Practical work using low-level radioactive materials available to the public
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The inspiration of Marie Curie
Practical work using low-level
radioactive materials available to
the public
Ralph 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 are generally available to the public and present
given at Association for Science Education negligible risk if used with straightforward safety
(ASE) regional and national conferences by precautions. However, if the items are misused
the ASE’s Safeguards in Science Committee to or damaged, it could lead to an unjustified
demonstrate practical work that can help with exposure. For those teachers looking to extend
teaching aspects of radioactivity. The low-level demonstrations or investigations beyond the
radioactive items described in these notes practical work described here, there are some
are currently conditionally exempt from the radioactive consumer items that are probably
Environmental Permitting (England and Wales) best avoided because radioactive material
Regulations 2010 and from the Radioactive could be easily released, even though in small
Substances Act 1993 in Scotland and Northern quantities, for example some types of old
Ireland. Thus buying and disposing of them thermionic valve and some types of compact
does not require permitting (or registration and fluorescent lamp.
authorisation in Scotland and Northern Ireland) The overall risk of detriment to human health
by the regulatory authority for the environment. from exposure to ionising radiation is assessed
However, their use does fall under the Ionising using a quantity called effective dose, expressed
Radiations Regulations 1999 and, although the in the unit sievert (Sv). The quantity takes into
risks are low, employers usually require you to account the relative biological effects of different
obtain their permission before you acquire new types of ionising radiation and the susceptibility
radioactive sources. Employers will also require of different parts of the body to radiation damage.
you to follow suitable risk assessments, such as As an example of this quantity, people in the UK
CLEAPSS L93 (2008), or, in Scotland, those receive an average annual effective dose from
available through the Scottish Schools Equipment background radiation of 2.2 mSv. The unit is
Research Centre (SSERC). Additional safety named after Rolf Sievert, a Swedish scientist and
notes are included in this article. a leading pioneer in radiation protection.
While teachers are usually aware that smoke Another radiological quantity is equivalent
detectors and radioluminescent watches have dose. This relates to radiation damage to an organ
radioactive components, there are other low-level or tissue where the relative biological effects
radioactive consumer items that are useful in of different types of ionising radiation have
demonstrating aspects of radioactivity. The been taken into account. This is also expressed
items featured in the following practical work in sieverts. As an example of this quantity, the
SSR June 2011, 92(341) 65
Practical work using low-level radioactive materials available to the public Whitcher
current legal limit of the annual equivalent dose of mantle are not dispersed. The mantle does not
to the lens of the eye for an adult employee is need to be taken out of the bag for use in this
150 mSv. Both effective dose and equivalent dose application. Do not use burnt mantles because
are used in setting regulatory control limits. they crumble into fine ash very easily.
Set a Geiger–Müller (GM) tube so that the end
1. Demonstrating randomness of window is uppermost and place the gas mantle
radioactive emissions using a thoriated in the bag directly on the end (Figure 2). Record
gas mantle the count rate at regular intervals. This can be
This activity demonstrates the random nature of done by recording the counts every 10 seconds
radioactive decay. A large variation in count rate and calculating the count rate over 10 seconds or,
can be shown clearly using a low-level radioactive even better, by using a datalogger to measure and
source such as a gas mantle. record the count rate at suitable short intervals.
Butane and paraffin camping lamps have gas Real-time datalogging gives an immediate
mantles that glow brightly when heated by the visual indication of the randomness, as shown in
burning fuel. Some brands of gas mantle, such Figure 3.
as the Tilley 164X gas mantle (Figure 1), include The variation can be surprising. In the
a small quantity of thorium oxide because it example shown here, 180 measurements were
gives a brilliant white light when hot. Although taken by logging the count rate at 1 second
thorium is radioactive, it has a low radiological intervals, and the range went from 9 counts s−1 to
risk because it has a relatively low specific 37 counts s−1.
activity. Thoriated gas mantles can be purchased
More advanced work
for under £2 each from hardware and camping
Calculate the mean and the standard deviation and
equipment stores. This ready availability for
see how the standard deviation compares with
public use implies that the risk is not high but
the square root of the mean. The data can also be
there is a small risk from inhalation or ingestion
grouped and the frequencies of each group shown
of gas mantle fragments, so keep the gas mantle
on a chart, using a spreadsheet (Figure 4).
in a sealable plastic bag to ensure that fragments
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
66 SSR June 2011, 92(341)
Whitcher Practical work using low-level radioactive materials available to the public
Figure 3 The count rate shown
by datalogging
Frequency
20
16
12
8
4
0
0 5 10 15 20 25 30 35 40
Count rate/s –1
Figure 4 The distribution of count rates
Table 1 Statistics from the data in Figure 4
Mean Maximum Minimum Standard Square root Mean + 3 × square Mean − 3 × square
deviation of the mean root of the mean root of the mean
22.43 37 9 4.95 4.74 36.6 8.2
Table 1 relates to the data displayed in the This gives a rule of thumb to check whether
frequency chart and shows how well this model a higher than expected count reading is no more
works for large numbers of data. The predicted than randomness. Take the square root of the
range based on the mean is 8.2–36.6, which fits expected (mean) value, multiply it by three and
well with the measured range of 9–37. this gives the likely range about the mean due to
SSR June 2011, 92(341) 67
Practical work using low-level radioactive materials available to the public Whitcher
randomness. For example, say the background Figure 5 shows the α particle tracks; they appear
count rate for a particular contamination monitor at a rate of about one or two every second. The α
was well established as 36 counts per minute. tracks vary in length, up to a maximum length of
If you were checking a surface and measured around 4 cm, because some α emissions come from
50 counts per minute with this monitor, is there just within the electrode surface and lose energy
likely to be contamination? The expected range before emerging. Occasionally, a V-shaped emission
is ± 18, so 50 counts per minute would be within appears from the rod surface. This is caused by
the range due to randomness and not necessarily the decay by α emission of 220Rn (a product in the
a sign of contamination, and another count would decay chain of 232Th) followed almost immediately
be advisable. by the decay by α emission of 210Po.
2. An alternative source for the diffusion More advanced work
(or Taylor) cloud chamber α particles are much more massive than electrons;
for a particular velocity, α particles have much
Diffusion cloud chambers are easy to use and have
greater momentum than β particles and are
great educational benefit in allowing students to
deflected less on collision. α particles tend to
observe directly the tracks of radioactive particles
travel in straight lines through air except for
as they are emitted. The diffusion cloud chambers
occasional collisions with nuclei of atoms (which
sold by school science equipment suppliers in the
produce observable deflections, as in the α particle
1960s and 1970s used small radium paint sources
track in the middle of Figure 6).
of nominal activity about 1 kBq. These sources
Low-energy β emissions produce tortuous
are no longer available and suitable new sources
tracks compared with α emissions because they are
in the UK are not easy to obtain. Fortunately,
deflected more readily by collisions. The tracks from
a thoriated tungsten electrode can be used as a
β radiation can be observed in the cloud chamber
cloud chamber source. These are designed for
but they are much fainter because β radiation is
TIG (tungsten inert gas) welding fabrication and
far less ionising than α radiation. A way to identify
are commonly available from welding supplies
β tracks is to take a number of photographs at
shops. The type used in the test experiments was
1 second intervals with a digital camera and flash,
an SWP brand 2% thoriated tungsten electrode,
say 20 or so, and then download the images to a
type WT20, with rod diameter 3.2 mm and length
computer. Zoom in on the photographs and you
150 mm, conforming to ISO 6848 (see Websites).
may be able to pick out some images that show β
There are also 4% thoriated tungsten electrodes,
radiation tracks. The contrast may be better if the
but these are uncommon. The price of an electrode
pictures are converted to greyscale.
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 A thoriated tungsten WT20 electrode used
as a source in a diffusion cloud chamber
68 SSR June 2011, 92(341)
Whitcher Practical work using low-level radioactive materials available to the public
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
Figure 6 The irregular track of a suspected low-
range manufactured from 1936 to 1973 (not
energy β emission, indicated by the upper arrow; the
continuously) that contained uranium in the glaze.
lower arrow indicates a deflection of an α emission;
The Fiesta-ware emits β radiation from the
the photograph has been digitally converted to 238
U decay chain. Little of the α radiation escapes
greyscale and the contrast and brightness enhanced
the surface glaze, and the γ emission from Fiesta-
ware is low compared with the β emission, which
Additional notes makes it suitable for this demonstration. The plate
Diffusion cloud chambers normally work reliably or saucer should be retained in a sealable plastic
if they are kept clean, so it is a good idea to bag so that if it breaks the fragments are retained;
store the chambers in sealed bags between use. this demonstration works satisfactorily with the
If they are allowed to become dusty they can saucer in the plastic bag.
be very difficult to get working. Before storing Use an end-window GM tube such as the
the chamber, allow the ethanol to evaporate ZP1481 (MX168) GM tube commonly used in
completely to prevent the plastic crazing over time. schools connected to a counter or ratemeter. Place
To store electrodes, they can be removed the GM tube on the bench so that the GM tube
from the cloud chambers and kept with other window is pointing upwards. It works better if
radioactive sources in a secure store. The you remove the spider-web protective cap, but be
electrodes are usually supplied in a handy plastic careful as the GM tube window is easily damaged.
storage case. Place the saucer flat on the bench (in its bag) next
The equivalent dose rate to the hand when to the GM tube. Take a count for 30 seconds,
holding the electrode is very low, no more than a or note the count rate. It will be a little above
few microsieverts per hour, and the dose received background. The β radiation does not enter the
during its use as a cloud chamber source will detector window. Although the GM tube detects
be insignificant. In standard WT20 type TIG photons principally through its tube wall, the
electrodes, thorium is evenly dispersed throughout efficiency is quite low, typically a few percent.
the rod. During manufacture, tungsten and Place a large sheet of aluminium about 5 cm
thorium oxide powder are sintered into a metal above the GM tube (Figure 7). A home-made
alloy rod and the thorium is firmly bound in the wooden holder is useful to do this. The aluminium
metal. It is almost inconceivable that thorium sheet should be large enough (approximately
could become released from the electrode, even 150 × 300 mm) so that its footprint covers the
if it were roughly handled. (Thorium is released saucer and GM tube. The count rate will rise
in small quantities when grinding the electrode, noticeably. Replace the aluminium sheet with a
or to a lesser extent during welding, neither of lead one (a piece of code 4 lead roof flashing from
SSR June 2011, 92(341) 69
Practical work using low-level radioactive materials available to the public Whitcher
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
Figure 7 The setup of the GM tube and uranium- something like a freezer bag resealing clip, attach
glazed saucer a piece of string and suspend it somewhere in the
room. It does not have to be high up, but you tend
a local builder is suitable) in the same position. to get better results if the balloon is hung in a place
The count rate rises even further. You can show away from draughts. Rub the balloon vigorously
that it is not the lead or the aluminium that is for a few moments with woollen gloves until the
radioactive by moving the saucer away. balloon is friction charged. Leave the balloon for
β backscatter from the sheet causes the about 30 minutes (although if you are really keen
detection rate to rise. The saucer is such a wide- to see the results, 15 minutes may be sufficient).
area source that there is plenty of scatter that gets Set up a counter with the GM tube upright.
to the GM tube window –no careful setting up Take a piece of rigid plastic, as in Figure 8, or
is needed. The scatter depends on the superficial card about 100 mm square with a 25 mm hole in
density of the sheet, so it is a good way of showing the centre, and place it so that the hole is about
a method for non-destructive testing of material 10 mm above the GM tube window. Take care as
thickness. You can extend the investigation by the GM tube end-window is fragile and easily
trying various materials for the sheet and various broken by carelessness. Take the background
thicknesses of the same material, and by changing count for 1 minute. Put on a pair of disposable
the height of the sheet above the GM tube. gloves (this is not for radiological protection;
see additional notes below) and take down the
Additional notes balloon. Deflate it by removing the clip, and put it
Saucers and small plates can be obtained from across the hole in the plastic sheet as in Figure 9.
chinaware antique dealers, including those on eBay. Avoid the balloon sagging and touching the GM
The modern orange colours do not use uranium. tube. Remove and discard your gloves into the
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. Figure 8 The setup for counting from the balloon
70 SSR June 2011, 92(341)
Whitcher Practical work using low-level radioactive materials available to the public
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
Figure 9 Counting emissions from the balloon wash your hands.
More advanced work
waste bin. Take a count for 1 minute to find the Using Excel’s ‘Add Trendline’ facility and
average count rate over the minute. The result can choosing the exponential trendline option, the
be quite astonishing. If time allows, take 1 minute exponential equation of best fit can be displayed
counts about every 20 minutes and plot the on the graph (Figure 10). The half-life can be
average count rate against time on a graph. determined from the exponent of base e, in this
The cause of the radioactive contamination of case −0.0119. The half-life is ln(2)/−0.0119,
the balloon which is 58.2 minutes in this example.
The radioactivity arises from the decay products Additional notes
of radon. Radon comes from the decay chains of The gloves are principally to stop nuisance
naturally occurring uranium and thorium in the contamination from the balloon onto the GM tube
environment. Measurements by Austen and Brouwer and counter. If you do contaminate the equipment
revealed that most of the radioactivity was due to with low levels of radon progeny and someone
214
Pb and 214Bi, both 222Rn progeny, and from 212Pb, else uses the equipment soon after, they are likely
one of the progeny of 220Rn. 214Pb has a half-life of to obtain erroneous results that can adversely
26.8 minutes, 214Bi 19.7 minutes and 212Pb 10.6 hours. affect their investigations or cause false alarm.
Figure 10 Graph of the decay of the contamination on the balloon
SSR June 2011, 92(341) 71
Practical work using low-level radioactive materials available to the public Whitcher
This experiment is an opportunity to introduce some of which can also appear brownish – the
a sense of proportion to the risks from low-level indicative tea-coloured tint only shows when
radioactivity. If you had a party at home, would looking through the lens.
you ask the people clearing away the party The lens is mounted in a holder with the radio
balloons to wear disposable gloves? Of course not! active lens element facing the GM tube (Figure 12).
Place a plate holder between the lens and the GM
5. Using a thoriated lens as a source to tube. Plates of material such as aluminium of
investigate emissions by absorption varying thickness (‘absorbers’) are placed between
Thorium was added to some camera lens elements the lens and the GM tube. Measure the counts
to give a high refractive index, For example, per minute detected by the GM tube for varying
versions of the Yashinon-DS 50 mm f1.7, Canon thickness or superficial density of plates.
FL 58 mm f1.2 and Pentax Super-Takumar A piece of paper placed between the lens and
50 mm f1.4 lenses. Such lenses have long been GM tube has a small effect on the count rate,
discontinued but they can be obtained from second- showing that the α radiation emission is small.
hand camera dealers and eBay. The rear element of Aluminium plates have a greater impact on
the Pentax Super-Takumar 50 mm f1.4 (Figure 11) count rate, showing that there is a considerable β
is radioactive and can be used as a source to radiation field from the lens. Suitably thick plates
investigate the emissions by placing plates of of aluminium will block all β radiation, but the
varying thickness between the source and detector. GM tube will continue to detect the γ radiation
A thoriated lens can generally be identified from the lens. A typical graph of count rate versus
by looking through the lens (but keeping it away plate superficial density (milligrams per square
from your eye) at white paper, which will appear centimetre of plate) is shown in Figure 13.
to have a slight tea-coloured tint. This is caused From these results, an aluminium plate of
by a gradual darkening of the glass from the roughly 800 mg cm−2 stopped the β field. This
self-irradiation of the lens. Ionising radiation would correspond to a maximum β energy of
displaces some electrons in the glass, forming about 2.0 MeV. (In fact, the most energetic β in the
defect sites that affect the absorption of light and 232
Th decay chain is 2.25 MeV from 212Bi.)
cause darkening within the glass (Speit, 1998). Thick lead stops β radiation but does not stop
Be careful not to confuse this with lens coatings, 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 Figure 12 The setup for placing absorbers in front
50 mm type II, manufactured between 1965 and 1971 of the thoriated lens
72 SSR June 2011, 92(341)
Whitcher Practical work using low-level radioactive materials available to the public
Count rate/min Ð1 the lens for a minute gives a trivial dose to the hand.
This measurement accords with the information
3000 in the report from the U.S. Nuclear Regulatory
Commission (2001). Do not bring the lens up to the
2500 eye, for example to use it as a magnifying glass,
because the eye is more susceptible to harm from
2000 radioactive exposure than the hand.
1500 6. Measuring the distribution of times
between emissions
1000 The distribution of times between emissions can
be determined using a low-level radioactive source
500 such as a gas mantle. The result is surprising to
many because it is not intuitive. People generally
0 expect some kind of bell distribution centred on a
0 500 1000 1500 2000 mean time between emissions.
Superficial density/mg cm Ð2 Keep the gas mantle in a sealable plastic bag
so that fragments of mantle are not dispersed. The
Figure 13 Graph of the count rate with absorbers of mantle does not need to be taken out of the bag for
differing superficial density use in this experiment.
The equipment is set up in a similar way
Count rate/min Ð1
to that measuring randomness. However, the
3000 practical demonstration needs a datalogger with a
function for measuring time between successive
2500 pulses, such as those from a slotted barrier going
through a light gate. The time between successive
2000 pulses from a GM tube can be very short, less
than 1 ms, and the datalogger needs to capture
1500 these. The Data Harvest EasySense datalogger
has a switched input for recording times between
1000 successive closures of the input. However, it is
not a good idea to connect the pulse output of the
500 GM counter directly to the datalogger switched
input, as the input could be damaged in some
0 circumstances. The switched inputs can be open
Figure014 Thoriated lens 1000
on a spark1500
counter, 2000 and closed safely using a photoswitch such as a
500
showing α emissions Panasonic (Matsushita) PhotoMOS relay.
Ð2
Superficial density/mg cm Some GM counters have a pulse output;
the three considered here are the Philip Harris
spark counter is better than a single-wire version
Digicounter (B8H29280), the Unilab GeigerTeller
for this demonstration. The detection was roughly
(F4H29371) and the Unilab modular GM EHT
1 spark per second. Placing a sheet of paper
unit (411.010; no longer available).
between the lens and detector wires stops the sparks
A PhotoMOS relay AQV251 connected to
and confirms that the α emissions are from the lens.
the pulse output from the Digicounter worked
Additional notes satisfactorily, and you can use the loudspeaker
The thorium is bonded in the glass so the release output. Connect a 1 kΩ resistor in series with the
of any significant thorium dust will be unlikely. PhotoMOS input (Figures 15 and 16).
Nonetheless, caution is needed to avoid breaking The pulse output from the Unilab GeigerTeller
the lens. In storage, keep the rear lens caps on until and the Unilab modular GM EHT unit is about
needed for use. In measurements carried out by the 100 µs, which is too short to switch the standard
author on a Pentax lens, the equivalent dose rate at type of PhotoMOS relay. The circuit shown in
the lens surface was roughly 15 µSv h−1, so handling Figure 17 uses a faster PhotoMOS relay, AQV259,
SSR June 2011, 92(341) 73
Practical work using low-level radioactive materials available to the public Whitcher
and a speed-up circuit. This worked reliably expected, and the relationship between the mean
(Figure 18). The transistor type is not critical – and standard deviation is interesting.
any high-gain general-purpose small-signal npn Using EasySense software version 2.2 and
transistor will work. connecting to the EasySense switch input labelled
The switched output in the PhotoMOS relay is ‘5A’, select ‘Timing’ (the ‘Time’ radio button will
connected to the switch sensor of the datalogger then appear selected) and then choose ‘From A to
and the times between successive switch opening A (stopwatch)’.
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
Figure 17 Circuit diagram for the outputs from the
Figure 15 Circuit diagram for the Digicounter output GeigerTeller and Unilab modular units
Figure 16 The setup with the Digicounter and Figure 18 Using a fast solid-state relay with the
datalogger Unilab GeigerTeller
References
Austen, D. and Brouwer, W. (1997) Radioactive balloons: radiological assessment of exemptions for source and
experiments on radon concentration in schools or homes. byproduct materials. NUREG 1717. Washington, DC: U.S.
Physics Education, 32, 97–100. Nuclear Regulatory Commission [www.nrc.gov/reading-
Speit, B. (1998). Special glasses for nuclear technologies. rm/doc-collections/nuregs/staff/sr1717/nureg-1717.pdf].
In The properties of optical glass (Schott series on glass
Websites
and glass ceramics), ed. Bach H. and Neuroth, N. pp.
343–348. Berlin: Springer. Practical Physics: www.practicalphysics.org.
U.S. Nuclear Regulatory Commission (2001) Systematic 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
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