivity” (radioactivité) appears.
1902
After years of processing tons of pitchblende waste from Joachimsthal (today Jáchymov in Czechia) they obtain about 0.1 g of pure radium chloride and determine the atomic mass of radium.
1903
Doctorate. The Nobel Prize in Physics — together with Pierre Curie and Henri Becquerel. Marie is the first woman to win a Nobel Prize.
1906
Pierre is killed after being struck by a horse-drawn wagon. Marie takes over his lectures — the first woman to teach at the Sorbonne.
1911
The Nobel Prize in Chemistry for the discovery of radium and polonium. The first person with two Nobel Prizes and to this day the only one honored in two different sciences.
1914
During World War I she organizes mobile X-ray units — soldiers called them “petites Curies” (“little Curies”). With her daughter Irène she trains the operators; about 20 vehicles and about 200 fixed X-ray rooms are set up.
1932
The Radium Institute in Warsaw opens; she founded it together with her sister Bronisława Dłuska, its first director. Today it is the National Cancer Institute that bears her name.
1934
She dies on July 4 in France of aplastic anemia — most likely the result of many years of working with radiation, whose dangers were little understood at the time.
1995
The remains of Marie and Pierre are moved to the Panthéon in Paris. Marie was the first woman buried there on her own merits.

Her laboratory notebooks still contain traces of radium-226 (T½ = 1,600 years). Reportedly they are kept in Paris in lead-lined boxes, and readers handle them wearing protective clothing. The element curium (Cm, 96) was also named in honor of Marie and Pierre.

Polonium (Po)Radium (Ra)Uranium (U)Spectroscopy — how elements are identified

Find the radium — an ore purification game

You have a ton of pitchblende waste. It contains only about 0.2 g of radium. The electrometer shows how strongly one gram of your sample radiates. Dissolve, precipitate and crystallize until only pure radium chloride is left in the flask.

Sample mass: 1,000 kg
Radium share: —
Steps: 0

The flask is waiting for radium.

The game is schematic: the numbers of steps and losses are chosen to show the idea of the method. The real work took years, and there were thousands of crystallizations. Radium behaves chemically like barium, so it travels with barium through each stage; radium chloride crystallizes a little more easily than barium chloride — and the separation relies on that tiny difference.

How Marie measured radiation

Radiation ionizes air, so a tiny current flows between two plates under voltage. Marie balanced it with charge from a quartz crystal squeezed by weights — the piezoelectric effect, discovered in 1880 by Pierre and his brother Jacques Curie. This let her compare the activity of samples in numbers, not “by eye” from how dark a photographic plate turned.

Chain reaction sandbox

subcritical

U-235 share in the grid 20%

Control rod insertion 75%

Neutrons now: 0
Factor k: —
Fissions: 0
Energy: 0 J

A toy model: distances, speeds and probabilities are chosen for clarity, and split nuclei “come back” after a moment so the fun can go on. In a real reactor the neutrons are slowed down by water (the moderator), and the reaction is also stabilized by delayed neutrons and automatic safety systems. The number of neutrons is capped at 600.

What happens in the nucleus

n + ²³⁵U → ¹⁴¹Ba + ⁹²Kr + 3 n + ≈ 200 MeVone of many possible fissions; on average about 2.4 neutrons are released

A neutron enters a uranium-235 nucleus, which wobbles and splits into two smaller nuclei. The fragments and new neutrons fly off with enormous energy. If on average exactly one of these neutrons causes another fission, the reaction is critical (k = 1) — that is how a reactor runs. When k < 1 the reaction dies out; when k > 1 it speeds up, so the control rods have to go in.

Fissioning all of 1 g of uranium-235 gives roughly as much heat as burning about 3 tons of coal.

1938
Otto Hahn and Fritz Strassmann in Berlin find barium after bombarding uranium with neutrons.
1939
Lise Meitner and Otto Frisch explain that the uranium nucleus has split in two and call it fission. Only Hahn received the Nobel Prize (1944).
1942
On December 2 in Chicago, Enrico Fermi's team starts up CP-1 — the first reactor with a self-sustaining chain reaction.

Power plant: fission, thorium, fusion Uranium in the periodic table

Radioactivitythe glowing secrets of Marie Skłodowska-Curie

Chart of nuclides

Selected nuclide

The uranium series: from uranium-238 to stable lead-206

Four kinds of decay

What a nucleus is made of

Radon in Polish homes

Source, shields and counter

Shields and distance

Geiger-Müller counter

Sample

Graph of N(t)

Radiocarbon dating (C-14)

Puzzle: how old is the sample?

Clocks written in rocks

From microseconds to billions of years

Dose ladder (logarithmic scale)

Smallest first

Gray, sievert, becquerel and curie

Becquerel (Bq) — activity

Gray (Gy) — absorbed dose

Sievert (Sv) — effective dose

Your yearly dose — an educational estimate

Where radiation helps

PET — positron emission tomography

Radiotherapy

Technetium-99m

Sterilization

Smoke detector

Power for space probes

Energy

Marie Skłodowska-Curie (1867–1934)

Find the radium — an ore purification game

How Marie measured radiation

Chain reaction sandbox

What happens in the nucleus

How does it work?

Three (and a half) ways for a nucleus to change

The decay law

Shields and distance

Where fission energy comes from

Challenges

Did you know…?

Keys