RReal-time spectroscopy of solar pp neutrinosusing Nd K. Zuber a a Institut f¨ur Kern- und Teilchenphysik, Technische Universit¨at Dresden, Germany
Abstract
The potential real-time spectroscopy of solar pp neutrinos using
Nd as target isinvestigated. The threshold of 196 keV would be the lowest of all solar neutrino ex-periments running so far. Experimental rates and parameters are discussed, about580 SNU can be expected from pp-neutrinos and another 367 SNU from Be. Fur-thermore, it is investigated whether charged current reactions might cause a newbackground component for future double beta decay experiments based on a largeamount of
Nd .
Key words: neutrino rare search
PACS: todo
In the last decade neutrino physics made terrific progress by establishing anon-vanishing neutrino mass. This result stems from various neutrino oscil-lation searches including reactors, accelerators, the atmosphere and the Sun,for a recent review see [1]. The latter, namely the problem of missing solarneutrinos was, after the pioneering observation of the Homestake chlorine-experiment [2], one of the longest standing problems in particle astrophysics.Various astrophysical and particle physics solutions were proposed, but theproblem was finally settled by the gallium experiments GALLEX and SAGE,Super-Kamiokande and the Sudbury Neutrino Observatory SNO to be due toneutrino oscillations in matter. Independently, the large mixing angle solutionwas singled out as the only solution with the KamLAND detector observ-ing spectral distortions in reactor antineutrinos events. Recently, the first real
Preprint submitted to Elsevier 21 October 2018 a r X i v : . [ nu c l - e x ] A ug ime detection of sub-MeV solar neutrinos in form of the Be line has beenpublished by Borexino [3,4]. This has opened the window of real-time obser-vations of sub-MeV solar neutrinos.While the basic solution of the solar neutrino problem has been found, thereare still a lot of issues for astrophysics and particle physics to be explored.First of all the satisfying agreement of solar neutrino observations with he-lioseismological measurements and the Standard Solar Model has recently beworsened by improved 3D fitting of photospheric lines [5]. The newly de-duced elemental abundances lead to a worse description of helioseismologicalobservations. Thus, one of the fundamental assumptions of stellar structurephysics, the homogeneous distributions of the elements in stars, is in ques-tion and the measurement of neutrinos from the CNO cycle can give uniqueinformation on the abundance of these elements in the solar interior. Alsoparticle physics will benefit from future solar neutrino measurements. Besidesmatter oscillations alternative scenarios like non-standard interactions (NSI),Mass Varying Neutrinos (MaVaNs) and potential contributions from Lorentz-and CPT-violatioin have been proposed as alternative solutions to the solarneutrino problem [6,7,8]. However, all of them propose a different survivalprobability for ν e as the function of energy. The effect is most prominent inthe transition between vacuum and matter oscillations, i.e. around 1-2 MeV.Thus, an accurate pep solar neutrino measurement is also very important. Nomeasurement exists yet but it could be done by SNO+ and will help to reducethe uncertainties on the mixing angle θ . Also the small flux of hep neutrinosextending to highest energies has not been observed yet and would completethe picture of our understanding of stellar energy production. Last but notleast, the all over fundamental pp-neutrino flux, directly coupled with solarluminosity has not been observed in real time. Its observation and monitoringwill shed some light on the dynamics of the solar interior and any other timedependent effect. All together, real time measurement of the individual fluxeswill also determine the ratios of the various branches of the fusion chains anda real-time measurement of the full solar neutrino spectrum is the ultimateinformation one can get. For a recent review see [9].By far the largest flux is that of pp-neutrinos originating from the fundamen-tal fusion of two protons into deuterium within the dominant pp-chain. It isdirectly coupled to the solar luminosity and Standard Solar Models predicta flux of 6 × neutrinos per cm − s − with an error of about 0.5% [10].Unfortunately for experiments this is also the flux with the lowest energyterminating at neutrino energies of 423 keV [11]. The only existing measure-ments are based on radiochemical methods using Ga namely GALLEX andthe follow up GNO [12] as well as SAGE [13]. For detection via neutrino-electron scattering this results in a maximal energy of the electrons of about233 keV. Potential real-time pp measurements via this process using existinglarge scale scintillators suffer from backgrounds like C , Ar and Kr . Es-pecially dangerous is the β -decay of C with a half-life of 5730 years and aQ-value of 156 keV. Thus, for a long time other options in form of nuclear2ransitions were explored who would allow radiochemical detection and spec-troscopy of low energy solar neutrinos in real-time. Among them are doublebeta emitters and long-living isotopes, the most promising one for the latteris
In with a threshold of 114 keV [14,15] currently under investigation forthe LENS experiment [17]. From double beta candidates
Mo(threshold 168keV), Se(threshold 173 keV),
Gd(threshold 244 keV)
Yb(threshold 301keV) and
Cd(threshold at 464 keV, just above the pp-flux) were proposed[16,18,19].In this paper a new candidate is explored for real-time spectroscopy of pp-neutrinos, namely
Nd , a system also studied and used for double betadecay searches.
Nd and estimated rates
A well known isotope of interest for double beta decay searches is
Nd . Itdouble beta decays via
Pm into
Sm . Astonishingly according to [20] noexcited state of
Pm is known and even the ground state quantum numbershave some uncertainty, very like being a 1 − -state. However, recently withincharge exchange reactions studies a single 1 + has been identified in Pmabout 0.11 MeV ±
10% above the ground state with a Gamow-Teller strengthof B GT = 0 . ± .
02 [21]. Furthermore, they suggest for the ground statea 2 − assignment of quantum numbers compared to the 1 − recommended in[20]. The important point is that the newly discovered 1 + -state will allow thedetection of solar neutrinos with an energy threshold of 196 keV by neutrinocapture on Nd , given that the fact the Q EC for the electron capture of Pm is 86 keV [22].To estimate a rate only solar pp-neutrinos and Be neutrinos are considered.The flux used for Be is from the latest Borexino measurement and given as4.87 × cm − s − [4]. The pp-flux above the threshold of detection wouldbe about 77% of the total pp-flux. This has to be folded with the survivalprobability of ν e coming from the Sun, which is according to latest survivalprobability fits about 54-55%. Hence, the pp-flux considered for detection is2.5 × cm − s − . The absorption cross section can be written as [23] σ = 1 . × − (cid:104) p e E e F ( Z, E e ) (cid:105) cm (1)with p e , E e as the momentum and energy of the electron in units of electronmass and F ( Z, E e ) as the Fermi function. The bracket takes into account aspectral averaging for the pp-neutrinos. For the relativistic Fermi-functionsthe equations given in [24] is used. Given the above values a rate of σ × φ =353 SNU for the 862 keV Be line (another about 14 SNU might come from the384 keV line) can be determined, with the solar neutrino unit SNU being 10 − σ × φ ≈
580 SNU from pp-neutrinos can be estimated. Thus, about 1000 kg of
Ndenriched to 90 % would result in roughly 104 events/yr. Even for a very smalldetector from the solar neutrino point of view, there is already a significantrate.
Like in several other radiochemical approaches discussed the neutrino capturewill result in a coincidence, which is very convenient. The signal is shown inFig. 1. First of all there will be an electron within an energy range of 0-227keV together with a 110 keV de-excitation gamma which will be followed bythe β -decay of Pm . Whether this time coincidence can be used dependscrucially on the type of detector used as the half-life of
Pm is 2.68 hours[20]. Otherwise one has to rely on one of the signal parts. In liquid scintillatorbased approaches the coincidence search is very unlikely, thus either the firstpart or the
Pm decay can be used.The produced radioisotope
Pm has a complex decay scheme and will pref-erentially decay into excited states of
Sm emitting further characteristicgamma rays, in 68% of all cases one of 333.92 keV. Other observable gammalines with more than 10% emission probability are at 1324.1 keV (17.5%),1165.73 keV (15.8 %) and 831.85 keV (11.9%). In the following it is assumedthat the
Pm decay will be used due to its higher energy release. The domi-nant decay mode will be in 26.4% of the cases into a 1 − -state at 1165.73 keV.Furthermore 19.7 % will decay into a (2 − )-state at 1658.41 keV and 17.8% ina (2 − )-state at 2070 keV with the accompanied electron, accounting for 64%of the total decays. It should be mentioned in 12.4% of the decays a totalenergy in form of gammas is emitted within an energy range of 2100 ≤ E γ ≤ Pm is complex and the de-excitation energy is releasedin several gammas. All transitions are allowed transition, independent of theuncertain spin-parity assignment of the
Pm ground state (except that inthe case of a 2 − ground state this would be purely Gamow-Teller type), thusthe energy spectrum of the electrons can be well described by the known form.4 Experimental considerations
As mentioned before the signal consists of a low energy electron (in case of Be neutrino capture it is in 90% a monoenergetic 666 keV and in 10% a188 keV electron) in coincidence with a 110 keV gamma possibly in a longtime coincidence with the beta decay of
Pm , resulting in a second electronand associated gammas. As it is questionable whether any experiment will bedesigned especially for this purpose it might be worthwhile to explore whatthe next generation of large scale experiments for double beta decay based on
Nd can do.Currently two kind of approaches are considered, Nd-foils spanned into TPCs(the experiments DCBA and SuperNEMO) or Nd-loaded scintillators (SNO+).Consider the case of SNO+ first, which is supposed to run in a first phase with1000 tons of liquid scintillator with a 0.1% natural Nd loading (total mass of760 kg) and in a later stage with enriched
Nd . As the produced gammasand electrons won’t be resolved spatially the signal will be two energy deposi-tions, the first part being the neutrino capture and thus an energy depositionin the range of 110-337 keV (for pp-neutrinos) and 298 and 776 keV (for the Be lines) respectively. The second part would be the
Pm decay with a Q-value of 3454 keV. Due to the relative long life-time of
Pm of 2.68 hrs itis unlikely that a coincidence search can be used due to potential convectionand an overwhelming 2 νββ -decay background. The known half-lives of
Ndground state and first excited 0 + -state of this decay mode [25] will lead to arate of 0.5 Bq. In addition, this will also swamp the low energy signal dueto neutrino capture. Thus the only potential hope could be to search for thehigh energy part of the Pm decay leading to events beyond 3 MeV. Rely-ing solely on that the clear solar signal is gone. Various other contributionswill produce events in this energy range like
Tl contaminations, electronsfrom neutrino-electron scattering produced by B solar neutrinos and directproduction of
Pm . For example
Pm can be produced by (p,n) reactionson
Nd . However, the in-situ production by protons will be small as protonsfirst of all have to be created inside the detector by nuclear reactions and afterthat the (p,n) reaction on
Nd has to occur, recently measured cross sectionsare about 30 mb for 10 MeV protons [26].In the described first phase of SNO+ the used amount of
Nd is anyhow toosmall for any detection, the described problems for detection remain the sameeven for an enriched phase. Here the background due to 2 νββ -decay is evenorders of magnitude higher. 5he second approach would be thin foils spanned within TPCs. Here the elec-trons could be tracked, even though at these low energies the energy measure-ments might be disturbed by energy losses in the foil itself and the observationcrucially depends on the threshold used for electron detection. An advantagewill be that the coincidence of the capture signal and the
Pm decay canbe used, resulting in two electron tracks originating from the same point of afoil within a few hours. Combined with the given energy constraints on theelectrons and the detection of gammas in the TPC a clear signal should beobserved. The disadvantage of this approach are the space requirements be-cause the foils must be very thin to allow the electrons to escape. Thus, at themoment it seems unrealistic to build a ton scale experiment based on enriched
Nd using foils and hence the reaction rate will be too low for solar neutrinospectroscopy.To sum it up, unfortunately planned large scale double beta decay experi-ments using
Nd might be not suitable for low energy solar neutrino detec-tion. Other detector concepts based on
Nd have to be developed perhaps ahighly granulated Nd-loaded scintillator could be an option.However, a relatively high rate of low energy solar neutrino captures couldcause a severe background for double beta searches on
Nd , especially if thecoincidence described before cannot be used. The neutrinoless double betapeak is expect at 3371 keV [27]. While this perhaps is not an issue for ex-periments using enriched Nd-foils due to their relatively low mass, it couldcause trouble for calorimetric approaches like Nd-loaded liquid scintillators.Here the different decay channels cannot be resolved and only the sum energyof the transition is measured. As the Q-value of the
Pm decay is 3454 keVthere is a significant overlay with the Nd double beta peak region. Howeverwith the above given estimate a detector with 500 kg of
Nd enriched to90%, which is a potential scenario for SNO+ phase 2, there would be only 52events per year in total. With the given branching ratios of the
Pm decaythe number of events in a region above 3000 keV up to 3454 keV will be muchless than one event per year.
Real time solar neutrino spectroscopy still offers a lot of information for particeand astrophysics. With the recent discovery of an excited 1 + state in Pmin charge-exchange reactions a new opportunity for low energy real time mea-surements of low energy solar neutrinos using
Nd has been opened. Rateswere estimated and revealed that even for a relatively small detector a signif-6cant numbers of events can be achieved. The solar neutrino capture will notcause a major worry for current or planned double beta decay experiments.
Acknowledgement
The author would like to thank R. Dvornicky for discussions and help withthe Fermi-function and O. Chvorets for valuable discussions. The help of B.Lehnert is also acknowledged.
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