Precise measurement of the 222Rn half-life: a probe to monitor the stability of radioactivity

0.35 MB·English

by  E. Bellotti

Precise measurement of the 222Rn half-life: a probe to monitor the stability of
radioactivity
E.Bellottia,C.Brogginib,∗,G.DiCarloc,M.Laubensteinc,R.Menegazzob
aUniversita`degliStudidiMilanoBicoccaandIstitutoNazionalediFisicaNucleare,SezionediMilano,Milano,Italy
bIstitutoNazionalediFisicaNucleare,SezionediPadova,Padova,Italy
cIstitutoNazionalediFisicaNucleare,LaboratoriNazionalidelGranSasso,Assergi(AQ),Italy
5
1 Abstract
0
2 Wegivetheresultsofastudyonthe222RndecayweperformedintheGranSassoLaboratory(LNGS)bydetectingthe
gammaraysfromtheradonprogeny. Themotivationwastomonitorthestabilityofradioactivitymeasuringseveral
r
a times per year the half-life of a short lifetime (days) source instead of measuring over a long period the activity of
M
a long lifetime (tens or hundreds of years) source. In particular, we give a possible reason of the large periodical
fluctuations in the count rate of the gamma rays due to radon inside a closed canister which has been described in
1 literature and which has been attributed to a possible influence of a component in the solar irradiation affecting the
1
nucleardecayrates.Wethenprovidetheresultoffourhalf-lifemeasurementsweperformedundergroundatLNGSin
] theperiodfromMay2014toJanuary2015withradondiffusedintooliveoil. Briefly,wedidnotmeasureanychange
x of the 222Rn half-life with a 8·10−5 precision. Finally, we provide the most precise value for the 222Rn half-life:
e
- 3.82146(16)stat(4)syst days.
l
c
Keywords: Radioactivity,Radon,GranSasso
u
n
[

1. Introduction particle being emitted from the Sun. In [4] the pos-
2
sibility that the coupling to a long range scalar field,
v
Apossibletimedependenceoftheradioactivenuclei
7 sourced by the Sun, might be the origin of the mod-
decay constant has been searched for since the begin-
5 ulation has been quantitatively discussed. The labora-
7 ningofthescienceofradioactivity. Forinstance,inthe toryconstraintsonthevariationofα andoftheelec-
em
7 Ph.D.ofM.Curie[1]onecanalreadyfindthedescrip-
trontoprotonmassratioonanannualtimescaleturned
0 tion of the search for a difference in the radioactivity
out to induce upper bounds to the relative variation of
.
1 ofuraniumbetweenmiddayandmidnight. Recently,in thedecayconstantnineordersofmagnitudelowerthan
0 particular since the year 2009 [2], various experiments the claimed per mil effect. Solar neutrinos have also
5 havereportedevidenceofatimemodulationofthede-
been proposed as responsible for the modulation, with
1
cay constant of several radioactive nuclei with period,
: acrosssectionseveralordersofmagnitudehigherthan
v inmostcases,ofoneyear(butalsoofaboutonemonth
expected.Inparticular,thepossibilityforanti-neutrinos
Xi oroneday)andamplitudeatthepermillevel. Thisan- affecting the β+ decay of 22Na has been very recently
nualmodulation,withthemaximuminFebruaryandthe
r studiedinareactorexperiment[6].
a minimuminAugust, hasbeencorrelatedtothechange
Sincewebelievethatdedicatedexperimentsarestill
oftheSun-Earthdistancebetweenaphelionandperihe-
needed to clarify the somehow contradictory situation,
lion;howeverdifferentmeasurementsexcludeanymod- in this letter we describe the results of a different ap-
ulationaslargeasthereportedones(forrecentreviews
proach we pursue to monitor the time dependence of
onresultswithandwithoutmodulation[3,4]).
radioactivity.
In [2, 5] the existence of new and unknown parti-
Generally,alonglifetimesource(tensorhundredsof
cleinteractionhasbeenadvocatedtoexplaintheyearly
years) is selected and its activity is measured for a pe-
variationintheactivityofradioactivesources,withthe
riod of at least one year. This way both the count rate
andthedeadtimearealmostconstant.Wealsofollowed
∗Correspondingauthor this approach in the study of 137Cs [7], 40K and 232Th
PreprintsubmittedtoElsevier March12,2015
Table1:The222Rndecaychain[11].Gammarayswithrelativeprob-
0.05
abilitysmallerthan3%areomitted.
Isotope Decay half-life Gamma Relative
type energy[keV] probability
222Rn α 3.8d uals 0
221184PPob αβ 236..18mm 242 7% d resid
295 18% malize
214Bi β 19.9m 365029 3465%% Nor-0.05
768 5% 0.005
934 3% 0
1120 15% -0.005
1238 6% 0 20 40 60 80 100 120
1378 4% Time [h]
1764 15% Figure 1: Normalized deviations from the exponential trend of the
2204 5% measuredcountrateasfunctionoftimeforradoninair:without(up-
214Po α 164µs perpanel)andwith(lowerpanel)thepolystyreneparticlesinsidethe
210Pb β 22.3y 46.5 4% glassspherecontainingairchargedwithradon.Theshadedareaslimit
210Po α 138d theregionwherefluctuationsarewithin±2σ.
206Pb
The set-up we used in the external Gran Sasso Lab-
oratoryduringthepreparatoryphaseoftheexperiment
[8,4]. However,duringsuchalongperiodoftimesev-
(tostudythesourceproduction)israthersimple:aglass
eralthingsmaychange,suchasthelaboratorypressure,
sphere(130mmdiameter)isconnectedthroughapipe
temperature and humidity, the radioactive background,
toastainlesssteelcylindercontaininga0.3kgrockrich
the electronic noise, and the performances of the elec-
in uranium. The radon from the radium decays in the
tronicchain.Itisnoteasytokeepalltheseundercontrol
rock fills by diffusion the glass sphere which, after 5-
iflookingforavariationatthepermillevel,orevenbe-
6 days, is isolated from the radon source by closing a
low. Tomitigatetheeffectofallthesepossibleinstabil-
Nupro(R)valve.
itieswedecidedtomeasuretimeaftertimethehalf-life
Gamma rays from radon progeny are detected by a
ofashortlifetime(days)sourceandforthispurposewe
selected 222Rn, a source relatively easy to produce. A 3”x3” NaI crystal placed at a few millimeter distance
fromthespheresurface(thegeometricalcentersofthe
similar approach has already been followed in [9] and
[10]withthemeasurementofthe198Auand214Pohalf- sphereandofthecrystalarealigned). TheNaIispow-
eredandreadoutthroughanOrtec(R)digiBASE(TM)
life, respectively, with a precision of a few parts over
104. whichalsopre-amplifyanddigitizethesignal. Boththe
detectorandtheglasssphereareenclosedinsidea5cm
In the next section we describe the experiment we
thickleadshield. Thedetectorcountrate,i.e. theinte-
performed with radon in a confined air volume, before
gralofthespectrumabove6keVthreshold,isofabout
giving in section 3 the results of the measurements we
performed underground at LNGS with radon diffused 1000 counts per second (cps) at the beginning of the
measurement, to be compared to a background, before
intooliveoil.
fillingwithradon,ofabout5cps.
The normalized residuals of the count rate, i.e. the
2. Radoninair difference between the measured rate and the expected
one, divided by the expected rate as function of time
222Rn is a noble radioactive gas coming from the is shown in Figure 1 (upper panel). Instead of having
decay of 226Ra (half-life: 1600(7) y [11] ) in the a statistical distribution around zero, there is clearly a
238U chain. The decay chain below 222Rn (half-life: 24 hour period. We have the same behavior if, instead
3.8232(8) d [12]) is given in Table 1, together with ofconsideringthewholeenergyspectrum,wetakeinto
the energy and the relative probability of the emitted accountonlythepeaksdueto214Pband214Bidecay. A
gamma rays. In our experiment we measure the 222Rn similar effect has already been observed in the gamma
half-lifewithgammaspectroscopy:thiswaywearesen- radiationfromradonprogenywithinairinconfinedcon-
sitivetovariationsbothinalphaandbetadecay. ditions [13, 14, 15] and it has been attributed to a pos-
2
sameeverywhereonthesurfaceoftheglasssphere. In
addition,wehadaclearcorrelation(Figure2)between
difference00..0024 tahnedctohuenttermatpeedraifftuerreendcieffeorfetnhceetwbeotwdeeteencttohresDtw1oapnodiDnt2s
malized 0 opfertahteursepThe1reanindfTro2nwt oerfethmeedaesutercetdor(sTw1haenrde Tth2ewteemre-
Nor-0.02 taken in front of D1 and D2, respectively). The count
0.06 rateofthedetectorclosetothehighertemperaturepoint
waslowerthantheoneofthedetectorclosetothelower
K]0.04 temperaturepoint(themaximumtemperaturedifference
T [ amounted to 0.05 K, while the maximum temperature
0.02
D excursion was from 13.5 to 17 ◦C). Finally, we were
0 abletoreproducethecountratemodulationbyswitch-
0 50 100 150 200 ingonandoffaheaterplacedat1mdistancefromthe
Time [h]
leadshield.
Figure2: Normalizeddifferenceofdetectorcountrates(D1-D2)as
Our conclusion is that large changes in the count
functionoftimewiththetwodetectorsplacedonoppositesidesof
theglassspherefilledwithradon(toppanel).Typical2σfluctuations rateinadditiontotheexponentialdecayareduetothe
amounttoabout0.001. Thetimedependenceofthetemperaturedif- displacement of radon and its progeny inside the glass
ference(T2-T1)measuredonthespheresurfaceisshowninthelower spherevolumetriggeredbyasmalltemperaturegradient
panel. Thehoursfrom100to150correspondtoaweek-end,when amongdifferentpointsonthespheresurface(asshown
laboratorytemperatureisverystable.
inFigure2,adifferenceof0.01Kisenoughtoproduce
asizableeffectonthecountrate). Thedisplacementof
sible influence of a component in the solar irradiation the radioactive atoms gives then rise to a different de-
affectingthenucleardecayrates. Inouropinion,allthis tectionefficiency.
couldsimplybeattributedtothedisplacementofthera-
dioactivenucleiinsidethegasvolume,withavariation
3. Radoninliquid
inthedetectionefficiency.
We triedto verifyour hypothesis bycompletely fill- For a precise study of the 222Rn half-life it is nec-
ingtheglassspherewithsphericalpolystyreneparticles essary to suppress the temperature effect described in
(diameter: 0.7-0.9 mm). This way the atoms cannot the previous section. Therefore, we selected an option
movefreelywithintheentirevolumebuttheyareessen- ableto’immobilize’radonanditsprogenybutallowing
tiallyconfinedtothesmallvolumewithintheinterstitial at the same time for a rather high radon concentration
space among polystyrene particles. The background, and,asaconsequence,ahighcountrate. Asamatterof
measuredduring20daysbeforefillingwithradon,had fact, astatisticalanalysisshowsthataprecisiononthe
still a stable rate of about 5 cps. The glass sphere was lifetimebetterthanonepartover104requiresaninitial
thenchargedwithradonfortwoweekstogiveaninitial count rate of at least 400 cps and a measuring time of
count rate of about 1500 cps. Spectra taken with this about 2 months. To achieve this we diffused radon in
configuration do not exhibit any modulation, as shown olive oil which has much higher viscosity than air and
in Figure 1 (lower panel). As a consequence, we con- which permits in equilibrium conditions a radon con-
clude that the displacement of radioactive atoms is the centration29timeshigherthaninair[16].
reasonoftheeffect. For the half-life measurement we minimized the
Searching for the trigger of this displacement, we background and its fluctuations by running the exper-
modified the experimental set-up by adding a second imentundergroundatLNGS,wherethemuonandneu-
3”x3” NaI detector (the two detectors were symmetri- tronfluxaresuppressedbysixandthreeordersofmag-
callyplacedonoppositesideoftheglasssphere).Inad- nitude, respectively, and by having a copper cube (10
dition,wemeasuredthetemperatureatdifferentplaces cm side, 2 mm thick walls) to contain the olive oil
onthespherebyusingprecisionintegrated-circuittem- chargedwithradon. Inparticular,thecoppercubefilled
perature sensors with output voltage linearly propor- with olive oil was charged with radon in the external
tional to the Centigrade temperature. In particular, we laboratory by slowly pumping and diffusing through it
measuredthetemperatureinthetwopointsonthesur- the air coming from the cylinder containing the ura-
facejustfacingthetwodetectors. niumrichrock. Afteraboutoneweekthecubewasiso-
We noticed that the temperature change is not the lated, detached from the cylinder, hermetically closed
3
ordertoimprovethestatistics.
Spectra are collected during consecutive time win-
1 dows of 3600 s each, with the timing provided by the
internalquartzoscillatorofthedigiBASE(TM).Itspre-
cisionandstability(betterthan5ppm/year)areenough
0.1
V] forourpurposes. Thetimestampofeach3600smea-
ke
nts/s/ surement is then converted into the UTC (Coordinated
ou0.01 UniversalTime)timescaleusingaperiodiccheckofthe
Rate [c internal time of the data acquisition PC (free running)
with respect to the UTC itself. This conversion gives
10-3
risetoarelativesystematicuncertaintyonthelifetime
measurementoflessthan2·10−6.
10-40 500 1000 1500 2000 2500 IntheperiodfromMay2014toJanuary2015wetook
Eg [keV] fourdifferentmeasurementsofthe222Rnactivitytoex-
Figure3:Spectracollectedundergroundbeforechargingtheoilwith tract the half-life and to measure its stability. Before
radon(lowergraph)andduringaradonrun(uppergraph). chargingwith222Rnwetooka20daylongbackground
runwiththecopperboxfilledwitholiveoilandclosed
inside the lead shield underground. We took this mea-
andmovedunderground. Thedetectorwasa3”x3”NaI
surementtopreliminarycheckthebackgroundstability
poweredandreadoutbyadigiBASE(TM).Theshield-
overtime.
ing to absorb the external gamma rays was made of at
Deadtime,providedbythedigiBASE(TM)with20
least15cmofleadandthelaboratorytemperaturewas
keptbetween12and13◦C. msresolution,amountsto0.2%atthebeginningofthe
radonmeasurement, linearlydecreasingwiththecount
rate,asshowninFigure4. FromthelowerpanelinFig.
4weseethat,foraconstantcountrate,thereisamaxi-
0.2 mumrelativefluctuationofthedeadtimeof4·10−3. As
aconsequence,themaximumrelativesystematicuncer-
0.15 tainty on the life time measurement due to dead time
%]
me [ fluctuations amounts to at most 8·10−6, one order of
Ti 0.1 magnitudesmallerthanthestatisticalerror.
d
Dea 0.185
0.05
0.18
410 420 430
30 0
1
x 0.5
uals 0 100
Resid-0.50 100 200 300 400 500 unts/s]
Figure4: DeadtimeasfuncRtiaoten [coofunctos/us]ntrateforthehalf-lifemea- Rate [co 10
surements.Adetailedviewatthebeginningoftherunsisgiveninthe
insert.Absolutedeviationsfromalineardependenceareshowninthe
1
lowerpanel.
Figure3showsthespectracollectedundergroundbe- duals0.05
forechargingtheoilwithradonandduringaradonrun. Resi-0.5
AllthepeaksduetothegammaraysgiveninTable1or 0 200 400 600 800 1000 1200 1400
Time [h]
to their sum are clearly visible. In addition, there is a
Figure5:Countrateasfunctionoftimeandnormalizedresidualsfor
small bump at 511 keV due to e+e − annihilation from
thesecondhalf-lifemeasurement.Theshadedareacorrespondstothe
gammaraysinteractingintheleadshield. Intheanaly- ±2σuncertaintyregion.
sisweconsidertheentireenergyspectrumabove6keV
and not only the full energy peaks. This because we Thecountrateasfunctionoftimeandthenormalized
want to avoid any inaccuracy coming from the fitting residualsforthesecondmeasurementareshowninFig-
procedureandwealsowanttoincreasethetotalratein ure 5 (for the other three measurements the figures are
4
Table2:TheparametersfromthefitA+B·e−t/Ctothecountrateasfunctionoftime.
Run Runningtime[h] A[cps] B[cps] C[d] reducedχ2
1 1301 1.0209(9) 452.98(5) 5.51335(46) 1.13
2 1462 1.0239(7) 457.98(5) 5.51302(44) 1.06
3 1185 1.0247(10) 463.16(5) 5.51352(46) 1.09
4 1357 1.0241(9) 354.82(4) 5.51289(50) 1.03
essentiallythesame). Foreachmeasurementwefitthe From the four measurements we obtain the follow-
countrate,afterdeadtimecorrection,withthefunction ingvaluesforthehalf-life: 3.82157(32)d,3.82134(30)
A+B·e−t/C. InTable2thevaluesoftheparametersand d, 3.82169(32) d and 3.82124(35). They are perfectly
ofthereducedchisquaredaregiven,withχ2 computed compatible and, as a consequence, the 222Rn half-life
bytakingintoaccountonlystatisticaluncertaintiesfrom hasbeenconstant, within8partsover105 (1sigmaer-
thePoissondistribution. ror for each measurement) during 9 months from May
The fit parameter A corresponds to the background 2014toJanuary2015. Wearenowplanningtocontinue
which, as shown in Table 2, is slightly increasing dur- thesehalf-lifemeasurementsforatleastanotheryear.
ing the measurements. Figure 6 shows the lowest en- Finally, if we average the results of the four in-
ergypartofthebackgroundspectrumtakeninonehour dependent and mutually consistent measurements we
before starting the radon measurement and during the obtain the most precise value for 222Rn half-life:
last hour of the third radon measurement, when radon 3.82146(16)stat(4)syst d, wherethevalueofthesystem-
hascompletelydecayed. Itisinterestingtonotethatthe aticuncertaintyisthesuminquadratureofallthepre-
46.5 keV peak due to 210Pb, which is the background viously described systematic uncertainties which are
component expected to increase with time during the given in Table 3. Our value has to be compared to the
radonmeasurementbecauseofitslonghalflife,hasin- most precise recent measurement of 3.8224(18) d, ob-
creasedbylessthan3·10−3cps(the46.5keVpeakinthe tainedwitha222Rnaqueoussolutioninliquidscintilla-
backgroundspectrumbeforefillingwithradonisdueto tor [17], and to the world average of 3.8232(8) d [12].
210Pbdecayintheleadshield). Ifweimposeintothefit Weobservethattheworldaverageisaweightedaverage
aslopeonthebackgroundof10−3 cps/60dayswethen of seven measurements [18], several of which are very
have a relative variation of the half life of 7·10−6, one old,withlimiteddescriptionofthesystematicsbutwith
orderofmagnitudesmallerthanthestatisticalerror. verysmallerror,muchsmallerthanin[17].
Table3: Therelativesystematicuncertaintiesonthe222Rnhalf-life
(in10−6units).
100
Timeconversion Deadtimefluctuations 210Pb Total
80 2 8 7 10.8
60
nts
u
o
C 4. Conclusions
40
We have studied the decay of 222Rn at the Gran
20 SassoLaboratorybydetectingthegammaraysfromits
progeny.Thepurposehasbeenthesearchfortimemod-
0 ulationsofradioactivitybypreciselymeasuringseveral
0 20 40 60 80 100 120 140
Eg [keV] times per year the half-life of a short lifetime (days)
Figure6: Thelowestenergypartofthebackgroundspectrumtaken sourceinsteadofmeasuringoveralongperiodoftime
inonehourbeforestartingtheradonmeasurement(shadedarea)and theactivityofalonglifetime(tensorhundredsofyears)
during the last hour of the third radon measurement (points). Sta- source. As source we have used radon diffused into
tisticaluncertaintiesareshownonlyfortheradonmeasurement. In
addition to the 46.5 keV peak from 210Pb decay, the L (around 12 olive oil: this way we removed the large fluctuations
keV)andK(around75keV)X-raysofleadareclearlyvisible. inthecountratetriggeredbyatemperaturegradientat
5
the surface of the volume containing air charged with
radon(adifferenceof0.01Kisenoughtoproduceasiz-
ableeffectonthecountrate). Ourresultsdonotsupport
the hypothesis of a possible influence of a component
inthesolarirradiationaffectingthenucleardecayrate.
Inparticular,weperformedfourhalf-lifemeasurements
in the period from May 2014 to January 2015 which
provide a constant 222Rn half-life with a 8·10−5 preci-
sion. Finally, we improve by one order of magnitude
the precision on 222Rn half-life providing the value of
3.82146(16) (4) d.
stat syst
Acknowledgments
We thank Prof. F. Borghesani of Padova University
for several enlightening discussions. The Director of
LNGS and the staff of the Laboratory are warmly ac-
knowledgedfortheirsupport.
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