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The inspiration of Marie Curie

Practical work using low-level radioactive materials available to

the publicRalph Whitcher

ABSTRACT These notes describe six practical activities for supplementing standard practical work in radioactivity. They are based on a series of workshops given at ASE regional and national conferences by the ASE’s Safeguards in Science Committee. The activities, which demonstrate aspects of radioactivity, feature consumer items that happen to be radioactive at a low level. It is an opportunity to show that radioactivity is in some materials we can all encounter in our lives; low-level radioactivity is not something special or alien.

These notes are based on a series of workshops given at Association for Science Education (ASE) regional and national conferences by the ASE’s Safeguards in Science Committee to demonstrate practical work that can help with teaching aspects of radioactivity. The low-level radioactive items described in these notes are currently conditionally exempt from the Environmental Permitting (England and Wales) Regulations 2010 and from the Radioactive Substances Act 1993 in Scotland and Northern Ireland. Thus buying and disposing of them does not require permitting (or registration and authorisation in Scotland and Northern Ireland) by the regulatory authority for the environment. However, their use does fall under the Ionising Radiations Regulations 1999 and, although the risks are low, employers usually require you to obtain their permission before you acquire new radioactive sources. Employers will also require you to follow suitable risk assessments, such as CLEAPSS L93 (2008), or, in Scotland, those available through the Scottish Schools Equipment Research Centre (SSERC). Additional safety notes are included in this article.

While teachers are usually aware that smoke detectors and radioluminescent watches have radioactive components, there are other low-level radioactive consumer items that are useful in demonstrating aspects of radioactivity. The items featured in the following practical work

are generally available to the public and present negligible risk if used with straightforward safety precautions. However, if the items are misused or damaged, it could lead to an unjustified exposure. For those teachers looking to extend demonstrations or investigations beyond the practical work described here, there are some radioactive consumer items that are probably best avoided because radioactive material could be easily released, even though in small quantities, for example some types of old thermionic valve and some types of compact fluorescent lamp.

The overall risk of detriment to human health from exposure to ionising radiation is assessed using a quantity called effective dose, expressed in the unit sievert (Sv). The quantity takes into account the relative biological effects of different types of ionising radiation and the susceptibility of different parts of the body to radiation damage. As an example of this quantity, people in the UK receive an average annual effective dose from background radiation of 2.2 mSv. The unit is named after Rolf Sievert, a Swedish scientist and a leading pioneer in radiation protection.

Another radiological quantity is equivalent dose. This relates to radiation damage to an organ or tissue where the relative biological effects of different types of ionising radiation have been taken into account. This is also expressed in sieverts. As an example of this quantity, the

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current legal limit of the annual equivalent dose to the lens of the eye for an adult employee is 150 mSv. Both effective dose and equivalent dose are used in setting regulatory control limits.

1. Demonstrating randomness of radioactive emissions using a thoriated gas mantle

This activity demonstrates the random nature of radioactive decay. A large variation in count rate can be shown clearly using a low-level radioactive source such as a gas mantle.

Butane and paraffin camping lamps have gas mantles that glow brightly when heated by the burning fuel. Some brands of gas mantle, such as the Tilley 164X gas mantle (Figure 1), include a small quantity of thorium oxide because it gives a brilliant white light when hot. Although thorium is radioactive, it has a low radiological risk because it has a relatively low specific activity. Thoriated gas mantles can be purchased for under £2 each from hardware and camping equipment stores. This ready availability for public use implies that the risk is not high but there is a small risk from inhalation or ingestion of gas mantle fragments, so keep the gas mantle in a sealable plastic bag to ensure that fragments

of mantle are not dispersed. The mantle does not need to be taken out of the bag for use in this application. Do not use burnt mantles because they crumble into fine ash very easily.

Set a Geiger–Müller (GM) tube so that the end window is uppermost and place the gas mantle in the bag directly on the end (Figure 2). Record the count rate at regular intervals. This can be done by recording the counts every 10 seconds and calculating the count rate over 10 seconds or, even better, by using a datalogger to measure and record the count rate at suitable short intervals.

Real-time datalogging gives an immediate visual indication of the randomness, as shown in Figure 3.

The variation can be surprising. In the example shown here, 180 measurements were taken by logging the count rate at 1 second intervals, and the range went from 9 counts s−1 to 37 counts s−1.

More advanced workCalculate the mean and the standard deviation and see how the standard deviation compares with the square root of the mean. The data can also be grouped and the frequencies of each group shown on a chart, using a spreadsheet (Figure 4).

The data distribution can be modelled as a Poisson distribution. The predicted value of the standard deviation is the square root of the mean. If the mean value is large, say more than 20, the distribution can be approximated to the normal (or Gaussian) distribution, which predicts that 99.7% of the data should fall within ± 3 standard deviations from the mean.

Figure 1 A Tilley type 164X gas mantle in a sealed plastic bag; some other brands contain yttrium instead of thorium and are not radioactive Figure 2 The setup of the GM tube and counter

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Table 1 relates to the data displayed in the frequency chart and shows how well this model works for large numbers of data. The predicted range based on the mean is 8.2–36.6, which fits well with the measured range of 9–37.

This gives a rule of thumb to check whether a higher than expected count reading is no more than randomness. Take the square root of the expected (mean) value, multiply it by three and this gives the likely range about the mean due to

Figure 3 The count rate shown by datalogging

Figure 4 The distribution of count rates

0 5 10 15 20 25 30 35 40

20

16

12

8

4

0

Frequency

Count rate/s–1

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Table 1 Statistics from the data in Figure 4

Mean Maximum Minimum Standard deviation

Square root of the mean

Mean + 3 × square root of the mean

Mean − 3 × square root of the mean

22.43 37 9 4.95 4.74 36.6 8.2

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randomness. For example, say the background count rate for a particular contamination monitor was well established as 36 counts per minute. If you were checking a surface and measured 50 counts per minute with this monitor, is there likely to be contamination? The expected range is ± 18, so 50 counts per minute would be within the range due to randomness and not necessarily a sign of contamination, and another count would be advisable.

2. An alternative source for the diffusion (or Taylor) cloud chamber

Diffusion cloud chambers are easy to use and have great educational benefit in allowing students to observe directly the tracks of radioactive particles as they are emitted. The diffusion cloud chambers sold by school science equipment suppliers in the 1960s and 1970s used small radium paint sources of nominal activity about 1 kBq. These sources are no longer available and suitable new sources in the UK are not easy to obtain. Fortunately, a thoriated tungsten electrode can be used as a cloud chamber source. These are designed for TIG (tungsten inert gas) welding fabrication and are commonly available from welding supplies shops. The type used in the test experiments was an SWP brand 2% thoriated tungsten electrode, type WT20, with rod diameter 3.2 mm and length 150 mm, conforming to ISO 6848 (see Websites). There are also 4% thoriated tungsten electrodes, but these are uncommon. The price of an electrode is about £3. The 2% thorium electrode has an identifying red colour tip.

The electrode is put through the cloud chamber and corks/bungs placed firmly on each end to keep it in place. If you are modifying a cloud chamber for this source, holes drilled in the sides should be set so that the electrode centre is about 7 mm above the chamber floor.

Put dry ice pieces in the lower chamber and replace the bottom lid. Dampen the felt inside the upper chamber with a few cubic centimetres of ethanol. Replace the lid and rub it clean with a soft duster. Within a few minutes you should see tracks. These are caused by condensation of ethanol. The low temperature in the chamber causes the ethanol vapour to become supersaturated and, in the absence of dust, the ions produced by the ionising radiation act as nucleation sites on which condensation forms.

Figure 5 shows the α particle tracks; they appear at a rate of about one or two every second. The α tracks vary in length, up to a maximum length of around 4 cm, because some α emissions come from just within the electrode surface and lose energy before emerging. Occasionally, a V-shaped emission appears from the rod surface. This is caused by the decay by α emission of 220Rn (a product in the decay chain of 232Th) followed almost immediately by the decay by α emission of 210Po.

More advanced workα particles are much more massive than electrons; for a particular velocity, α particles have much greater momentum than β particles and are deflected less on collision. α particles tend to travel in straight lines through air except for occasional collisions with nuclei of atoms (which produce observable deflections, as in the α particle track in the middle of Figure 6).

Low-energy β emissions produce tortuous tracks compared with α emissions because they are deflected more readily by collisions. The tracks from β radiation can be observed in the cloud chamber but they are much fainter because β radiation is far less ionising than α radiation. A way to identify β tracks is to take a number of photographs at 1 second intervals with a digital camera and flash, say 20 or so, and then download the images to a computer. Zoom in on the photographs and you may be able to pick out some images that show β radiation tracks. The contrast may be better if the pictures are converted to greyscale.

Figure 5 A thoriated tungsten WT20 electrode used as a source in a diffusion cloud chamber

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Additional notesDiffusion cloud chambers normally work reliably if they are kept clean, so it is a good idea to store the chambers in sealed bags between use. If they are allowed to become dusty they can be very difficult to get working. Before storing the chamber, allow the ethanol to evaporate completely to prevent the plastic crazing over time.

To store electrodes, they can be removed from the cloud chambers and kept with other radioactive sources in a secure store. The electrodes are usually supplied in a handy plastic storage case.

The equivalent dose rate to the hand when holding the electrode is very low, no more than a few microsieverts per hour, and the dose received during its use as a cloud chamber source will be insignificant. In standard WT20 type TIG electrodes, thorium is evenly dispersed throughout the rod. During manufacture, tungsten and thorium oxide powder are sintered into a metal alloy rod and the thorium is firmly bound in the metal. It is almost inconceivable that thorium could become released from the electrode, even if it were roughly handled. (Thorium is released in small quantities when grinding the electrode, or to a lesser extent during welding, neither of

which happens when using the electrode as a cloud chamber source.) There are many welding accessory outlets and a search using the internet should find one in your area. When you purchase electrodes, check that they are type WT20 with the thorium sintered homogeneously in the tungsten and conforming to ISO 6848.

Where permitted, it would be justifiable for responsible students under 16 to use the cloud chambers with this source fitted because the activity is low and the radiation risk is negligible.

This activity was first explained by the author on the Practical Physics website (see Websites).

3. Using a uranium-glazed saucer or small plate to demonstrate β backscatter

This demonstration uses a saucer (or small plate) manufactured by the Homer Laughlin China Co. It is the red/orange type from the Fiesta-ware range manufactured from 1936 to 1973 (not continuously) that contained uranium in the glaze.

The Fiesta-ware emits β radiation from the 238U decay chain. Little of the α radiation escapes the surface glaze, and the γ emission from Fiesta-ware is low compared with the β emission, which makes it suitable for this demonstration. The plate or saucer should be retained in a sealable plastic bag so that if it breaks the fragments are retained; this demonstration works satisfactorily with the saucer in the plastic bag.

Use an end-window GM tube such as the ZP1481 (MX168) GM tube commonly used in schools connected to a counter or ratemeter. Place the GM tube on the bench so that the GM tube window is pointing upwards. It works better if you remove the spider-web protective cap, but be careful as the GM tube window is easily damaged. Place the saucer flat on the bench (in its bag) next to the GM tube. Take a count for 30 seconds, or note the count rate. It will be a little above background. The β radiation does not enter the detector window. Although the GM tube detects photons principally through its tube wall, the efficiency is quite low, typically a few percent.

Place a large sheet of aluminium about 5 cm above the GM tube (Figure 7). A home-made wooden holder is useful to do this. The aluminium sheet should be large enough (approximately 150 × 300 mm) so that its footprint covers the saucer and GM tube. The count rate will rise noticeably. Replace the aluminium sheet with a lead one (a piece of code 4 lead roof flashing from

Figure 6 The irregular track of a suspected low-energy β emission, indicated by the upper arrow; the lower arrow indicates a deflection of an α emission; the photograph has been digitally converted to greyscale and the contrast and brightness enhanced

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a local builder is suitable) in the same position. The count rate rises even further. You can show that it is not the lead or the aluminium that is radioactive by moving the saucer away.

β backscatter from the sheet causes the detection rate to rise. The saucer is such a wide-area source that there is plenty of scatter that gets to the GM tube window –no careful setting up is needed. The scatter depends on the superficial density of the sheet, so it is a good way of showing a method for non-destructive testing of material thickness. You can extend the investigation by trying various materials for the sheet and various thicknesses of the same material, and by changing the height of the sheet above the GM tube.

Additional notesSaucers and small plates can be obtained from chinaware antique dealers, including those on eBay. The modern orange colours do not use uranium.

When uranium is extracted chemically, it contains equal activities of 238U and 234U, with a small percentage of 235U (the percentage of 235U is even lower in depleted uranium, which was used by Homer Laughlin from 1959 onwards). The β radiation comes principally from 234Th and 234mPa in the 238U decay chain. The 238U decay chain pretty much comes to a stop after 234U because there are two successive radionuclides with long half-lives. On measurements taken from a saucer by the author, the equivalent dose rate to the hand from the β and γ radiation was roughly 30 µSv h−1. The information in a report from the U.S. Nuclear Regulatory Commission (2001) indicates that the dose rate measurements vary, with an equivalent dose rate as high as 320 µSv h−1 being measured from the surface of a tea cup. However, handling this chinaware for less than a minute will only give a trivial dose to the hand.

4. Detecting radon in a building using a charged rubber balloon and showing radioactive decay

This experiment was explained by Austen and Brouwer (1997). It can be used to show how the air has naturally low levels of radioactive material in it, and that this decays over time. It is also a demonstration that our environment has low levels of radioactivity to which we are exposed continuously.

Inflate a rubber balloon, clamp the neck with something like a freezer bag resealing clip, attach a piece of string and suspend it somewhere in the room. It does not have to be high up, but you tend to get better results if the balloon is hung in a place away from draughts. Rub the balloon vigorously for a few moments with woollen gloves until the balloon is friction charged. Leave the balloon for about 30 minutes (although if you are really keen to see the results, 15 minutes may be sufficient).

Set up a counter with the GM tube upright. Take a piece of rigid plastic, as in Figure 8, or card about 100 mm square with a 25 mm hole in the centre, and place it so that the hole is about 10 mm above the GM tube window. Take care as the GM tube end-window is fragile and easily broken by carelessness. Take the background count for 1 minute. Put on a pair of disposable gloves (this is not for radiological protection; see additional notes below) and take down the balloon. Deflate it by removing the clip, and put it across the hole in the plastic sheet as in Figure 9. Avoid the balloon sagging and touching the GM tube. Remove and discard your gloves into the

Figure 7 The setup of the GM tube and uranium-glazed saucer

Figure 8 The setup for counting from the balloon

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waste bin. Take a count for 1 minute to find the average count rate over the minute. The result can be quite astonishing. If time allows, take 1 minute counts about every 20 minutes and plot the average count rate against time on a graph.

The cause of the radioactive contamination of the balloonThe radioactivity arises from the decay products of radon. Radon comes from the decay chains of naturally occurring uranium and thorium in the environment. Measurements by Austen and Brouwer revealed that most of the radioactivity was due to 214Pb and 214Bi, both 222Rn progeny, and from 212Pb, one of the progeny of 220Rn. 214Pb has a half-life of 26.8 minutes, 214Bi 19.7 minutes and 212Pb 10.6 hours.

If you record the count over 1 minute at 20 minute intervals without disturbing the balloon and GM tube equipment, you should obtain a decreasing count rate. The initial decay is roughly exponential. The initial decay curve, mainly from the decay of 214Pb and 214Bi, gives an average ‘half-life’ of about 50 minutes. If there is sufficient activity on the balloon such that there is a measurable count rate from it after 24 hours, the decay of the 220Rn progeny 212Pb and 212Bi predominate to give an average half-life of about 11 hours.

When you have finished, discard the balloon in the waste bin, wipe down the plastic and then wash your hands.

More advanced workUsing Excel’s ‘Add Trendline’ facility and choosing the exponential trendline option, the exponential equation of best fit can be displayed on the graph (Figure 10). The half-life can be determined from the exponent of base e, in this case −0.0119. The half-life is ln(2)/−0.0119, which is 58.2 minutes in this example.

Additional notesThe gloves are principally to stop nuisance contamination from the balloon onto the GM tube and counter. If you do contaminate the equipment with low levels of radon progeny and someone else uses the equipment soon after, they are likely to obtain erroneous results that can adversely affect their investigations or cause false alarm.

Figure 9 Counting emissions from the balloon

Figure 10 Graph of the decay of the contamination on the balloon

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This experiment is an opportunity to introduce a sense of proportion to the risks from low-level radioactivity. If you had a party at home, would you ask the people clearing away the party balloons to wear disposable gloves? Of course not!

5. Using a thoriated lens as a source to investigate emissions by absorption

Thorium was added to some camera lens elements to give a high refractive index, For example, versions of the Yashinon-DS 50 mm f1.7, Canon FL 58 mm f1.2 and Pentax Super-Takumar 50 mm f1.4 lenses. Such lenses have long been discontinued but they can be obtained from second-hand camera dealers and eBay. The rear element of the Pentax Super-Takumar 50 mm f1.4 (Figure 11) is radioactive and can be used as a source to investigate the emissions by placing plates of varying thickness between the source and detector.

A thoriated lens can generally be identified by looking through the lens (but keeping it away from your eye) at white paper, which will appear to have a slight tea-coloured tint. This is caused by a gradual darkening of the glass from the self-irradiation of the lens. Ionising radiation displaces some electrons in the glass, forming defect sites that affect the absorption of light and cause darkening within the glass (Speit, 1998). Be careful not to confuse this with lens coatings,

some of which can also appear brownish – the indicative tea-coloured tint only shows when looking through the lens.

The lens is mounted in a holder with the radio-active lens element facing the GM tube (Figure 12). Place a plate holder between the lens and the GM tube. Plates of material such as aluminium of varying thickness (‘absorbers’) are placed between the lens and the GM tube. Measure the counts per minute detected by the GM tube for varying thickness or superficial density of plates.

A piece of paper placed between the lens and GM tube has a small effect on the count rate, showing that the α radiation emission is small. Aluminium plates have a greater impact on count rate, showing that there is a considerable β radiation field from the lens. Suitably thick plates of aluminium will block all β radiation, but the GM tube will continue to detect the γ radiation from the lens. A typical graph of count rate versus plate superficial density (milligrams per square centimetre of plate) is shown in Figure 13.

From these results, an aluminium plate of roughly 800 mg cm−2 stopped the β field. This would correspond to a maximum β energy of about 2.0 MeV. (In fact, the most energetic β in the 232Th decay chain is 2.25 MeV from 212Bi.)

Thick lead stops β radiation but does not stop the detection completely when placed between the lens and GM tube, showing the penetrating characteristics of the γ radiation from the lens.

The emission of α radiation can be demonstrated with a spark counter (Figure 14). The spark counter detects only the α emissions at the surface of the glass lens that escape the glass. A two-dimensional

Figure 11 The thoriated rear lens element of the Pentax Super-Takumar lens M42 screw-mount f1.4 50 mm type II, manufactured between 1965 and 1971

Figure 12 The setup for placing absorbers in front of the thoriated lens

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spark counter is better than a single-wire version for this demonstration. The detection was roughly 1 spark per second. Placing a sheet of paper between the lens and detector wires stops the sparks and confirms that the α emissions are from the lens.

Additional notesThe thorium is bonded in the glass so the release of any significant thorium dust will be unlikely. Nonetheless, caution is needed to avoid breaking the lens. In storage, keep the rear lens caps on until needed for use. In measurements carried out by the author on a Pentax lens, the equivalent dose rate at the lens surface was roughly 15 µSv h−1, so handling

the lens for a minute gives a trivial dose to the hand. This measurement accords with the information in the report from the U.S. Nuclear Regulatory Commission (2001). Do not bring the lens up to the eye, for example to use it as a magnifying glass, because the eye is more susceptible to harm from radioactive exposure than the hand.

6. Measuring the distribution of times between emissions

The distribution of times between emissions can be determined using a low-level radioactive source such as a gas mantle. The result is surprising to many because it is not intuitive. People generally expect some kind of bell distribution centred on a mean time between emissions.

Keep the gas mantle in a sealable plastic bag so that fragments of mantle are not dispersed. The mantle does not need to be taken out of the bag for use in this experiment.

The equipment is set up in a similar way to that measuring randomness. However, the practical demonstration needs a datalogger with a function for measuring time between successive pulses, such as those from a slotted barrier going through a light gate. The time between successive pulses from a GM tube can be very short, less than 1 ms, and the datalogger needs to capture these. The Data Harvest EasySense datalogger has a switched input for recording times between successive closures of the input. However, it is not a good idea to connect the pulse output of the GM counter directly to the datalogger switched input, as the input could be damaged in some circumstances. The switched inputs can be open and closed safely using a photoswitch such as a Panasonic (Matsushita) PhotoMOS relay.

Some GM counters have a pulse output; the three considered here are the Philip Harris Digicounter (B8H29280), the Unilab GeigerTeller (F4H29371) and the Unilab modular GM EHT unit (411.010; no longer available).

A PhotoMOS relay AQV251 connected to the pulse output from the Digicounter worked satisfactorily, and you can use the loudspeaker output. Connect a 1 kΩ resistor in series with the PhotoMOS input (Figures 15 and 16).

The pulse output from the Unilab GeigerTeller and the Unilab modular GM EHT unit is about 100 µs, which is too short to switch the standard type of PhotoMOS relay. The circuit shown in Figure 17 uses a faster PhotoMOS relay, AQV259,

Figure 13 Graph of the count rate with absorbers of differing superficial density

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Figure 14 Thoriated lens on a spark counter, showing α emissions

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and a speed-up circuit. This worked reliably (Figure 18). The transistor type is not critical – any high-gain general-purpose small-signal npn transistor will work.

The switched output in the PhotoMOS relay is connected to the switch sensor of the datalogger and the times between successive switch opening and closing are recorded. The recorded data can then be copied to an Excel spreadsheet, grouped into time intervals and displayed as a frequency chart. The distribution is not what might be

expected, and the relationship between the mean and standard deviation is interesting.

Using EasySense software version 2.2 and connecting to the EasySense switch input labelled ‘5A’, select ‘Timing’ (the ‘Time’ radio button will then appear selected) and then choose ‘From A to A (stopwatch)’.

ReferencesAusten, D. and Brouwer, W. (1997) Radioactive balloons:

experiments on radon concentration in schools or homes. Physics Education, 32, 97–100.

Speit, B. (1998). Special glasses for nuclear technologies. In The properties of optical glass (Schott series on glass and glass ceramics), ed. Bach H. and Neuroth, N. pp. 343–348. Berlin: Springer.

U.S. Nuclear Regulatory Commission (2001) Systematic

radiological assessment of exemptions for source and byproduct materials. NUREG 1717. Washington, DC: U.S. Nuclear Regulatory Commission [www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1717/nureg-1717.pdf].

WebsitesPractical Physics: www.practicalphysics.org.SWP TIG Welding catalogue: www.specialisedwelding.

co.uk/tig-welding.html.

Ralph Whitcher is chair of the ASE Safeguards in Science Committee. He is one of the CLEAPSS radiation protection advisers and a chartered radiation protection professional. He is also the chair of the Research and Teaching Sectorial Committee of the Society for Radiological Protection. He was formerly an advisory teacher for science. Email: rwhitcher@btinternet.com

Figure 15 Circuit diagram for the Digicounter outputFigure 17 Circuit diagram for the outputs from the GeigerTeller and Unilab modular units

Figure 16 The setup with the Digicounter and datalogger

Figure 18 Using a fast solid-state relay with the Unilab GeigerTeller

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