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Geiger Counters and Measuring Radiation Radioactivity is all around us, both from natural and artificial sources. But it is usually invisible, so how do we tell if it is there? There are quite a few passive and electronic methods for detecting and classifying radiation. This article investigates radioactivity, radioactive sources and ways to measure radioactivity electronically, including Geiger counters. By Dr David Maddison N atural sources of radioactivity include soil and rocks (terrestrial radiation) and radiation from space (cosmic radiation). Artificial sources include atomic bombs, nuclear reactors, the concentration of natural radioactive materials by mining and the refinement or irradiation of non-radioactive materials such as in particle accelerators. Radioactivity may be referred to as “radiation” but as well as nuclear radiation, that term also covers non-­ ionising electromagnetic radiation like radio waves plus visible and infrared light. Image Source: https://unsplash.com/photos/sS5TcHkSxe8 expressed as a half-life. This is the time required for the radioactive atoms to decay to half the original number. Less common forms of radioactive decay include neutron emission, when a nucleus loses a neutron; electron capture, in which a nucleus captures an electron causing a proton to convert to a neutron; and cluster decay, in which a nucleus other than an alpha particle is emitted. Fig.1 & Table 1 show the penetrating 14 Silicon Chip Atoms and isotopes Atoms are a basic building block of matter that form chemical elements and compounds. They consist of a nucleus comprising positively charged protons and neutral neutrons, with a surrounding cloud of negatively Table 1: Characteristics of the three main types of radiation Alpha (α) (4He) Beta (β) Gamma (γ) Electromagnetic energy Nature A helium nucleus – two protons and two neutrons An electron (e−) or a positron (e+) Electric charge +2 -1 or +1 (positron) 0 Mass Relatively large Very small None Speed Slow Fast Speed of light Ionising effect Strong Weak Very weak Most dangerous Inside the body Outside the body Outside the body What is radioactivity? Put simply, radioactivity is the spontaneous emission of sub-atomic particles known as alpha and beta particles, or gamma rays, from the nuclei of unstable atoms. While individual radioactive decay events are random, when a great many atoms are involved, the decay process becomes predictable and can be power of the common types of radioactivity. Alpha and beta radiation are most easily stopped, while gamma radiation requires robust shielding. Australia's electronics magazine siliconchip.com.au α β γ Paper Aluminium I Period Fig.1: the penetrating ability of different common forms of radiation, as investigated by Rutherford. Alpha particles are stopped by paper (or human skin), a sheet of aluminium stops beta particles, while gamma rays are only stopped by a substantial thickness of dense matter such as lead. Source: Wikimedia user Lead Group Stannered (CC BY 2.5) II III 1 1 H 2 3 Li 4 Be 3 11 Na 12 Mg 4 19 K 20 Ca 21 Sc 22 Ti 23 V 24 Cr 25 Mn 26 Fe 27 Co 28 Ni 29 Cu 5 37 Rb 38 Sr 39 Y 40 Zr 41 Nb 42 Mo 43 Tc 44 Ru 45 Rh 46 Pd 47 Ag IV V VI VII VIII 2 He Half-lives 6 55 Cs 56 Ba 7 87 Fr 88 Ra stable over 4 million years between 800 and 34,000 years between 1 day and 130 years highly radioactive; between minutes and a day extremely radioactive; no more than a few minutes Fig.2: the traditional (Bohr) model of a carbon atom. 5 B 6 C 7 N 8 O 9 F 10 Ne 13 Al 14 Si 15 P 16 S 17 Cl 18 Ar 30 Zn 31 Ga 32 Ge 33 As 34 Se 35 Br 36 Kr 48 Cd 49 In 50 Sn 51 Sb 52 Te 53 I 54 Xe * 72 Hf 73 Ta 74 W 75 Re 76 Os 77 Ir 78 Pt 79 Au 80 Hg 81 Tl 82 Pb 83 Bi 84 Po 85 At 86 Rn ** 104 Rf 105 Db 106 Sg 107 Bh 108 Hs 109 Mt 110 Ds 111 Rg 112 Cn 113 Nh 114 Fl 115 Mc 116 Lv 117 Ts 118 Og stable 1014 yr 160 1012 yr 1010 yr 140 108 yr 106 yr 120 104 yr 100 yr 100 1 yr Z=N 80 100 s 60 1s 40 * Lanthanides 57 La 58 Ce 59 Pr 60 Nd 61 Pm 62 Sm 63 Eu 64 Gd 65 Tb 66 Dy ** Actinides 89 Ac 90 Th 91 Pa 92 U 93 Np 94 Pu 95 Am 96 Cm 97 Bk 98 Cf Fig.3 (left): a periodic table of the elements showing the properties of the most stable isotope of each element. Source: Wikimedia user Armtuk (CC BY-SA 2.5) charged electrons. The overall charge of an atom is neutral unless the atom is chemically combined or ionised, such as in extremely hot gas (plasma). Fig.2 is a representation of a carbon atom. Although this is not what an atom looks like according to current understanding, it illustrates the basic structure of a typical atom. The nuclei of most common atoms are stable and are not subject to radioactive decay over short periods. Still, some are unstable and decay over periods from tiny fractions of a second to billions of years. Most elements also have one or more isotopes. Isotopes are chemically (almost) identical, but they vary by the number of neutrons in the nucleus, hence the atomic mass. Isotopes can be separated by techniques exploiting their slight mass difference, such as in a mass spectrometer or centrifuges. There can be very slight differences in the chemical behaviour of different isotopes of the same element; these are more pronounced in isotopes of siliconchip.com.au 106 s 104 s 67 Ho 68 Er 69 Tm 70 Yb 71 Lu 20 99 Es 100 Fm 101 Md 102 No 103 Lr N 10−2 s 10−4 s 10−6 s Z 20 40 60 80 100 10−8 s no data Fig.4 (right): a ‘nuclide chart’ showing the half-lives of various isotopes by their colour. The horizontal axis indicates the element number (Fig.3), and the vertical axis is the number of neutrons in each isotope. Each element has many isotopes; darker colours represent more stable ones, with blue indicating less stable isotopes. lighter elements such as hydrogen; in this case, protium (1H), deuterium (2H or D) and tritium (3H). Deuterium (2H) atoms have roughly twice the mass of ordinary hydrogen (1H). So deuterium compounds behave quite differently than regular hydrogen compounds. Deuterium combined with oxygen makes heavy water or D2O. It has various scientific uses, including moderating nuclear reactions such as in ‘heavy water reactors’. Sometimes you will see isotopes written with a number after the element, eg, U235, U-235 or uranium-235 for 235U, but we will stick with the latter scientific notation in this article for clarity. The periodic table A periodic table is a common way to list and show the relationship between the chemical elements. The one shown in Fig.3 colour codes the chemical elements by their half-lives. The longer the half-life, the more stable the element or isotope is and the Australia's electronics magazine less radioactive. Notice that it’s mostly the higher numbered, less common elements that are less stable. A similar relationship is shown in the ‘nuclide chart’, Fig.4. Such charts in their full versions are highly detailed and contain thousands of entries and data. A popular one is the Karlsruhe Nuclide Chart. There are no stable elements or isotopes above element 82 (lead). The highest numbered natural element is 92, uranium. Elements above 92 do not exist in nature in any significant quantity because of their instability. The discovery of radioactivity It started with Henri Becquerel (1852-1908). In 1896, he used naturally phosphorescent compounds such as potassium uranyl sulfate to investigate X-rays (discovered by Wilhelm Roentgen the previous year). The uranium compound caused photographic plates to become exposed. When it was noticed that even non-phosphorescent uranium April 2022  15 Fig.6: an alpha particle being emitted from an atomic nucleus. Source: https://commons.wikimedia.org/wiki/ File:Alpha_Decay.svg Fig.5: the apparatus used by Becquerel to show the particles he discovered were influenced by a magnetic field. In this diagram, the magnetic field is perpendicular to the page. compounds did this, they realised that they must be emitting something similar to light but invisible. In fact, much earlier in 1861, Abel Niépce de Saint-Victor wrote that uranium salts produce “a radiation that is invisible to our eyes”. Becquerel’s father made similar written observations; however, Becquerel is credited with the discovery. Becquerel used an apparatus similar to that shown in Fig.5 to demonstrate that the particles had a charge, as they were deflected in different directions by a magnetic field. But other particles went straight ahead, like X-rays, meaning they were electrically neutral. Marie Curie (1867-1934) and her husband Pierre (1859-1906) started investigating the phenomenon reported by Becquerel. They coined the term radioactivity. Marie’s investigation was the subject of her PhD thesis. They used a quadrant electrometer, which measures electric charge, to measure radioactivity (see https://lamethodecurie.fr/ en/article23.html). They extracted uranium from its ore but then found that the leftover ore was more radioactive than the extracted uranium, and concluded there must be other radioactive elements present. They eventually discovered polonium and radium, but these were present in minute quantities, and many tonnes of ore had to be processed to get usable amounts. One tonne of pitchblende ore had to be processed to obtain 1g of radium, which was one million times more radioactive than uranium. Marie also co-discovered independently that previously-discovered thorium was radioactive. 16 Silicon Chip Ernest Rutherford (1871-1937) from New Zealand is regarded as the “father of nuclear physics”. In 1899, he coined the terms for two of the three common types of radiation: alpha and beta. Alpha and beta particles were influenced by a magnetic field, while gamma rays were not. He is credited with the discovery of alpha and beta particles. Then, in 1903, he investigated and named gamma rays, the third common type of radiation. However, these had been discovered by Frenchman Paul Villard in 1900 but not named at the time. Rutherford classified the three types of radiation according to their penetrating power. He also discovered the concept of radioactive “half-life”. Common types of radiation Alpha particles consist of two protons and two neutrons (a helium nucleus) and have a charge of +2 (see Fig.6). An alpha particle with an energy of 5MeV can travel a few centimetres in air. Beta particles are electrons with a charge of -1 or antimatter positrons with a charge of +1. A beta particle with an energy of 0.5MeV can travel about 1m in air. Gamma rays are high-intensity electromagnetic radiation. These are the shortest waves of the electromagnetic spectrum, with a frequency of 3 × 1019Hz. They are highly penetrating and can travel long distances in air. Thick, dense shielding such as lead or concrete are required to stop them. Gamma rays usually originate after alpha or beta emission leaves a nucleus in an excited state, which then emits a gamma ray when it relaxes to a lower Australia's electronics magazine Example images of Beta and Gamma decay can be respectively viewed at https://w.wiki/4ma6 and https://w. wiki/4ma7 energy state. Gamma rays also originate in nuclear explosions and fission and fusion processes, thunderstorms (a terrestrial gamma-ray flash), solar flares, cosmic rays and other processes. Intense neutron radiation can be generated during fission or fusion reactions or in particle accelerators, and due to a lack of charge, penetrate similarly to gamma rays. Measuring radioactivity Geiger counters are a common way to measure radioactivity, but there are other methods such as scintillation counters, proportional counters, ionisation chambers, semiconductor detectors, dosimeters (which can be worn) and particulate air monitors in nuclear facilities. Radiation may need to be monitored for reasons such as health and safety, use of medical isotopes for medical imaging (see August & September 2021; siliconchip.com.au/Series/369), scientific research, some types of smoke alarms, product sterilisation, evaluation of the density of materials, elimination of static electricity, tracing of groundwater flows and more. The Geiger counter The Geiger counter is probably the most well-known type of radiation measuring device. The detecting component is a Geiger–Müller tube. This is a tube filled with a low-pressure inert gas with a central anode and outer cathode, with about 400-900V applied between them – see Fig.7. As a radiation particle enters the window, which may be at the end or around the circumference, it causes the gas in its path to become ionised and siliconchip.com.au Fig.7: how a Geiger counter works. Source: Wikimedia user Svjo-2 (CC BY-SA 3.0) Fig.8: how an ionisation chamber works. Original Source: Wikimedia user Dougsim (CC BY-SA 3.0) conductive. This results in a cascading discharge known as a Townsend Avalanche, causing a large, easy-to-­ measure current pulse. This makes Geiger counter electronics cheap and simple to manufacture. The limitations are that they cannot measure a high radiation rate or determine the energy level or identity of the incident radiation. Ionisation chambers Ionisation chamber radiation measuring devices are widely used in nuclear industries. They have a good response over a wide range of radiation energies, and are the preferred method of detecting and measuring high-­energy gamma rays. These devices typically have two parallel plates with an electric field (typically 100-400V) between them and a chamber, usually at air pressure – see Fig.8. When a radiation particle enters the chamber, it disassociates gas molecules along its path into ion pairs that drift to the chamber’s anode or cathode. This creates an ionisation current, and the more pairs produced, the greater the current and thus radiation dose. The current is usually tiny, on the order of femtoamperes to picoamperes, so electrometer circuitry is needed to sense it. A domestic smoke detector of the type that uses a radiation source, as shown in Fig.9, is an example of an ionisation chamber. Most Cold War era devices for radiation surveys after a nuclear attack were based on an ionisation chamber rather than a Geiger-Müller tube. The latter tends to saturate at high radiation levels, giving a falsely low reading. An example is shown in Fig.10. siliconchip.com.au Fig.9: an ionisation-type smoke detector sensor, which uses an ionisation chamber and alpha-emitting 241Am (americium) to detect smoke. Fig.10: a US radiation survey meter of the Cold War era, the Victoreen Instrument Co. model CDV-715 (1961-1974). It is an ionisation chamber device and is most sensitive to high range gamma rays for radiation surveys after a nuclear attack. These are sold on eBay and elsewhere as collector’s items. Source: Wikimedia user Mrcomputerwiz (CC BY 3.0) Australia's electronics magazine April 2022  17 The boy who built a nuclear reactor In 1994, David Hahn (USA), aged 17, scavenged vast amounts of radioactive materials from sources such as smoke alarms, lantern mantles, radium-faced clocks and watches, uranium from Czechoslovakia and any other radioactive materials he could find. He also obtained the required lithium for his device from US$1000 worth of batteries. He researched and tried to build a breeder reactor with the hope of creating fissionable isotopes from thorium and uranium. It is widely reported that he made a reactor, but it was more correctly a neutron source that he managed to construct. At one point, he found that the radiation levels kept on rising and could even be detected from a long distance away from his bedroom. When he discovered that he could detect radiation from five houses down the street, he started to get worried and wanted to dismantle the device. When trying to load it into his car, his neighbours called the police because they thought he was stealing something. The boy warned police not to search the car as the material was Scintillation counters A scintillation counter uses a scintillation crystal that turns incident radiation into light photons, which can be detected with a photomultiplier, charge-coupled device (CCD) or photodiode – see Fig.11. Examples of scintillator materials are sodium iodide with thallium, zinc sulfide, lithium iodide or anthracene. Proportional counters A proportional counter combines features of both the Geiger-Müller tube and an ionisation chamber in a single device. It generates a pulse proportional to the radiation energy detected, and is typically used when accurate energy levels must be known. Semiconductor detectors Semiconductor detectors use a material such as doped silicon, germanium, cadmium telluride and cadmium zinc telluride to detect radiation. They work on the principle that Ionisation track High energy photon radioactive. The police thought he had an atomic bomb, so they called the bomb squad. Government authorities argued over whose job it was to clean up the site. A book was written about him by Ken Silverstein called “The Radioactive Boy Scout: The true story of a boy and his backyard nuclear reactor” (2004). There was also a 2003 movie made about him titled “The Nuclear Boy Scout” – see www.eagletv. co.uk/projects/the-nuclearboy-scout.html Also see the video “Radioactive Boy Scout – How Teen David Hahn Built a Nuclear Reactor” at https://youtu.be/ G0QMeTjcJDA radiation striking the semiconductor causes charge carriers to be spontaneously created, increasing the material’s conductivity briefly and causing spikes of extra current to flow above the baseline. Radiation hardening of electronics We have previously written about the need to provide radiation hardening for chips in military and space applications; see the article in the July 2019 issue titled Radiation Hardening (siliconchip.com.au/Article/11697). Electronics operating in high-­ radiation environments like space or a nuclear reactor need significant amounts of shielding and must be designed to tolerate radiation harmlessly, with larger and more robust semiconductor junctions etc. But there is also the problem of radiation emanating from within electronic devices, including solder and the material used to package the devices. Photomultiplier tube (PMT) Photocathode Focusing electrode Low energy photons Scintillator Primary electron Secondary electrons Connector pins Dynode Anode Fig.11: a scintillation counter using a photomultiplier tube. Source: Wikimedia user Qwerty123uiop (CC BY-SA 3.0) 18 Silicon Chip Australia's electronics magazine High component density devices like modern CPUs need to be made from silicon with no radioactive isotopes present; otherwise, radioactive decay can trigger unwanted state changes in the device. Onboard ECC (error checking and correction) is another vital tool for handling cosmic rays and other sources of spontaneous radiation. Radiation measurement units SI units are typically used for radiation measurements in Australia, New Zealand, Europe and most other countries. A few countries like the USA use non-SI units. Radioactivity is measured in terms of how many particles or photons (in the case of wave radiation such as gamma rays) are emitted per second. The SI unit is becquerel (Bq) while the US unit is the curie (Ci). For example, a Geiger counter giving two counts per second means the substance has a radioactivity of 2Bq (becquerel). The use of the curie unit is discouraged (even in the USA), but 1Ci is about 37GBq. Some Geiger counters give measure counts per second for a direct readout in Bq. A related measurement is particle flux, which is typically counts per square metre per second. The radiation exposure of humans is of particular importance. For this, there are three parameters to consider: • Absorbed Dose, which is the energy deposited by the radiation into the person • Equivalent Dose, which is the siliconchip.com.au Living near nuclear power station annually <0.01mSv Mammogram procedure 0.42mSv Fig.12: a radon detector as used to monitor radon levels in the basements of homes in radon-rich areas of the United States of America. Absorbed Dose with a weighting factor taking into account the relative harm of different types of radiation in a person • Effective Dose, which is the Equivalent Dose with a weighting factor taking into account the susceptibility of different tissues to radiation The Roentgen (R) is an obsolete unit of radiation exposure for X-rays and gamma rays in air. It has been replaced by rads (USA) and gray (Gy; SI). 1Gy = 100rad. The units of Equivalent Dose are sievert, Sv (SI units) or rem (USA) for “roentgen equivalent man”. 1Sv = 100rem. The weighting factor for x-rays, gamma rays and electrons absorbed by human tissue is 1, while for alpha particles, it is 20. To establish the Equivalent Dose, multiply the Absorbed dose in grays by the weighting factor, giving a result in sieverts. The units of Effective Dose are sievert, with a weighting factor for different organs, with organs having the most rapidly dividing cells being the most sensitive with the highest weighting factor. For more details, see www.epa.gov/radiation/radiationterms-and-units Natural sources of radiation Natural radiation is usually nothing to worry about, with rare exceptions. As mentioned above, it is either of terrestrial or space origin. Natural radioactive materials are often referred to as Naturally Occurring Radioactive Material (NORM). Natural radioactivity is one of the causes of mutations in living organisms that siliconchip.com.au Chest X-ray procedure 0.1mSv Terrestrial Radioactivity annually 0.21mSv Radiation in the body annually 0.29mSv Cosmic radiation living at sea level (low elevation) annually 0.3mSv Cosmic radiation Head CT Radon in average Upper gastroWhole body CT living in Denver procedure 2mSv US home intenstinal X-ray procedure 10mSv (high elevation) annually 2.28mSv procedure 6mSv annually 0.8mSv Fig.13: radiation exposure for people living in the USA; the main differences in Australia is that we don’t live at high elevations, have virtually no nuclear reactors, and Australian homes do not usually have radon-accumulating basements. Note the figure for radiation from within the body, caused by naturally occurring radioactive elements. lead to genetic diversity. Examples of natural radiation that can be harmful include the accumulation of radon in certain buildings or mines, which must be monitored and controlled by appropriate ventilation measures (see Fig.12), and the possibility of exposure of flight crews to excessive cosmic radiation. Exposure of flight crews is not generally considered a serious problem, but it is monitored and restricted by following certain recommendations. These include limiting flights over the poles or high latitudes where there is more cosmic radiation and avoiding flying during solar flare events. The Equivalent Dose in a commercial airliner at high altitudes (around 40,000ft/12,192m) can be close to 60 times that at ground level; about 4.5μSv/h compared to 0.08μSv/h. Some recommendations for flight crew safety are at siliconchip.com. au/link/abcy Radiation in space is usually hazardous to both humans and electronics, and special measures must be taken to protect against its effects. Fig.13 shows some of the primary sources of radiation we are exposed to and how they compare in terms of Equivalent Dose. tends to accumulate. Basements need to be monitored and ventilated to prevent the accumulation of radon. Australian rates of radon exposure are low by world standards. According to a 1990 report by ARPANSA, the average concentration for indoor exposure was 1/4 the world average. In Australian homes, the average level was found to be about 10Bq/m3 compared to a worldwide indoor average of 40Bq/m3. Levels are higher along the Great Dividing Range than the coastal plain – see Fig.14. Cigarette radiation exposure Fertilisers contain naturally occurring radium. This decays into radon and sticks to the hairs called trichomes Radioactive basements According to the US EPA, 1 in 15 homes in the USA have more than the recommended amount of radon. It is believed to be responsible for 20,000 lung cancer deaths per year in that country. Since it is heavier than air, it Australia's electronics magazine Fig.14: an interactive radon map of south-east Australia from www. arpansa.gov.au/understandingradiation/radiation-sources/moreradiation-sources/radon-map April 2022  19 beneath tobacco leaves. The radon decays into lead-210 and polonium210, with polonium-210 being more hazardous. The radiation in tobacco depends to a certain extent on the soil in which the plant was grown and the origin of the fertiliser. Over time, these isotopes accumulate in smokers’ lungs, causing radiation damage on top of the damage from the smoke. A typical smoker is exposed to 40 times the annual radiation dose limit imposed on radiation workers (see www.bmj.com/rapid-­ response/2011/10/28/radioactivity-­ cigarettes). Cosmic radiation Cosmic radiation includes high-­ energy photons and atomic nuclei moving through space that originate in the sun, our galaxy or distant galaxies. When these particles hit the upper atmosphere, they induce showers of secondary particles including x-ray photons, muons, protons, antiprotons, alpha particles, pions, electrons, positrons, and neutrons. Cosmic rays are detected by dedicated cosmic-ray observatories (see Fig.15). You can see a video of a simulated cosmic-ray shower at https:// youtu.be/Wv0CtPskhus Artificial sources of radiation Non-natural sources of radiation include radiation associated with nuclear medicine, certain household products (eg, ionisation smoke detectors), food irradiation, industrial uses Fig.15: cosmic rays and gamma-ray air showers on Earth can be measured by various means. Original Source: Konrad Bernlöhr (CC BY-SA 3.0) of radiation (eg. radiography), scientific experiments (eg. those requiring a neutron source from a reactor for investigations into the structure of matter) and radioactive waste. A brief nuclear history of Oz Australia has a long nuclear history. We have vast deposits of radioactive minerals containing both uranium and thorium. We have had atomic explosions on our territory, and we have a medical isotope reactor at Lucas Heights, NSW. Australia has never committed to civilian nuclear power (sadly, in the author’s opinion). However, in the The Gilbert U238 Atomic Energy Laboratory This educational toy was sold in the USA in 1950-51 to teach children about radioactivity. The set contained a Geiger–Müller counter, electroscope to detect electric charge, a spinthariscope to observe individual nuclear disintegration events, a Wilson cloud chamber with an alpha source, four samples of different uranium ores, radioactive sources: betaalpha (210Pb), pure beta (possibly 106Ru – ruthenium) and gamma (65Zn – zinc), spheres to make a model alpha particle and various literature. Imagine trying to sell such an educational set today! The Gilbert U-238 Atomic Energy Laboratory from 1950-51. Source: Wikimedia user Tiia Monto (CC BY-SA 3.0) 20 Silicon Chip Australia's electronics magazine 1960s, two sites were identified for possible reactors, at Jervis Bay, NSW and French Island, Vic. Preliminary construction was undertaken at Jervis Bay. Also, we have now committed to purchasing nuclear submarines for the Navy. Australia’s first mine for radioactive minerals was at Radium Hill, SA, which operated from 1906-1961 and produced radium for medical purposes and uranium for glass and glazes. Here is an extraordinary quote from The Advertiser newspaper, 13th May 1913, about the radium mined there, long before nuclear energy was fully understood or appreciated (the full article is at https://trove.nla.gov.au/ newspaper/article/5404770): That one ounce of it is equal to one hundred thousand nominal horsepower, and that small quantity would be sufficient to drive or propel three of the largest battle ships afloat for a period of two thousand years; ...It will mean that foreign nations will be obliged to seek from us the power wherewith to heat and light their cities, and find means of defence and offence. In 1950-1971, uranium was mined in Rum Jungle, NT, and the ore was sent to the USA and UK to support nuclear weapons programs. Australia currently has several active uranium mines – see Figs.16 & 22. Thorium is not directly produced, but it is present in the mineral monazite, which is incidentally unearthed during the mining of mineral sands. siliconchip.com.au Fig.16: nuclear and radiation sites in Australia. This map was prepared by an anti-nuclear group; we do not necessarily support their views but the map is reasonably comprehensive. (CC BY-SA 3.0) April 2022  21 Australia's electronics magazine siliconchip.com.au Your body is radioactive Our bodies are naturally radioactive because we ingest natural radioactive materials found in the environment. The primary radioactive element in people is 40K (potassium), which emits beta particles 11% of the time and gamma rays 89% of the time. In a typical 70kg person, around 5000 atoms undergo radioactive decay each second, 550 of which emit gamma rays. Other radioactive isotopes in the body include alpha emitters 238U (uranium), 232Th (thorium) and their decay products and beta emitters 14C (carbon, hence carbon-14 dating) and 87Rb (rubidium). Other radioactive elements found in the body are 210Po (polonium) and 210Pb (lead). 40K is 0.0117% of all potassium, and the human body is about 0.2% potassium, so a 70kg person would have 16.38mg of radioactive potassium. One in 1,000,000,000,000 carbon atoms are radioactive, and a 70kg person is 23% carbon by weight, so 16.1ng of that carbon would be 14C. Despite all this, the dose rate is insignificant. It requires extremely sensitive and specialised instrumentation to measure. While gamma rays can be detected emanating from our bodies, alpha and beta emissions cannot be detected because the body absorbs them. However, gamma rays from decay products after alpha and beta emission can be detected. For more details, see http://hps.org/publicinformation/ate/faqs/ faqradbods.html The unwanted monazite is returned to the ground after the other minerals have been extracted. Nuclear tests in Australia Atmospheric nuclear weapon tests in Australia left radioactive soil contamination, which has since been cleaned up. Radioactive clouds also caused people to suffer medical conditions many years after ingesting radioactive materials. 12 British nuclear weapons were detonated between 1952 and 1957 (kt = yield in kilotonnes of TNT): • Montebello Islands: 1952 (25kt), 1956 (15kt & 60kt nominal, with the true yield claimed to be 98kt – see Fig.17) • Emu Field: 1953 (10kt & 8kt) • Maralinga: 1956 (12.9kt, 1.4kt, 2.9kt & 10.8kt), 1957 (0.93kt, 5.67kt & 26.6kt) That doesn’t include a series of minor tests involving conventional explosives and highly radioactive materials, including plutonium, polonium, beryllium and uranium, to improve bomb designs and test how radioactive materials dispersed. These tests were at Emu Field and various locations around Maralinga. Detecting nuclear explosions and materials Nuclear explosions can be detected by seismic, hydroacoustic and infrasound methods but of interest for this article are radiation measurements. One reason for detecting such explosions is to enforce international arms control treaties. Radiation is detected through ground-based or airborne atmospheric sampling, looking for 241Am (americium), 131I (iodine), 137Cs (caesium), 85Kr (krypton), 90Sr (strontium), 239Pu (plutonium), 3H (tritium), 133Xe and 135Xe (xenon); all signature isotopes of nuclear explosions. During the Cold War, the USA had a system of 12 satellites known as Vela, which had X-ray, neutron and gamma-­ ray detectors. These satellites were decommissioned around 1980. Their function has now been replaced with the Nuclear Detection System (NDS) as an auxiliary payload on US GPS satellites. The NDS sensors consist of a global burst detection (GBD) suite of instruments and a space environment dosimeter (BDD) – see Fig.18. The GBD consists of: • the BDY (bhangmeter), to detect an optical flash from the fireball of a nuclear detonation • the BDX, an X-ray sensor to discriminate between terrestrial and space explosions • the BDW, an electromagnetic receiver that detects the electromagnetic pulse (EMP) from a nuclear explosion (a signal is only reported if it is consistent with an optical flash from the BDY instrument) • the BDP (burst detector processor), which coordinates and controls measurements from the other instruments The BDD detects particulate radiation and gamma radiation. Australia helps monitor compliance with the Comprehensive Nuclear-­TestBan Treaty (CTBT) via several monitoring stations in Australian territories, shown in Fig.19. EMP Low-Band Antenna (BDW) L-Band Space Environment Dosimeter (BDD; under) Fig.17: the largest atomic explosion in Australia at the Montebello Islands on 19th June 1956. It had a nominal yield of 60kt but was claimed by journalist Joan Smith to actually have been 98kt. Public domain image 22 Silicon Chip S-Band X-ray Sensor (BDX) EMP HighBand Antenna (BDW) Optical Sensor (BDY) Fig.18: the Nuclear Detection System sensors on US GPS satellites. Visit siliconchip.com.au/link/abd0 for more detail on the sensors. Source: ilrs.gsfc. nasa.gov/missions/satellite_missions/past_missions/gp35_general.html Australia's electronics magazine siliconchip.com.au Concealed nuclear material in locations like shipping containers can be detected by techniques such as neutron-­ gamma emission tomography (NGET). For details on this, see our article on Advanced Imaging, September 2021, page 21 (siliconchip.com.au/ Article/15021). All materials have a particular ‘isotopic signature’ with slightly different ratios of different isotopes depending upon their origin. The isotopic signature of nuclear materials can typically be used to determine their origin. This general area is known as ‘nuclear forensics’. Low-background steel Certain applications for steel such as Geiger counters, radiation counters in medical imaging devices, scientific equipment and air/space sensors require steel produced before atmospheric atomic detonations. These started on 16th July 1945 and continued until China’s last known atmospheric nuclear test in 1980. This is because modern steel production uses atmospheric gases contaminated with radioactive particles from nuclear testing. The levels are exceptionally low, but the presence of any unwanted radioactive elements can affect extremely sensitive radiation measurements. Another source of unwanted radiation in steel is 60Co (cobalt), which is used in the refractory lining of steel furnaces as a wear indicator. Small amounts of cobalt are embedded at various depths in the lining of a furnace. As the furnace lining wears out and reaches the depth of the cobalt, it shows up in the steel product, which indicates the extent of wear. This causes unwanted radiation in the steel, although it is not a safety concern at the levels used. Low-background steel has been sourced from German World War 1 ships scuttled in Scapa Flow in the Orkney Islands of Scotland, old railway lines and vehicles, and World War 2 surplus ship armour from the Norfolk Navy Shipyard (USA). Atmospheric radioactivity peaked at 0.11mSv/year in 1963 when the Partial Nuclear Test Ban Treaty was passed and has now declined to just 0.005mSv/year above natural levels. Present levels of artificial radioactive products in the atmosphere are siliconchip.com.au Interesting links Experimental demonstration of the radiation inverse square law: www. csun.edu/scied/6-instrumentation/inverse_square_law/demonstration_ equipment.htm 2. A Geiger counter project for advanced constructors: www.instructables. com/New-and-Improved-Geiger-Counter-Now-With-WiFi/ 3. An excellent free book full of nuclear experiments you can do: www. imagesco.com/geiger/pdf/geiger-counter-experiments-book.pdf Some experiments require low-level “license-exempt” nuclear sources, which private citizens can freely purchase in the USA, but you would have to establish their legality in Australia. Some of the experiments do not require special nuclear sources. 4. Detection of cosmic rays of extraterrestrial origin using the technique of coincident detection: https://physicsopenlab.org/2016/01/02/cosmic-rayscoincidence/ 5. A 2017 Australian project with 16 detectors to demonstrate how cosmic rays arrive as showers: https://core-electronics.com.au/projects/cosmicarray 6. An Australian website for amateur cosmic-ray astronomy: https:// cosmicray.com.au/ (there is an earlier version of the site at https:// hardhack.org.au/book/export/html/2). 7. Cosmic-ray muon detector projects for amateurs: https://quarknet.fnal.gov/ toolkits/new/crdetectors.html 8. A video titled “The tunnel where people pay to inhale radioactive gas”: https://youtu.be/zZkusjDFlS0 9. A video titled “Radioactive camera lens”: https://youtu.be/FW2rM1kaRug 10. Software for a variety of compatible Geiger counters: ● https://sourceforge.net/projects/geigerlog/ ● www.mineralab.com/GeigerGraph/ ● https://medcom.com/product/geigergraph-software/ ● www.amazon.com/dp/B00WAK68U4 11. A real-time world radiation map by Geiger counter company GQ Electronics: www.gmcmap.com 12. Software examples for the RadiationD-v1.1(CAJOE) Geiger counter board available online: ● https://github.com/RuzgarErik/I2Cgeiger/ (will drive an I2C LCD) ● www.instructables.com/Arduino-DIY-Geiger-Counter/ ● https://github.com/SensorsIot/Geiger-Counter-RadiationD-v1.1-CAJOE1. Fig.19: Australian monitoring stations for the Comprehensive Nuclear-Test-Ban Treaty: RN04 (Melbourne); RN06 (Townsville); RN07 (Macquarie Island); RN08 (Cocos Islands); RN09 (Darwin); RN10 (Perth) and PS05 (Mawson). Source: DFAT Australia's electronics magazine April 2022  23 The fascinating RadiaCode-101 The RadiaCode-101 (siliconchip.com.au/link/abcr) is both a detector of ionising radiation and a gamma-ray spectrometer based on a scintillation radiation sensor. It is said to be able to detect “Gamma, high energy Beta, and continuous X-rays in the energy range 0.05...3.0MeV and in the power range 0.1-1000μSv/h” – see below. It can also overlay radiation measurements on Google Maps. It can identify various isotopes by their gamma-ray spectra. The RadiaCode-101 spectrometer. The RadiaCode-101 display as seen on a linked smartphone. sufficiently low that steel produced today is considered satisfactory for use in all but the most sensitive radiation measurement applications. Lead from before the atomic bomb era Lead is another metal used in sensitive radiation measurement instruments and is susceptible to radioactive contamination from the modern era. So there is a demand for lead from before 1945 (see Fig.20). Sources include 3t of lead recovered from the pipes of Boston’s wastewater system and now in storage at the US Government’s Los Alamos National Laboratory, where the atomic bomb was first developed. Another source was from a 300-year-old British shipwreck. Contamination of gold jewellery In the USA in the 1930s and 1940s, radioactive gold that was used as a ‘seed’ to hold radon for medical treatment was recycled into gold for jewellery. The radium decay products contained 210Pb (lead) which contaminated the gold. Fly ash radioactivity Fly ash is the non-combustible material left over after burning coal. It has various applications, such as being added to concrete, or if unused, it is buried in a landfill. Concerns have been raised that it is radioactive and constitutes a health hazard because there are trace amounts of uranium in coal, as with many other minerals. The concern has been shown to be Fig.20: very old “low activity lead” from a company that specialises in the sale of such material. It can be made into radiation shielding for sensitive instruments. Source: www.nuclearshields.com/low-activity-lead.html 24 Silicon Chip Fig.22: the location of uranium and thorium deposits in Australia. without foundation; see siliconchip. com.au/link/abcz Uranium extraction from fly ash has shown to be technically possible, although the economics are questionable; see siliconchip.com.au/link/abck A natural nuclear reactor Around 1.7 billion years ago in what is now Oklo, Gabon in Africa, a natural nuclear reactor formed that ran for several hundred thousand years, Fig.21: an ancient natural nuclear reactor in Oklo, Gabon. Source: Robert D. Loss (https://apod.nasa. (https://apod.nasa. gov/apod/ap100912.html)) gov/apod/ap100912.html Australia's electronics magazine siliconchip.com.au Bananas are radioactive Bananas are relatively high in potassium. Some figures we saw were for different sized bananas are 362mg (small), 422mg (medium), 487mg (large) and 544mg (extra large). Natural potassium contains around 0.012% of the radioactive isotope 40K. In the video titled “Potassium Metal From Bananas!” at https://youtu.be/ fmaZdEq-Xzs the experimenter chemically processes 6.5kg of bananas to extract 9g of potassium metal. At 16m 18s, he measures the radioactivity of the extracted potassium and establishes that it is about twice the background level of radiation. So, it is true that bananas are radioactive. However, a medium-sized banana with 450mg of potassium will expose you to 0.01mrem of radiation. A chest X-ray is about 10mrem, so 10,000 bananas would have to be consumed to produce the same radiation exposure as one chest X-ray. In any case, the human body contains about 120g of potassium, so the extra dose is negligible. Feel free to enjoy a banana! Note that as a home experimenter without extremely sensitive laboratory equipment, you are unlikely to be able to measure the extra radioactivity of a single banana above the background radiation. That’s why so many bananas had to be processed and the potassium purified to get even a doubling of the background count. producing about 100kW from a self-­ sustaining fission reactor. The discovery was made in 1972 – see Fig.21. Such a phenomenon could not occur today because there is insufficient fissile 235U in natural uranium ore today; only about 0.72%, which is not enough for a self-sustaining fission reaction. In a much younger Earth, uranium ore had about 3.1% 235U, comparable to what is used in civilian nuclear reactors (typically 3-5%). There is a lower percentage of 235U in ore today due to radioactive decay over the Earth’s history. Conclusion There is radiation all around us but it’s generally far below the level of concern. Various instruments exist that allow you to confirm that, with Geiger counters being one of the simplest and cheapest. Still, they are quite limited in terms of accuracy and sensitivity. If you really want to explore the radioactivity that might be around you then the RadiaCode-101 shown opposite is one of the best consumer-­ grade pieces of electronics that you could use. While somewhat expensive with an RRP of US$275 (about $400), its capabilities far exceed those of a basic Geiger counter that you could purchase for around $80 (such as the one shown overleaf). Continued on page 26 Radioactive isotopes used for industrial purposes Isotope Uses 241Am Backscatter gauges for smoke detectors, fill height detectors & ash content sensors 90Sr Thickness gauging up to 3mm 85Kr Thickness gauging of thinner materials like paper, plastics etc 137Cs 60Co 226Ra, 255Cf 192Ir, 169Yb, 60Co Density and fill height level switches Density and fill height level switches, monitoring of furnace wear Ash content sensors Industrial radiography Safety Note Use common sense when dealing with radioactive materials. Although plenty of videos and web pages show it, we do not recommend you disassemble smoke detectors to obtain the radioactive source unless you know what you are doing and follow appropriate safety precautions. Source: Non-Destructive Testing and Radiation in Industry by Colin Woodford and Paul Ashby – https://inis.iaea.org/collection/NCLCollectionStore/_ Public/33/034/33034305.pdf siliconchip.com.au Australia's electronics magazine April 2022  25 Measuring radiation and experiments for the enthusiast Geiger counters for measuring radiation can be bought relatively inexpensively. As a general rule, the more expensive the Geiger counter, the more sensitive it will be and the more types of radiation it will be sensitive to. Some Geiger counters are less sensitive or insensitive to alpha and beta radiation (which are more common in natural settings). Geiger counters cannot distinguish between alpha, beta and gamma rays. A different type of instrument is required for this; some can even identify specific isotopes, such as scintillation counters and proportional counters. A typical Geiger counter will click about 10 to 30 times per minute from natural background radiation, but this varies depending on geographic area, cosmic ray activity, and the detector’s sensitivity. Cheaper Geiger counters frequently come with SBM-20 type tubes (see siliconchip.com.au/link/abcl). These were initially developed in the Soviet Union. J305 tubes are also relatively common. The website at siliconchip.com.au/link/ abcm lists all common tube types. J305 tubes have a clear glass tube with a central conductor. The outer conductor is a coating of the transparent electrical conductor indium tin oxide. As Geiger counter tubes run at high voltages, be careful when experimenting with them, especially if using unenclosed circuit boards. One inexpensive Geiger counter we looked at is the RadiationD-v1.1(CAJOE), shown in Fig.23; it comes without a case. Other popular fully-enclosed Geiger counters of interest are made by GQ Electronics (siliconchip.com.au/link/abcn). Depending on airline rules, you might be able to bring a Geiger counter on a plane to see how altitude affects its measurements. You can also examine granite such as in benchtops or other stonework to see if it is radioactive, as it may contain uranium or thorium. This has been confirmed in some cases, but it is unlikely to be harmful; see the following videos for details: ● “Radioactive Granite” at https://youtu.be/jKIXKo5QgT8 ● “Special Report: Radioactive Kitchen Counters” at https:// youtu.be/8tgxXOqCwTI Other items which might be radioactive include: ● some Brazil nuts, due to their radium content (see the video “Are Brazil Nuts Radioactive?” at https://youtu.be/ Pt-SMAVN898) ● antique “uranium glass”, also known as “Vaseline glass” (see Fig.24) ● “static elimination” brushes (see Fig.25, siliconchip.com. au/link/abcp and siliconchip.com.au/link/abcq) ● uranium ore (www.amazon.com/dp/B000796XXM) ● luminous markings in old clocks and watches ● tritium vials as used on certain watches, gun sights and compasses ● lantern gas mantles that contain thorium ● salt substitutes with potassium instead of sodium ● some camera lenses from 1950-70s which use 232Th (thorium) to alter the index of refraction ● some Fiesta Ware brand dinnerware from the mid 20th century use uranium glazes, especially red; these are collectable and not harmful ● thorium concentrated from certain beach sands, often black sands (see siliconchip.com.au/link/abco) 26 Silicon Chip Fig.23: an inexpensive Geiger counter board labelled RadiationD-v1.1(CAJOE). It uses a J305 Geiger-Müller tube and is primarily sensitive to beta and gamma radiation. It also supports M4011, STS-5 and SBM-20 tubes. It can be interfaced to an Arduino or work in a standalone mode where it beeps for every radiation event detected. Fig.24: antique uranium glass vases fluoresce under UV light as well as being slightly radioactive. Source: Wikimedia user Realfintogive (CC BY-SA 3.0) Fig.25: static elimination brushes typically contain alpha-emitting polonium-210. They generate charged particles in the air, making the staticcharged item electrically neutral so it will no longer attract dust (until it becomes charged again). Source: Oak Ridge Associated Universities (ORAU) Museum of Radiation and Radioactivity Fig.26: you can buy ionisation chambers for smoke detectors online for $4-6 delivered to Australia. Although not considered harmful, we don’t recommend opening one of these. If you want to see the radioactive ‘pill’ inside, there are photos at www.instructables.com/ How-to-Obtain-and-ExtractAmericium/ ● ionisation chamber smoke detectors containing 241Am (americium), producing alpha particles – see Fig.26 ● ordinary glass if it has enough 40K (potassium) or 232Th (thorium) ● some fertilisers with potassium or phosphorous from SC certain sources Australia's electronics magazine siliconchip.com.au