Estimation of reactor neutrino fluxes

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by  Daniel A. Dwyer

Estimation of reactor neutrino fluxes
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LawrenceBerkeleyNationalLaboratory
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x E-mail: [email protected]
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Reactorantineutrinoshavebeenindispensableforourunderstandingofneutrinomassandmixing.
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n At thesame time, discrepanciesbetweenthe observedandpredictedreactorn rate andenergy
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spectrahavegrownastheprecisionofthesemeasurementshasimproved. Measurementsofthe

1 electronsemittedfollowingfissionresultinthemostprecisepredictionsforthecorrespondingn
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2 flux,andourunderstandingofthepotentialsystematicdifferencesbetweenthefissione− andn
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fluxeshasimproved. Measurementsofindividualfissiondaughterisotopesandtheirdecaysare
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8 fraughtwithuncertainties,yetstillprovideinsightintothesediscrepancies.Detailedcomparisons
0 ofn measurementsamongreactorsarealsosheddingnewlightonthistopic.
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NeutrinoOscillationWorkshop
4-11September,2016
Otranto(Lecce,Italy)
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Speaker.
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommons
Attribution-NonCommercial-NoDerivatives4.0InternationalLicense(CCBY-NC-ND4.0). http://pos.sissa.it/
Estimationofreactorneutrinofluxes DanielA.Dwyer
1. Introduction
Antineutrinos emitted bynuclear fission reactors have served asapowerful tool forthe study
of these weakly-interacting particles. The intense flux from reactors, roughly 1020 n per second
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perGW ofreactorpower, wasusedforthefirstdetection oftheseelusive particles [1]. Measure-
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mentsofreactorantineutrinoshavealsorevealedthedistinctsignatureoftheoscillationofneutrino
flavor[2,3]. Ontheotherhand,precisemodelsofreactorn emissiondonotagreewiththesemea-
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surements. Thepredictedrateis6%higherthanthatobserved, afeaturethatiscommonlyreferred
toasthereactorantineutrinoanomalyandhasbeenconsideredpossibleevidenceforsterileneutri-
nos[4]. Morerecently,precisemeasurementsofn energyspectrahavealsoshowna∼10%excess
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relative toprediction intheregionof5to7MeV[5,6,7]. Intheseproceedings Iwillexaminethe
details behindthesediscrepancies, anddiscuss thesubstantial recentdevelopments inthisfield.
The process of reactor n production is well understood. Fission of actinides, in particu-
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lar 235U, 238U, 239Pu, and 241Pu, produce unstable neutron-rich fission fragments. These fission
daughter isotopes undergo successive b -decays until reaching stability, with an average of 6 de-
cays per initial fission. Thetotal n e emission from areactor, S(En ),isthe sumofthe n e’semitted
bythesedecays,
n m
(cid:229) (cid:229)
S(En )= Ri fijSij(En ), (1.1)
i=0 j=0
where R is the rate of decays of the i’th fission daughter isotope, f isthe relative probability for
i ij
the j’thdecaymodeofthisdaughter (alsoreferredtoasthebranching fraction), andSij(En )isthe
n energy spectrum for the j’th decay mode. There are more than 1300 known fission daughter
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isotopes, andcombinedtheyincludemorethan10,000uniquedecaymodes.
2. Current measurements
Three sets of measurements are particularly relevant to the assessment of reactor n produc-
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tion:
1. directmeasurements ofreactor n emission,
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2. measurements ofelectronemissionfollowingfission,and
3. measurements ofthefissionyieldsanddecaymodesoffissiondaughters.
Directmeasurementscommonlyinvolven detectionviainversebetadecay(IBD)inlargeorganic
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scintillator detectors. Calorimetry of the positrons produced by IBD allow accurate estimation of
the rate and energy spectra of those n with energies above the interaction threshold of 1.8 MeV.
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Subsequent detection of the neutrons produced by IBD allows for effective background rejection.
Themostrecent generation ofdirectmeasurements haveobserved morethan 1millionn interac-
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tions, and obtained percent-level uncertainties in both the rate and energy spectra [5, 6, 7]. This
precision has been putting pressure on the field to obtain more accurate predictions of the reactor
n flux.
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Estimationofreactorneutrinofluxes DanielA.Dwyer
Themostprecisepredictions havebeenbasedoncorresponding measurements oftherateand
energy spectra of electrons emitted following fission. Dueto the kinematic symmetry ofthe elec-
trons and n produced in b -decay, their rate and energy spectra are highly correlated. The fission
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electron spectra were measured at the 2%-level in a series of experiments at the ILL research re-
actor in Grenoble in the 1980’s [8, 9, 10, 11]. In these measurements, foils of actinides (235U,
239Pu, 241U) were exposed to the neutron flux in the ILL reactor and the emitted electrons were
measured. Bymeasuring the cumulative electron spectra due to all the fission daughters and their
decay modes, one avoids the need to know the detailed aspects of each daughter. Modeling the
electronspectrumasthesumofalargenumberofb -decays,thecorresponding n spectrumcanbe
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calculated [12]. Nuclearcorrections tob -decaydointroduce slightasymmetriesbetweentheelec-
tron and n spectra, assummarized in [13]. Ahybrid approach that uses data onfission daughters
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toinformthesenuclearcorrections givesasimilarresult[14]. Overall,thisb -conversion approach
provides aprediction forthe reactor n rate and energy spectra withuncertainties atthe 3%-level,
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andhasservedasthede-facto standard forthepastthirtyyears.
Unfortunately, thedirectn measurements andb -conversion predictions disagree onboththe
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rate and energy spectra, as discussed in the introduction. The origin of these discrepancies are
unclear, although potential explanations have been explored [15]. Antineutrinos from the decay
ofneutron-activated reactormaterials,spectraldistortionsfromforbiddendecays,andnon-thermal
fission of 238U do not seem to be large enough to explain the differences. The energy spectra of
the neutrons producing fission in the ILL electron measurements differ slightly from that in the
commercial reactors used in the direct n measurements. This could result in a slightly different
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distribution of fission daughters, which is difficult to rule out as a source for the discrepancies
between the electron and n data. Another option could be an unknown systematic in the ILL
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electron measurements, although this is difficult to confirm given that these are the only set of
electron measurements todate.
What guidance can the past century of measurements of nuclear fission and decay provide?
These measurements, which are collected in nuclear databases such as ENDF,JEFF,and JENDL,
can be used to calculate the n flux according to Eq. 1.1. Examples of such calculations can be
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found in [16, 17, 18, 19]. Given the large uncertainties of such calculations, one might conclude
thatthese databases canprovide littleguidance. Forexample, 70%oftheknownfissiondaughters
lack decay mode data (although these tend to be those daughters which are rarely produced, and
henceonlyamountto∼6%ofthetotalfissionyield). Thefissionyielddataprovidedbythevarious
databases are inconsistent with each other, and gross errors have been identified [20]. Decay data
are generally only known for the most prominent decay modes, and are susceptible to systematic
biasesfrommeasurementtechniques (e.g.thePandemonium effect).
Despite these obstacles, the shape of reactor n energy spectrum calculated from the ENDF
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database isunexpectedly similar to thedirect n measurements [19]. Thismaynot bewholly sur-
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prising, since the spectral shape seems to be dominated by a small number of prominent fission
daughters anddecay modeswhicharewell-measured. Manyoftheuncertainties impactdaughters
andmodeswhicheachcontribute atmost1%oftheoveralln flux,andhencehavelittleinfluence
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on the spectral shape. Consequently, there is potential to improve the calculation of the spectral
shape through a targeted program of measurement of the most prominent fission daughters. Un-
fortunately, the rate calculation will likely continue to suffer from large uncertainties due to the
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Estimationofreactorneutrinofluxes DanielA.Dwyer
cumulative effectofthemanyrarebutpoorlyknownfissiondaughters.
3. Looking forward
A targeted program of measurements of the decay modes of prominent fission daughters is
beingpursued,andhasbeguntoyieldresults. Inparticular, measurementsof92Rband142Csusing
total absorption spectroscopy have already reduced the largest uncertainties in the calculation of
the5to7MeVdiscrepant regionofthen spectrum [21,22]. Comparison oftheENDFandJEFF
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databasessuggestanotherimportantstepwillbeimprovedmeasurementsofthefissionyieldsofthe
most prominent daughters, of which 96Y is the most critical [15]. To directly address the tension
between the electron and n measurements, a repeat of the ILL electron measurements is being
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considered atLANL.
Recent work comparing the direct n measurements between different nuclear reactors has
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also been fruitful. A global analysis of n rate measurements has shown that the rate discrepancy
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cannot be attributed solely to the minor fission parents such as 239Pu or 238U, and instead shows
that 235U electron and n data are in tension [23]. A double ratio of the Daya Bay and NEOS
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observed over expected n spectra also suggests tension between the 235U electron and n energy
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spectra [24]. Data from the upcoming generation of short-baseline direct n measurements, such
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as PROSPECT [25], should continue to elucidate. The impressive precision of recent n mea-
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surements also suggests interesting potential for reactor characterization and non-proliferation. In
general, our understanding of reactor n emission is advancing rapidly and I expect substantial
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improvements overthecomingyears.
I would like to thank the organizers of the 2016 Neutrino Oscillation Workshop for the invi-
tation to come speak on this topic. I owe Patrick Huber, Bryce Littlejohn, and Patrick Tsang for
thoughtful discussions on these topics. This work was supported under DOE OHEP DE-AC02-
05CH11231.
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