for Teaching and Environmental Applications R. G. ~ ~ o n s '
Department of Geography and Oceanography, Australian Defence Force Academy, University of New South Wales, Northcott Drive, Campbell, ACT 2600, Australia P. C. Crossley
Department of Geography, University of Auckland, New Zealand D. Fortune
Department of Physics, University of Auckland, New Zealand In teaching radiation chemistry and the environmental significance of radiation, it is highly desirable to have some "hands-on" instrument for measuring radiation. As well as the obvious pedagogical advantages of experimental learning that is fundamental to scientific training, radiation dosimetry is a highly topical environmental issue. An appreciation of the natural variability in environmental radiation and its methods of measurement WP an important part of a ch~mistryor environmental science student's trainine ( I ) . However, "hot" radiation sources often are neither available nor desirable for use in tcachine the fundamentals of radiation chemistry, particularly at senior school level, nor are they the best means of teaching an understanding of environmental dosimetry. This paper describes a small portable and, most importantly, inexpensive GeigerMuller (GM) instrument which, unlike most GM instruments, is designed to measure low-level background dose rates. The instrument described here, which we called the "physmeter", originally was developed to measure background gamma radiation in caves, typically half that of surface levels, which is required as an input parameter to dating of speleothems and archeological artifacts by electron-spin resonance or thermoluminescent dating. It also has been used to measure background radiation in houses and buildings, for estimating the radionuclide properties of different soils and rock types and as a general purpose gamma dosimeter in laboratory and field work. It is sufficiently sensitive to detect minor radioactive contamination and pick up "hot spots" in the laboratory or environment and can be used to demonstrate the interaction of matter with radiation, shielding (using laboratory sources) and the screening of cosmic radiation (2). General descriptions of different methods for measuring gamma radiation may be found in texts such as Wang, Willis, and Loveland (3) and Friedlander and Kennedy (41, and of low-level environmental dosimetry in Aitken (5). For teaching purposes the method chosen must give immediate results, which eliminates methods such as film badges and thermoluminescent dosimeters. Instruments are availahle commorcially that are designed for low-level use in the environment. They fall into two main categories: gamma spectrometers and GM detectors. Gamma spectrometers with a large crystal may record a measurement in as little as 20 min which, after processing, can yield valuable data on the energies and concentrations of gamma-emitting radionuclides present. However, where low capital cost, portability, environmental (and student)
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'Current Address: Department of Physics. University of Auckland, Private Bag 92019. Auckland. New Zealand. 524
Joumai of Chemical Education
robustness, and technical simplicity are high priorities, spectrometers may have significant disadvantages for in situ measurements. Because GM detectors register only the total number of photons interacting with the detector, they are not energy discriminating and the counts recorded will be a function of the energy spectrum as well as the total dose. However, they are simple to use, much more robust than gamma ~~ectrnmeters, require much simpler supporting electronics and hence can bc made more ponable. The detectors themselves are relativelv inexoensive. so that if sufficient sensitivity can be achreved to allow rapid in situ measurements and the secondary design considerations met, a GM system potentially offers a viable, affordable low-level gamma dosimetry method with wide environmental and teaching applications. Design and Construction
The principal desim .. reauirement is to Drovide immediatt? m~asurehenta;i.e., f0.r high sensitivky. Construction and operation should be straiahtforward and costs minimal. secondary design considerations are a low-power demand, low voltage, low electronicbackground noise and independence from supply voltage fluctuations. Both the detector and the electronics need to be compact, light, and robust. Selection of Detectors Two detectors, ZP1220 (Mullard) and 78014 (LND) were selected for trial principally because they have a relatively high nominal "background". This might suggest they were unsuitable for low-level measurements, but it seemed possible that this "background" was, in fact, what we desired to measure; that is, the low-level"noisenin the more usual measurements of elevated radiation dose rates was the actual environmental dose rate that was the target for measurement. This view was reinforced bv the observation that the backgrounds quoted for the ranie of solid wall detectors available were almost directly proportional to the surface area of the detectors. It was not possible to obtain s e ~ a r a t eestimates of environmental radiation and electronic background from the manufacturers, but the detectors selected on this remise oroved to have the ~redicted high sensitivity to backgro&d radiation, with llectronic noise acceptably low. It is suggested that if these particular detectors are not available, detectors with a high surface area should be selected. Both detectors have a relatively low voltage requirement (500 V, compared to some detectors that require 1500 V), which minimizes the likelihood of condensation occurring in humid environments as well as placing less demands on
co-axial cable and BNC connector to the electronic circuit and readout. The electronics are housed in a lightweight, commercially available plastic food container, sealed with an O-ring (a Click-Clack box, manufactured by CPI Custom Moulders, Auckland, New Zealand). Because the circuit is operated by external magnetic switching and the Dower reouirements are low. there is wnerallv no~ need toaccess the electronicsin the held, an &vantage in adverse field conditions or when the instrument is o~eratedbv students. The instrument has proved extremkly robu2 and reliable in field use, with only one malfunction (a loose connection in the detector) in five years of demanding field work and casual handling. It should, thus, be suitable for routine student use.
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Fiaure 1. The circuit desion for the ohvsmeter adaDted from Tilbrwk ~~,- -. ( G j ~ h o"g.nal e design nas been modifiedby changing the readout to gave total couni instead of count me, whm enables oata to be aaumulared to give significant readouts. This mod11cabon, together with removlng the soma enects ano automattc t ming lac lilies, also reduces the power requirements, me size and the cost substantially, The switching mechanism has been replaced by a magnetic reset and start switch operated by an external magnet. In other respects thecircuficorresponds with that of Tilbmok,whoseguidelinesfor construction may therefore be followed. ~
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the cimuitry and power supply. Their Stainless Steel walls give them robustness. Circuit Design The circuit desien used is piven in Fieure 1. The basic circuit was designid by Tilbmik (6)and Kas been modified to meet the design requirements outlined above. The power supply is capable of delivering a voltage well within the operating requirements of the detectors for more than 200 h f?om four alkaline AAbatteries. In order to keep the instrument as simple and cheap as possible and to minimize the demands on the power supply, no provision is made for automatic timing, although this is part of Tilbrook's original design: readout is in the form of total counts and is converted to a count rate by independent timing. Because of the specialized requirements of field work in adverse conditions, the controls have been simplified and modified so that the instrument is switched on and the counter zeroed by external magnetic switching. The circuitry can, thus, be kept sealed in the field to minimize condensation, induced by the high voltage, and intrusion of dust.
The completed instruments give typical aboveground within-building count rates of 3.5 cps and 2.5 cps for ZP1220 and 78014, respectively; that is, a reading with a statistical counting uncertainty of f5% can usually be obtained in less than 2 and 3 min, respectively, and one of 2% in 12 or 15 min. The sensitivity of the physmeter allows a large number of measurements to be taken and gives considerable experimental flexibility; for example, students can carry out comparative tests for sites on different rock types and sediments or examine differences between and within buildings during a normal laboratory or field session. Cost The total cost of this instrument should not exceed $US300 (June 1992).Actualconstruction time by a competent technician would not normally exceed 5-6 h. Laboratory Calibration
Calibration can be obtained either by cross-calibration with factory calibrated instruments or directly by comparison with known standards. The probe also may be calibrated by using the radioactivity of K-40 present in any natural potassium salt such as KC1. This, being a low activity source, will need no shielding to protect the operators. A hollow annulus is made that will take the probe, about 5 1 0 cm in wall thickness and sufficiently long to extend beyond the ends of the probe. The annulus is then filled with KC1. If there is no other commercially calibrated monitor against which the instrument may bk calibrated, an approximate dose rate may be obtained as follows from the basic equations of radioactive decay. This calibration could be used as a class exercise.
One Bequerel is defined as 1disintegration per second, therefore, for 1Bq we require l/h atoms:
Instrument Housing The GM tubes themselves are protected by encasing them in W C plastic with an area density of 200-250 mg cm-'. The ends of the tubes are sealed with a gasket system as in Figure 2, which gives an excellent seal without sharp pressure points liable to fiacture the connecting wire under repeated stress. The tube is linked via a
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Figure 2. The GM detector housing. The compressedgasket Seal provided no sharp flexure points which might weaken the cable. Volume 71 Number 6 June 1994
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whereN = number ofatoms; k = decay constant (= (In 2Wtos); A =atomicweieht:andAN= Avoeadru's number = 6.023~loz3 ~ubstitutingtheappropriatevalues for h and A, we obtain: 1Bq = 3.872 ( K 4 0 ) Allowing for the relative abundances of the different isotopes of U and K, we have: 1Bq g-' = 3.16%K = 3.79% KzO = 0.766 m ~ y a d . The conversion of ppm to Gy is taken from Nambi and Aitken (19861, using a relative abundance of K-40 of 0.117%. Fmm the relative atomic weights of K and C1, KC1 has 52.4% K; i.e., 16.6 Bqg', and the dose rate due to an infinite matrix would be 12.7 mGya-'. This is an order of magnitude greater than typical background values. Because the annulus is not infinite the environmental dose rate will be overestimated by approximately 15%(for a 15-m annulus). For most teaching and environmental applichtions, relative dose rates will be sufficient. If absolute dose rates are required, then the implications of the dependence of the sensitivity of GM detectors on the energy of the gamma radiation must be considered and the limitations appreciated. The manufacturers' calibrations of GM tubes are given with respect to Cs-137 and Co-60 sources at much greater dose rates than natural background rates and are for the detectors without additional shielding. The use of GM detectors for absolute environmental dose rate measurements must allow for the degraded composite energy suedrum that includes a much hieher ~ercentageof lower energy radiation, the 47~geome&andthe additional protective housine: these wnsiderations are not trivial. Absolute caGbratious have been carried out for these detectors with respect to (1) a factory calibrated Mini-Monitor,Model 5-10G,rnanufactured by MINI-INSTRUMENTS,United Kingdom, using Cs-137 and Ca-60 laboratory standards and areas of raised radiation levels due to contamination as sources, and (2) the NZ Natiodal Standards at the National Radiation Laboratory, Christchurch, using Cs-137 and Co-60 sources (7). Depending on geometry and energy of the irradiatin gamma spectrum, 100 cps corresponds to 50-60 mGyw , with the more sensitive response corresponding to lower calibration energies.
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Field Calibratlon
The physmeter, usingdetector ZP1220, was compared to thermoluminescent dosimeters (TI.DI for a wide varietv of field sites. The TLD used for comparison were suppiied and analyzed by the National Radiation Laboratory of New Zealand. Descriptions of the methodology may be found in Robertson and Tucker (8)and Robertson, Tucker. and Randle (9). The ~hvsmeteris more sensitive. bv a fador of about two, to &ironmental spectra thaneto the Cs and Co sources in the laboratow. Overall. the ~hvsmetereave count rates of approximatdy 100 cps z 30 m ~ y a - '( ~ i g3). . This increased sensitivitv is. no doubt. due to the lower energy primary photons that occur in the environmental spectra and to the greater build-up of lower energy secondary photons in the effectively infinite matrix of the environment compared to the unshielded source used in laboratory calibr&ions. Lower energy gammas will contribute disproportionately more to the physmeter that does not
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Joumal of Chemical Education
0.2
0.4
TLD
0.6
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dose rate (mGya-')
Figure 3. Comparative Physmeter and TLD data for various caves. The error bars represent the approximate 0 (solid iine) and 20 (dashed line) uncertaintv limits. o Stes n &es in ~enla;ylhmestone,Wanomo. New Zea ana (Aranul. UrenJl. Co!nchdence Cavem, Te Ana HohonL). Soild I ne, 100 cps z 30 mGya-'. A Sites in iimenite-rich Pleistocene sand-dunes. Waiuku, West Coast, North island. New Zeaiand (dashed iine. 100 cps 26 r n ~ ~ a - ' ) . 0 Sites in caves in Ordovician marble, Nelson. New Zealand (Buimer Cavem System) The GM instrument show higher sensitivities (100 cps- 20 m~ya-l)for these sites, which are in rock with an unusually high thoriumluranium ratio, i.e., a higher proportion of low energy photons in the environmental spectrum. (Bulk analyses for uranium and thorium were carried out by National Radiation Laboratory, Christchurch, New Zealand.)
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discriminate between photon energies when counting pulses, than to the TLD, which measure actual dose. Recognizing that field calibration may not be within the scope of users of this instrument, and that generally teachreauire onlv relative measurements. it is ing - adications -. suggested that exp&mental calibration focus, not on the absolute values obtained. but be used rather to demoustrate calibration methods. If different energy spectra are available; e.g.. Cs-137 and Co-60, it also will be possible to demonstratethis fundamental energy dependence, which should, in any case, be stressed. A Further Application
In addition to the obvious demonstrations of calibration methods and environmental radiation dosimetry meutioned above, the physmeter can be used to demonstrate accumulation of radon in different sites. Radon is a highly topical issue because it has significant health implications and has been shown to accumulate to hazardous levels in situations where emanation from uraniferous rocks and sediments are not dis~ersedreadilv bv ventilation:, e.e.. inbasements of houseeon granite or in poorly ventilated caves and mines (10).Because radon is an alpha emitter,
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when digging disclosed that the grassed area was underlain bv a substantial concrete slab. uresumablv also containi; fly ash. (This means of dispbsing of t h e k a l power station flv ash is not restricted to China. and unusuallv high dose estimates in concrete buildings elsewhere m& be due to similar ~ractices.which students could investigate.) Conclusion
Figure 4. Gamma radiation monitor. its direct measurement requires relatively sophisticated equipment and is beyond the scope of most teaching applications. However, radon forms an efficient condensation nucleus and is known to "plate-out" readily on surfaces. Thus, high radon sites can be expected to have high concentrations of radon daughters on the surfaces. Because 97% of the gamma energy in the U-238 decay chain comes from radon daughters, such sites can be expected to have a high gamma dose and can be detected, at least qnalitatively, by the physmeter (11). A Tidbit The versatilitv of this instrument was demonstrated in 1987, when it &as used for environmental dosimetry in Guiyang Normal University, Guizhon, People's Republic of China. Routine measurements were camed out in a number of indoor and outdoor sites and revealed that some concrete buildings and one of the grassed areas had abnormally high count rates, three to four times the low rates expected for similar sites in New Zealand where the field trials reported above were carried out. The only sites with comparable radiation levels were the heaps of coal stored around the campus for heating and cooking. It seemed probable that the coal was uraniferous, and on enquiry, it was established that fly ash from thermal power stations was incorporated frequently in building concrete, creating elevated radiation levels. This hypothesis was supported
The instrument developed here, the physmeter, satisfies the maior desim criteria outlined above. In ~articnlar.it is snffici~ntlysensitive for readings to be takkn in a matter of minutes, enabling it to be used flexibly in monitoring low-level laboratory and field conditions and in student experimentation. The physmeter gives a good broad correspondence to environmental dose rates as determined by TLD. It is an effective tool for determining relative dose rates and sensitive in detecting regions of slightly elevated rates. Its use as an absolute dosimeter is more limited because of its increased sensitivity to lower doses, and, although the physmeter is competitive with commercially available GM systems, it would be wise to employ auxiliary techniques if absolute dose rate data is required. The instrument can, however, be most useful in gaining an appreciation of the variability within a site and in targetting further analyses. The physmeter has proved to be extremely reliable and robust. It is highly portable, weighing less than 1kg. It is relatively cheap, easy to construct, and simple to operate, requiring minimal technical expertise. In summary, the physmeter has considerable potential in teaching and environmental dosimetry applications. Literature Cited Ronneau. C. J. Chem. Educ. 1890.67736733.
Friedlander, G.; Kennedy J.WNuckorandRodiochemlaiq. Wlley:NewYork, 1955. Aitken, . I T ~ m o l u m i w % e n c r D a t i n gAcademic Re*: London, 1985 Tllhrwk, D. "AGeiger Counter You Can Build."Eleefmnies T a b j Inlrmafioml, 1880.(Aueuatl. lqona, R. ~ & t r i h ~ t i to ~ mESR dadng with specia referenee to New Zealand ape1oothems.PhD thesis; Uniuer6tytyfAuckLmd,Ne~Zealand, 1990. Robertson,M.K.; lhcker, L. J. A High Sensitivity Photon Counting TLD System; R e ~ o rNo. t NRL 1980110.NationalRadiat~n Iahmatmv: Christehureh.New Zea-
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