XenonPaul R.J. Saey

Vienna University of Technology, Vienna, Austria

1 Summary 1792 Occurrence 1793 Radioxenon in the Atmosphere 1824 Separation and Analytical Characterization

Techniques 1835 Conclusions 1866 Glossary 1877 End Notes 1878 Related Articles 1889 Abbreviations and Acronyms 188

10 References 188

1 SUMMARY

Radioxenon isotopes are noble gases mainly pro-duced in nuclear fission, e.g., that of 235U. They have twomain applications.

In nuclear medicine, 133Xe isotopes are used formeasuring the physiological parameters of lung ventilationand to image the lungs. They are further used in isotonicsolutions to image blood flow, particularly cerebral blood flow.

Most radioactive isotopes of this element areproduced by a nuclear fission reaction, e.g., that of 235U,238U, or 239Pu. To verify the comprehensive nuclear-test-bantreaty (CTBT) certain radioxenon isotopes are measured bya global network to detect clandestine (underground) nuclearexplosions that vented these gases in the atmosphere. Therelevant isotopes for this application are 131mXe, 133mXe,133Xe, and 135Xe.

Radioxenon isotopes are currently most frequentlymeasured with β –γ coincidence spectrometry, high-resolution γ -spectrometry, or with proportional counters butalso with gas chromatography-mass spectrometry.

2 OCCURRENCE

Xenon (Xe) is a noble gas and therefore chemicallyinert in the environment. The Earths’ atmosphere contains

approximately 0.087 ppm of stable xenon. The name derivesfrom the Greek xenon which means ‘‘the stranger’’. Xenonwas discovered in 1898 by Sir William Ramsay and MorrisTravers in residues left after evaporating liquid air. It is aheavy, odorless, colorless, tasteless, and nonflammable gaswith element number 54 and is around 4.5 times heavier thanair. When it is excited by an electrical discharge in a vacuumtube, it produces a blue glow. Some principal characteristicsare presented in Table 1.

Naturally occurring xenon consists of seven stableand two radioactive isotopes (124Xe and 136Xe, both with verylong half-lives). Beyond these stable and semistable forms,34 other radioactive isotopes and meta-stable states with half-lives above 0.1 s have been found. Nearly half of these arefission products of uranium and plutonium. The major part ofradioxenon isotopes is manmade—however, the spontaneousfission of uranium in nature produces very-low levels ofradioxenon.3,4 The stable as well as the known radioisotopesare listed in Table 2.

Radioxenon isotopes are artificial isotopes that arecreated during fission of heavy atoms, like 235U, 238U, or239Pu or during nuclear reactions, like (n,p) reactions. Theycan be created in or released from, among others, nuclearpower plants (NPPs), nuclear research reactors (NRRs),radiopharmaceutical production facilities (RPFs), or nuclearexplosions (NEs).5 A very small amount of radioxenon isalso created in the atmosphere in cosmic ray reactions withstable xenon gas. In areas with high uranium concentrations

Radionuclides in the Environment. Edited by David A. Atwood. 2010 John Wiley & Sons, Ltd. ISBN 978-0-470-71434-8

180 RADIONUCLIDES IN THE ENVIRONMENT

Table 1 Some principal characteristics of xenon1,2

Characteristic Value

Molecular weight 131.3 g mol−1

Melting point −111.75 ◦CBoiling point −108.04 ◦CGas density at boiling point 9.86 kg m−3

Gas density at STP 5.761 kg m−3

Atomic diameter in crystal 3.94 ACritical pressure 57.64 atmCritical temperature 16.058 ◦CCritical volume 118 cm3 mol−1

Solubility in water at STP 203.2 ml l−1

Solubility in water at 20 ◦C 108.1 ml l−1

Thermal conductivity at STP 5.5 mW (mK)−1

underground, spontaneous fission also creates small amountsof radioxenon.

Around 24% of the uranium or plutonium fissionproducts (sum of recommended cumulative yields) are noblegases, mainly xenon isotopes. Figure 1 and Table 3 showthe fission yield for several nuclear fission relevant nuclides:235U, 238U, and 239Pu. The radioxenon isotopes that are createdafter the fission of uranium are produced directly as fissionproducts and indirectly as daughters of fission products witha higher neutron number. The number of atoms produced asfission products per fission is the independent fission yield,whereas the direct fission yield and the sum of all atoms ofthe isotope produced from the radioactive decay of the otherfission products per fission are the cumulative fission yield.

Figure 2 shows the isobaric decay chains for themasses 131, 133, and 135, of which the radioxenon isotopesdiscussed later are a part.

2.1 Common Applications

Xenon-133, with a short half-life (5.243 days) andlow-energy γ -rays (81 keV), is used for measuring thephysiological parameters of lung ventilation and for imagingthe lungs. It is also used in an isotonic solution to imageblood flow, particularly cerebral blood flow.7,8 However,many hospitals are replacing 133Xe with the newly developed99mTc gas (half-life, 9.14 h) (see Technetium). The importanceof 133Xe in medicine and its commercial production are,therefore, decreasing.

Xenon-135 is of considerable significance in theoperation of nuclear power reactors. It is the daughter of 135I(half-life, 6.7 h). Because of the large neutron absorption crosssection of 135Xe (2.65 × 106 barn for thermal neutrons—thesimilar figure for 133Xe is 190 barn), 135Xe is converted tostable 136Xe during the irradiation period. After the irradiationhas ended and the neutron flux stops, 135I keeps on decayingand producing new 135Xe. It therefore acts as a neutronabsorber or ‘‘poison’’ that can slow down or stop the chainreaction after a period of operation. This was discovered in

Table 2 The different xenon isotopes and their characteristics

IsotopeNatural

abundance (%) Half-lifeMajor

decay mode

Majordaughternuclide

111Xe n.a. 0.74 s EC 111I112Xe n.a. 2.7 s EC 112I113Xe n.a. 2.74 s EC 113I114Xe n.a. 10 s EC 114I115Xe n.a. 18 s EC 115I116Xe n.a. 59 s EC 116I117Xe n.a. 61 s EC 117I118Xe n.a. 3.8 min EC 118I119Xe n.a. 5.8 min EC 119I120Xe n.a. 40 min EC 120I121Xe n.a. 40.1 min EC 121I122Xe n.a. 20.1 h EC 122I123Xe n.a. 2.08 h EC 123I124Xe 0.095 1.6 × 1014 a 2EC 124Te125mXe n.a. 56.9 s IT 125Xe125Xe n.a. 16.9 h EC 125I126Xe 0.089 stable — —127mXe n.a. 69.2 s IT 127Xe127Xe n.a. 36.4 days EC 127I128Xe 1.91 stable — —129mXe n.a. 8.88 days IT 129Xe129Xe 26.4 stable — —130Xe 4.07 stable — —131mXe n.a. 11.84 days IT 131Xe131Xe 21.2 stable — —132Xe 26.9 stable — —133mXe n.a. 2.19 days IT 133Xe133Xe n.a. 5.243 days β− 133Cs134mXe n.a. 0.29 s IT 134Xe134Xe 10.4 5.80 × 1022 a — —135mXe n.a. 15.29 min IT 135Xe135Xe n.a. 9.14 h β− 135Cs136Xe 8.86 3.60 × 1020 a — —137Xe n.a. 3.818 min β− 137Cs138Xe n.a. 14.08 min β− 138Cs139Xe n.a. 39.68 s β− 139Cs140Xe n.a. 13.6 s β− 140Cs141Xe n.a. 1.73 s β− 141Cs142Xe n.a. 1.22 s β− 142Cs143Xe n.a. 0.3 s β− 143Cs144Xe n.a. 1.15 s β− 144Cs145Xe n.a. 0.188 s β− 145Cs146Xe n.a. 0.1 s β− 146Cs

the earliest nuclear reactors built by the American ManhattanProjecta for plutonium production.

Another application of radioxenon is the measure-ment of 129Xe/129I isotopic ratios in meteorites. They are apowerful tool for studying the age difference between theearth and the oldest meteorites found. This gives informationon the formation of the solar system.9

Environmental radioxenon gas monitoring is afundamental and highly sensitive technique for the detectionof underground or underwater NEs. Of all the technologies to

10−3

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101

Fis

sion

yie

ld (

%)

60 80 100 120 140 160

Mass number

Fission by fissionspectrum neutrons

Fission by highenergy neutrons

235Uf238Uf

235UHE238UHE

239Puf239PuHE

131Xe 135Xe

133Xe

Figure 1 Fission yield in percent for several nuclear fissionrelevant nuclides: 235U, 238U, and 239Pu, for fission induced byfission spectrum (thermal) neutrons (f) and high-energy neutrons(14.7 MeV) (he) respectively6

verify the CTBTb,10 it is, together with radionuclide particulatemonitoring, the only technique that has the potential to provideunmistakable proof of an NE.11,12 The noble gas radioisotopesthat are useful for identifying an NE are 131mXe, 133mXe,133Xe and 135Xe5 —they are produced in significant quantitiesand have half-lives that are long enough to be measured aconsiderable time after any release. Depending on the fissionmaterial (235U, 233U, or 239Pu), between 1.08 × 1016 Bq and1.33 × 1016 Bq of 133Xe will be created in a 1 kton NE.3

To establish a global noble gas monitoring network,as part of the International Monitoring System (IMS) toverify the CTBT, fully automated radioxenon measurementsystems had to be developed, as no commercial systemswere available when the treaty was opened for signature.13

Four countries, France, Russia, Sweden, and USA, all withexperience of atmospheric xenon measurements, offeredto develop such systems, which are described later. Withthe Provisional Technical Secretariat (PTS) for the CTBTOrganisation (CTBTO) and the German Federal Office forRadiation Protection (Bundesamt fur Strahlenschutz, BfS),they participate in the International Noble Gas Experiment(INGE) project.14 These systems are now being installed at

XENON 181

Sn Sn

Sb Sb

Te Te

I I

Xe Xe

91 9

22.2

78.8

1.298.8

Cs

Sb

Te

I

Xe

Cs

Ba

83

83.4

17

17.5

70 12.5

97.1 2.9

135133131

16.8

Figure 2 Isobaric decay chains for the masses 131, 133, and 135with the branching ratios (in percent)—the gray dots are metastablestates (isomers)

up to 40 worldwide locations and they send their results to theInternational Data Centre (IDC) in Vienna for processing andanalysis.5

After the announced NE in North Korea in October2006, the Swedish Defence Research Agency (FOI) couldconfirm out of the ratio 133mXe/133Xe measured in the northof the Republic of Korea and using longer term backgroundmeasurements that the explosion was nuclear.15,16 Air sampledindependently above the Japanese Sea after the event contained133Xe and 135Xe in a ratio that also confirmed the nuclearorigin of the explosion. Also, the increased 133Xe activityconcentration measured at the Yellowknife IMS station inNorth Canada in late October 2006 was consistent with leakscenarios assumed for a low-yield underground NE on theKorean peninsula.17 This demonstrates the importance of twofactors: radioxenon activity concentration ratios can identifythe nuclear origin of a source if several isotopes are measuredduring consecutive days or if different isotopes are foundin one or more measurements and the knowledge of theradioxenon background can help identify such an event evenin the case where only one isotope is detected.

Table 3 The cumulative fission yields in percent for six fission modes relevant to nuclear fission, induced by fission spectrum(thermal) neutrons (f) and high-energy neutrons (14.7 MeV) (he)6

IsotopeFission yield

235Uf (%)Fission yield

235Uhe (%)Fission yield

238Uf (%)Fission yield

238Uhe (%)Fission yield

239Puf (%)Fission yield239Puhe (%)

131mXe 0.05 0.06 0.05 0.06 0.05 0.07133mXe 0.19 0.29 0.19 0.18 0.24 0.42133Xe 6.72 5.53 6.76 6.02 6.97 4.86135Xe 6.6 5.67 6.97 5.84 7.54 6.18

182 RADIONUCLIDES IN THE ENVIRONMENT

2.2 Production of Xenon

Xenon gas is recovered on a commercial scale byliquefying and the fractional distillation of liquid air and is, ingeneral, a by-product during the production of liquid oxygenand liquid nitrogen. It is collected in the liquid oxygen fraction,together with krypton and other noble gases that are present inthe air. Xenon is absorbed on a silica gel at low temperatureand then separated from the other noble gases by selectiveabsorption and desorption from activated charcoal.2

2.3 Production of Radioxenon

The most common and efficient way (more than 95%)to produce radioxenon isotopes, e.g., for medical applicationsis by neutron irradiation of highly enriched uranium (HEU;uranium with up to 97% of 235U) or low-enriched uranium(LEU; uranium with less than 20% 235U) (see Uranium).18,19

Uranium targets (in most cases, uranium pressedbetween two aluminum plates) for the production ofradioisotopes are irradiated in a nuclear reactor for2–20 days with a nuclear thermal flux between 1013 and5.1014 n cm−2 s−1. The targets are irradiated as long as isnecessary to create enough fission products, limited by theattainment of steady-state production and the formation ofundesirable isotope by-products. After irradiation, the decay-at-rest technique is adopted for a short while to removeshort-lived isotopes, in order to reduce the total radiation. Theuranium is then base or acid dissolved in heavily shielded hotcells.20 During the dissolution process, which takes around1–2 h, all the noble gases that were created during the fissioninside the targets, or that were since formed by their precursordecay, are drawn off and taken care of in varying ways.21

Then, the rest of the different fission products are separatedand purified.

Depending on the goal of the facility, some noblegases are recovered and carried with helium to krypton, xenon,and/or iodine recovery cells.22 In these cells, the gases arefrozen out with liquid nitrogen and during warming up theyare separated from each other, trapped on a molecular sieve,and further trapped on copper clippings. Subsequently, they arepurified and shipped to the end customer. At other facilities,the noble gases are treated as waste. These facilities sendthe noble gases into charcoal traps where they pass throughslowly as they decay. When leaving one trap, depending ontheir activity, the gases will flow into another trap or they willbe released into the atmosphere.23

Radioxenon isotopes are further produced in nuclearpower reactor operations. Being fission products, they arepresent within the nuclear fuel rods once the reactor is startedup and the fission process is initiated. If there is a crack in oneor more of the fuel rods, the gas will leak out and enter theventilation system of the facility, followed by a release intothe atmosphere.

During reprocessing of nuclear fuel rods, these rodsare dissolved to separate the isotopes. If this process takes

place within a few weeks after the fuel rods were irradiated,there still will be enough radioxenon isotopes present thatwould be discharged into the atmosphere. In most facilities,however, fuel is stored first for a few years and thereforethe radioxenon isotopes would have decayed away belowmeasurable activities.

Another source of atmospheric radioxenon is 239Pu,whose spontaneous fission yields of the masses 131, 133, and135 range between 4 and 8%. Plutonium-239 is continuouslyproduced in nuclear fuel elements storage ponds from238U: 238U (n, γ ), 239U, and 239U 239Pu + β. Radioxenonisotopes originating from 239Pu could therefore be present innuclear reactors, nuclear fuel reprocessing facilities, or badlycontained waste storages.

3 RADIOXENON IN THE ATMOSPHERE

The most common environmental radioxenon isotopeis 133Xe. Its half-life of 5.243 days is ideal for environmentaldetection systems since it is not much accumulated in theatmosphere, it is not washed out by precipitation, and itremains long enough to be detectable after atmospherictransportation to a monitoring station. This isotope is,therefore, typically detected in various environmental samples,originating from fission in different kinds of nuclear facilities.

The worldwide environmental background for thelonger lived noble gas isotope 85Kr was well defined inthe 1990s. The background of the shorter lived radioxenonisotopes, however, was not known accurately in the late1990s and early 2000 because of nonavailability of globaland well-resolved timely data and their regional variation. Todistinguish globally a civilian radioxenon release from nuclearfacilities with the signal from a possible NE was a complicatedissue. Recent long-term environmental measurements ofradioxenon isotopes measured down to very low levels and athigh-time resolution have shown that they are all lognormallydistributed. It was shown that, e.g., in Europe, there is anincrease in 133Xe activity concentration between 2000 and2008, which can be attributed to an increase in the productionof radiopharmaceutical isotopes (in which radioxenon gasesare, in most cases, a waste product) during that period.13,24

It has further been shown in recent studies that a partof the low background present in the northern hemisphere aswell as most extreme values measured are not attributed toNPPs, as believed in the 1990s,11,14,25–27 but to the releasesfrom a very few large radiopharmaceutical isotope productionfacilities.20,23,28,29 In these facilities, xenon radioisotopes area by-product created during the dissolution of the irradiateduranium targets for the production of 99Mo. It was shownthat during the production of radioisotopes for pharmaceuticalpurposes, a significant amount of radioxenon gases is releasedinto the atmosphere. Such an individual release is likelyto be 100–10 000 times higher than typical releases from

a single NPP. These facilities release in routine operationsbetween 200 and 315 times per year, whereas NPP releasesare dominated by a puff once or twice per year. Literatureindicates that all NPPs worldwide release about 0.74 × 1015

Bq of 133Xe per year.30 In Ref. 23 it has been shownthat the three largest radiopharmaceutical isotope productionfacilities alone release in total 11 × 1015 Bq of 133Xe peryear. It can, therefore, be concluded that these few largefacilities are the major contributors to the global radioxenonbackground.

A good method to distinguish a radioxenonmeasurement originating from an NPP from an NE wasdeveloped by Kalinowski et al.31 If three or all four relevantisotopes are measured, they can be plotted as the followingratios: 135Xe/133Xe versus 133mXe/133Xe, or 135Xe/133Xe versus133mXe/131mXe as shown in Figure 3. This figure also showshow the ratios of two different NE scenarios (235U and 239Pu(see Uranium; Plutonium)) move over time (from upper rightto lower left). The indicated ratios of RPF releases show thatfor short irradiation of the uranium target, feeding the RPF,and a late separation (more than a day) of the ‘‘explosion’’-xenon from its precursors produces very similar ratios. Thisis natural as a short irradiation very much resembles an NE,which, in turn, can be seen as a very short irradiation. Itshould be noted, however, that in most environmental samplesin areas where nuclear facilities are present, most samplescontain only one or two different radioxenon isotopes.

Table 4 summarizes the typical order of magnitude ofradioxenon release from different nuclear facilities.23 It shouldbe noted that facility releases, although given in Bq d−1, donot imply that emissions happen every day, as they of coursedepend on local work schedules. For NPPs, the releases areoften correlated with revision periods.

4 SEPARATION AND ANALYTICALCHARACTERIZATION TECHNIQUES

Being a noble gas and having a very lowconcentration in the atmosphere, it is demanding to separatexenon from environmental air. Once xenon is separated, theradioxenon isotopes can be measured in different ways, asdescribed below.

4.1 Atmospheric Radioxenon Sampling andMeasurements in the 1940s–70s

The first measurements of environmental radioactivexenon reported in the literature took place in 1944 as apart of the Manhattan Project intelligence efforts. It was theidea of Luis Walter Alvarez (later a Nobel Prize laureate) tosample gas above Nazi Germany and try to find 133Xe tracesof any possible nuclear fission activities performed there.32

The gases were trapped on cooled activated charcoal in the

XENON 183

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239Pu explosion Target irradiation: 48 h235U explosion Target irradiation: 220 h

Discrimination line

135 X

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t = 0

Dissolution after36 h of cooling

Irradiation stopt = 48 h

t = 0

t = 0

Irradiationstop t = 220 h

Dissolution after36 h of cooling

Irradiationstop t = 220 h

Figure 3 Xenon isotopic ratio plots for two RPF feed irradiationtimes (48 and 220 h) and two types of explosions (235U and 239Pu).The upper plot uses three isotopes (excluding 131mXe) and the lowerone all four. Such plots follow the ratio dependence from zero time(sample out of reactor and explosion time respectively—upper rightstarting point of all curves). They are normally useful up to arounda week. The explosion curves are shown for two cases: immediateprecursor separation (dashed lines) and no precursor separation (fulllines). Other separation times fall in between but after 1 or 2 daysthey are quite close to the ‘‘no-separation’’ line. For the isotopeproduction, a specific separation line is used and there the lines gofrom full to dashed

Table 4 Order of magnitude of releases of radioxenon at differentnuclear facilities24

Type of releaseTypical order of magnitude

of 133Xe release

Hospitals ∼ 106 Bq d−1

Nuclear power plants ∼109 Bq d−1

Radiopharmaceutical facilities ∼109 to ∼1013 Bq d−1

1 kt nuclear explosion underground 0 to ∼1015 Bq1 kt nuclear explosion atmospheric ∼1016 Bq

184 RADIONUCLIDES IN THE ENVIRONMENT

bomb bay of Douglas A-26 airplanes. Back at the laboratory,these charcoal traps were heated to extract the gases. Xenonwas separated out using its boiling point. The radioactivemeasurement of the radioxenon gas was then performed witha standard Geiger–Muller counter.33

During the late 1950s, high-pressure gas-samplingsystems, collecting gas in stainless steel spheres, in mostcases, mounted in the bomb bays of airplanes, were developedfor the US Defence Atomic Support Agency by the AirForce Technical Applications Centre (AFTAC) to evaluateworldwide fallout from NEs.34–36 In the laboratories, the gaswas then analyzed using Geiger–Muller counters and laterproportional counters, sodium iodide (NaI) detectors, andmore recent high-resolution high-purity germanium (HPGe)detectors. From the 1960s on, it is also reported that in Sweden,Germany, and the Soviet Union radioxenon measurementswere performed to identify signals of NEs.

4.2 Current Measurement Methods for RadioxenonIsotopes

Most xenon isotopes, in general, can be identifiedusing gas chromatography-mass spectrometry (GC-MS),which is, however, both time and cost intensive.

The important radioxenon isotopes for environmentalmonitoring and for NE verification (131mXe, 133mXe, 133Xe, and135Xe), all emit photons (X-rays and/or γ -rays) in coincidencewith β- or conversion electrons (see Table 5). The β-spectrumhas a continuum (defined by its maximum energy) from theβ-decay of 133Xe and 135Xe and defined peaks from themonoenergetic conversion electrons from 133mXe and 131mXe,which are immediately followed by X-rays.

X-rays are in the 30-keV range and have a totalbranching ratio of about 50%, except for 135Xe, which hasjust a 5% X-ray branch. The strongest associated conversionelectrons in coincidence with the X-rays are 129.4, 198.7,45.0, and 213.8 keV for 131mXe, 133mXe, 133Xe, and 135Xe,respectively (see Figure 4). Other strong coincident decaymodes are the 346-keV endpoint energy β-decay of 133Xein association with an 81.0-keV γ -decay, and the 901-keVendpoint energy β-decay in 135Xe, which is followed by a249.8-keV γ -ray (see Table 5).

131mXe 133mXe

131Xe 133Xe

135Xe

163.9 keV (2.0%)

233.2 keV (10.0%)

T1/2 = 11.934 d

T1/2 = 5.243 d

Ec = 129.4 keV (61.0%)X-rays (av) 30.41 keV(54.2%)

Ec = 45.0 keV(55.1%)X-rays (av) 31.64 keV(49.7%)

Ec = 213.8 keV (5.7%)X-rays (av) 31.64 keV(5.2%)

T1/2 = 2.19 d

T1/2 = 9.14 h

Ec = 198.7 keV (64.0%)X-rays (av) 30.41 keV56.5%)

133Xe

133Cs

81.0 keV (38.0%)

249.8 keV (90.0%)b

max = 346.4 keV

(99.2%)

bm

ax = 901.0 keV

(96.0%)

135Cs

131m 133m

135133

Figure 4 The strongest decay modes for 131mXe, 133mXe, 133Xe,and 135Xe5

Several new measurement methods are in place or areunder development to measure the radioxenon isotopes. Theyare based on short sampling periods (8, 12, or 24 h) and high-sensitive radioactive measurements (β –gated γ -coincidence,high-resolution γ -spectroscopy, and proportional counting)that can measure environmental 133Xe as low as 0.1 mBq m−3.

4.2.1 β —γ Coincidence Spectrometry

The nuclear measurement component of theSwedish SAUNA (Swedish Automatic Unit for Noble GasAcquisition)39 developed by the Swedish Defence ResearchInstitute (FOI) near Stockholm, Sweden, of the US ARSA(Automated Radioxenon Sampler and Analyzer) developedby Pacific Northwest National Laboratory (PNNL), Rich-land, USA40,41 and of the new Russian ARIX-IV (ARIX,Analyzer of Radioactive Isotopes of Xenon) developed andcommercialized by the Khlopin Radium Institute (KRI) basedin St. Petersburg, Russian Federation, are systems based onβ –γ -coincidence spectrometry. This technique is used tosuppress the noncoincident background and to achieve highsensitivity to the coincidence events characteristic of theradioxenons of interest (see Table 1).

Table 5 The four radioxenon isotopes discussed in this section, their half-lives, and their most intense γ -ray and X-ray(from Ref. 37)

Isotope Half-lifeEnergy X-ray (keV)

(Kα1 and Kα2) Intensity(a) (%) Energy γ -ray (keV) Intensity (%)

131mXe 11.84 days 29.62 44.4 163.930 1.91133mXe 2.19 days 29.62 46.1 233.22 8.2(b)

133Xe 5.243 days 30.80 40.9 80.997 38.0135Xe 9.14 h 30.80 2.1 249.77 90.0

(a)These values are the weighted averages of the Kα1 and Kα2 X-rays. The intensities are the sum of these two Kα lines.(b) From Ref. 38.

In most of these systems the xenon purificationmethod is similar. Environmental air is sampled with anairflow that is larger than 0.4 m3 h−1. The sampled air iscleaned from aerosols, water, Rn, Ar, N2, O2, CO2, and xenonis adsorbed by activated charcoal. This is followed by thermaldesorption of the xenon into a helium or nitrogen carrier. Thestable xenon volume of the concentrated gas is quantified bygas chromatography and via an in-line thermal conductivitymeasurement.

The SAUNA-II system samples air in 12-h cycles.Then, the collected xenon fraction is purified and concentratedfor about 7 h before it is counted with the (plastic and NaI(Tl))β –γ coincidence detector for around 12 h. The ARIX-IVcollects air as well for 12 h, while the ARSA samples for 8 h.

The β –γ detector has a NaI crystal with a drilledhole, where the gas flows in. The hole is coated with aplastic scintillator layer. On top and underneath the NaI cellis a photomultiplier to count the γ -pulses and there are twophotomultipliers at the end of the scintillator cell to count theβ-pulses. The electronic system counts the γ -, the β-, andthe coincidence pulses for around 12 h. A typical spectrum isshown in Figure 5. The two diagrams on the right side indicateclearly the advantage of a gated spectrum versus a nongatedspectrum: the background is a few orders of magnitude lowerwhen using the coincidence mode.

XENON 185

Before each sample measurement, a quality controlsource (e.g., 125Eu) enters the cell and is measured, to verifythe stability of the detector. Then a gas background of theempty cell is measured for 11 h to count the possible memoryeffect of a previous sample. Memory here refers to xenon thathas diffused into the plastic cell wall, where it will contributeto subsequent measurements. A typical memory effect in thecurrent β –γ systems is some 5%. Around 2 ml of stable xenonmay be extracted per sample, depending on the system usedand a minimum detectable concentration (MDC; the minimumconcentration that, with a given risk, can be expected to bedetected by a given process) of 0.1 mBq m3 for 133Xe in a 12-hmeasurement can be reached.

4.2.2 β-Gated γ -Coincidence Spectrometry

The nuclear measurement component of the ARIXI, II, and III systems is based on β-gated γ -coincidencespectrometry. The system collects air in 12-h cycles, whichis then purified and concentrated for around 4 h before itis counted with the (plastic and NaI) β-gated γ -detectorfor around 18 h.42 The detectors of the ARIX I–III consistof a low-resolution γ -detector (NaI detector) and a plasticscintillator β-detector, which are operated in β –γ coincidencemode. To minimize the memory effect, the plastic scintillator

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214Pb

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eV]

0

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nts

g -energy [keV]

b -energy [keV]

NongatedGated

NongatedGated

Cou

nts

Figure 5 This β –γ coincidence spectrum originates from an environmental air sample collected between 7 a.m. and 3 p.m. on June 10,2002 with an ARSA system in Charlottesville, USA. It shows the presence of (0.77 ± 0.11) mBq m−3 of 133mXe and (4.26 ± 0.35) mBq m−3

of 133Xe. The Region of Interest (ROI) boxes are marked in white. The color code indicates the counts5

186 RADIONUCLIDES IN THE ENVIRONMENT

used is so thin that the β-energy cannot be measured and onlyβ-gated γ -spectra but no γ -gated β-spectra can be recorded.The spectra are, therefore, a contraction of the β-axis, i.e.,a summation of all β-energy channels of the correspondingγ -energy channel. The γ -peaks at 80 keV and 250 keVcontain only counts from 133Xe and 135Xe, respectively andhence can be used to quantify these two isotopes separately. Asa consequence of the β-energy contraction however, the peaksof 131mXe, 133Xe, and 133mXe at 30 keV cannot be separated byenergy spectral analysis, but by performing decay rate analysisof the two hourly preliminary spectra. The MDC for 133Xereach 0.2 mBq m−3 for a 12-h measurement—the MDC’s forthe other isotopes, however, are much higher.

In early 2007, KRI decided to stop the productionof β-gated γ -coincidence detectors for their ARIX systemsand changed to β –γ coincidence spectrometry (see Section4.2.1).

4.2.3 High-Resolution γ -Spectrometry

A system for sampling and analyzing small amountsof radioxenon in ambient air was developed around 1980 bythe Swedish Defence Research Agency (FOI, formerly FOA).This was a forerunner to the SAUNA system but at that timeused charcoal adsorption of the xenon gas at −80 ◦C andhigh-resolution γ -spectroscopy for nuclear detection.43 TheMDC for 133Xe was in average around 1 mBq m−3.

The French Commissariat a l’Energie Atomique(CEA) started developing the SPALAX system (Systeme dePrelevement d’air Automatique en Ligne avec l’Analyse desradio-Xenons) in the late 1990s. This equipment continuouslysamples air for 24 h per cycle. At the end of such a collectioncycle and of the final purification, the xenon gas (around7 ml) is transferred into the counting system, which is ap-type broad energy high-purity germanium γ -ray (HPGe)detector for around 23 h.44 The gas sample cell is made oflow background aluminum, on top of the germanium crystal.At some stations, the standard HPGe end cap, which hasan aluminum window, has been replaced by a carbon fiberwindow to give improved X-ray transmission. The newestversions reach an MDC of around 0.2 mBq m−3 for a 24-hmeasurement of 133Xe.

4.2.4 Proportional Counting

The method of proportional counting has beenused since the 1970s by the noble gas laboratory at theGerman Federal Office for Radiation Protection (BfS) inFreiburg to continuously monitor the 85Kr and 133Xe activityconcentrations in ground level air in a global network. Thesample collection time during routine operation is 7 days. Thetotal volume sampled is around 10 m3 of air. The proceduresfor sampling, enrichment, and purification of the noblegas fractions are all manual. The integral β-activity of thesamples is measured in proportional counters using methane

as additional gas component. This integral counting methodgives the total activity of all radioxenons but a separation ofthe components can be done by decay analysis.45

Xenon-133 is the most abundant of the radioxenonsobserved in environmental samples, although contributions of131mXe and 135Xe can be determined down to a few percent ofthe total β-activity. The MDC for 133Xe in routine samples isabout 1 mBq m−3.

4.2.5 New Developments

Some portable systems that measure the fourradioxenon isotopes 131m, 133, 133m, and 135 in theatmosphere, have been developed at Argonne NationalLaboratory in collaboration with the University of Cincinnati,but were not built commercially. These integrated systemsconsist of a fluid-based concentration subsystem and adetection subsystem, based on NaI(Tl) photon detectorsalong with either gas proportional plastic scintillator orpassivated implanted planar silicon detectors to distinguishradioxenon signature emissions and discriminate against radonbackground.46,47

At the University of Coimbra, Portugal, a β –γ

coincidence system to measure the metastable isotopes 131mXeand 133mXe in high resolution (1.4 keV X-ray and 25 keVβ-emission) has been developed in cooperation with theLos Alamos National Laboratory. It is based on two gasproportional scintillator counters and a multiwire proportionalcounter with two silicon charged particle detectors, all builtin a beryllium box, which absorbs β-signals from outside thedetector.48,49

A group at the PNNL is currently developing andevaluating a simpler detector system than the existing ones,named PhosWatch, consisting of a CsI(Tl)/BC-404 phoswichwell detector with digital readout electronics and pulse shapeanalysis algorithms implemented in a digital signal processoron the electronics. This system uses a single phoswich detectorin which β –γ coincidences are detected by pulse shapeanalysis.50 Different prototypes are currently under testing.

5 CONCLUSIONS

Most radioisotopes of xenon are anthropogenic andcreated principally in nuclear fission of 235U, 238U, or 239Pu.They are released into the atmosphere from NPPs, RRs, RPFs,reprocessing facilities, and NEs.

Xenon-133 used to be an important isotope in nuclearmedicine, however, it is being replaced by 99mTc gas, whichhas the same physiological characteristics as 133Xe, but a muchshorter half-life, which is, of course, in favor of the patient(see Technetium).

The current most important application is themeasurement of radioxenon isotopes in the environment to

detect nuclear test explosions, to monitor and to verifycompliance with the CTBT. These newly developed high-sensitive measurement techniques can also serve other nuclearnonproliferation applications. Several ultralow measurementsystems have been developed and improved in the last 10 years.

To perform a waterproof verification of compliancewith the CTBT, especially to detect underground NEsfrom long distances and remote stations, the globalenvironmental radioxenon background needs to be knownand understood. This also includes the expected radioxenonactivity concentrations as the expected and calculated ratiosof these isotopes as an advanced atmospheric transport model.If three or four isotopes are measured, their ratios can give anindication of the source, namely, where and how they weremade and in which process: e.g., during a long irradiationof fuel rods in an NPP, in a short irradiated 235U target forradiopharmaceutical purposes, or during a very short reactionin an NE.

In future projects, the theoretical releases fordifferent scenarios should be confirmed by performing onlinemeasurements of all four xenon isotopes in the stack ofradiopharmaceutical facilities. Further, the releases of othernuclear facilities such as research reactors should be studied.

Furthermore, a total reduction of the emissions alsoappears possible, including the use of retention lines withcharcoal traps or other methods. A reduction of emissionsby a factor of 1000 or more is technically possible forseveral of these known RPFs and would bring the releases tothe same level as in the NPPs (∼109 Bq d−1). The benefitsof such reductions should be accordingly communicatedto the radiopharmaceutical producing community, especiallykeeping in mind that medical isotope production is predictedto increase in the future.

6 GLOSSARY

Absorber: Any material that stops ionizing radiation. Lead,concrete, and steel attenuate γ -rays. A thin sheet of paper ormetal will stop or absorb α-particles and most β-particles.

Intensity: Fraction of a decay event that results in theradiation(s) (e.g., a γ -line at a specific energy or a β –γ

coincidence pair). Intensity is sometimes used to meanabundance.

β-particle; β-radiation; β-ray: An electron of eitherpositive charge (ß+) or negative charge (ß−) that has beenemitted by an atomic nucleus or neutron in the process of atransformation. β-particles are more penetrating thanα-particles but less than γ -rays or X-rays. May also refer toother electron radiations, e.g., a conversion electron.

β –γ coincidence event: Nuclear decay producing both aγ -ray and a β-particle that are registered in a detector within

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a very short timescale. May also refer to the detection ofother photon–electron coincidence events such as an X-raywith a conversion electron.

β -gated γ -spectrum: A γ -spectrum, in which all photonsregistered with its energy were in coincidence with anelectron irrespective of its energy.

Concentration: For example, activity per unit volume of air(e.g., Bq m−3).

Electron capture: A radioactive decay process in which anorbital electron is captured by and merges with the nucleus.The mass number is unchanged, but the atomic number isdecreased by one as the process involves the transmutation ofone proton into a neutron.

Fission products: Radionuclides formed by the fission ofheavy elements. They are of medium atomic weight andalmost all are radioactive, for examples, 90Sr, 133Xe, and137Cs.

Fission: The splitting of a heavy nucleus into two majorparts of high kinetic energy, a few neutrons, and γ -energy.

Germanium detectors: In order for a significant absorptionof a γ -ray to take place, the material must have a highenough absorption coefficient, which can be provided by amaterial of high atomic number. It must also have a lowbandgap for conduction to occur, and must also have lowlevels of impurities in order to satisfy the conductionrequirements. This leaves only a few possible options—thetwo main candidates are silicon and germanium.

K-capture: The capture by an atom’s nucleus of an orbitalelectron from the innermost shell (K) surrounding thenucleus.

Scintillation counter: An instrument that detects andmeasures γ -radiation by counting light flashes (scintillations)induced by the radiation.

7 END NOTES

a. The Manhattan Project was the code name for theproject to develop the first atomic bombs during World WarII.

b. The CTBT was opened for signature in 1996 andis a key element in the nonproliferation of nuclear weaponsand a crucial basis for the pursuit of nuclear disarmament as itbans any kind of nuclear explosion.

188 RADIONUCLIDES IN THE ENVIRONMENT

8 RELATED ARTICLES

Anthropogenic Radioactivity.

9 ABBREVIATIONS AND ACRONYMS

AFTAC = Air Force Technical Applications Cen-tre; ARIX = Analyzer of Radioactive Isotopes of Xenon;ARSA = Automated Radioxenon Sampler and Analyzer;BfS = Bundesamt fur Strahlenschutz; CEA = Commissariata l’Energie Atomique; CTBT = comprehensive nuclear-test-ban treaty; CTBTO = CTBT Organisation; GC-MS =gas chromatography-mass spectrometry; HEU = highlyenriched uranium; IDC = International Data Centre; IMS =International Monitoring System; INGE = InternationalNoble Gas Experiment; KRI = Khlopin Radium Institute;LEU = low-enriched uranium; MDC = minimum detectableconcentration; NaI = sodium iodide; NE = nuclear explo-sion; NPP = nuclear power plant; NRR = nuclear researchreactor; PNNL = Pacific Northwest National Laboratory;PTS = Provisional Technical Secretariat; ROI = Region ofInterest; RPF = radiopharmaceutical production facilities;SAUNA = Swedish Automatic Unit for Noble Gas Acquisi-tion; SPALAX = Systeme de Prelevement d’air Automatiqueen Ligne avec l’Analyse des radio-Xenons.

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