Detection of Supernova Neutrinos

Transcription

1 Nuclear Physics B Proceedings Supplement 00 (2011) 1 6 Nuclear Physics B Proceedings Supplement Detection of Supernova Neutrinos Mark R. Vagins The Institute for the Physics and Mathematics of the Universe, University of Tokyo, Kashowanoha, Kashiwa, Chiba JAPAN Abstract Prospects for the future detection of supernova neutrinos are considered, with primary emphasis on water Cherenkov detectors. In particular, for the last seven years R&D on a potential upgrade to the Super Kamiokande detector has been underway. Enriching Super K with 100,000 kilograms of a water-soluble gadolinium compound thereby enabling it to detect thermal neutrons and dramatically improving its performance, particularly as a detector for supernova neutrinos and reactor neutrinos is discussed, as are some of the studies both ongoing and planned to help make this ambitious proposal a reality, including the construction of a large-scale, dedicated gadolinium testing facility in the Kamioka mine. Keywords: neutrino, supernova, gadolinium, GADZOOKS!, EGADS, Super Kamiokande 1. The Waiting Is The Hardest Part Who doesn t love supernova neutrinos? They carry unique information about one of the most dramatic processes in the stellar life-cycle, a process responsible for the production and dispersal of all the heavy elements (i.e., just about everything above helium) in the universe, and therefore a process absolutely essential not only to the look and feel of the universe as we know it, but also to life itself. As a gauge of the community s level of interest in these particular particles, it is worth noting that, based upon the world sample of twenty or so neutrinos detected from SN1987A (by Kamiokande, IMB, and BAKSAN), there has on average been a paper published once every ten days... for the last twenty-three years! After a quarter of a century, this handful of events remain the only recorded neutrinos known to have originated from a more distant source than our own Sun (by an easily-remembered factor of ). The detectors which stand ready to record the next nearby burst have changed only very slowly over the years. BAKSAN is still in business, while the teams behind Kamiokande and IMB joined forces nearly two decades ago to build the still-operational Super Kamiokande [SK, Super K] detector. LVD has been running since 1992, while SNO has already come and gone without having the opportunity to observe a galactic supernova. Nature is not always kind, and this endless waiting is maddening! True, there are a few (relatively) new players in the game. KamLAND and Borexino are both fine supernova neutrino detectors, as is the nearly complete (but already taking data) IceCube. SNO s 3 He counters are in the process of being recycled as a small but dedicated supernova neutrino detector called HALO, and the redesigned SNO+ currently under construction may yet get a chance to see what eluded its predecessor. In the neutrino world one has to dream big, so perhaps in the future we will see a true next-generation project like Hyper Kamiokande in Japan, LBNE in the US, or one of the LAGUNA detectors in Europe finally get funded and get built. Any one of these would be a spectacular device for detecting supernova neutrinos, but they are all a decade away from collecting data, at the very least. That darned waiting again... but what to do about it? It would certainly be great if there was a reliable way to collect some more supernova neutrinos. Unfortu-

2 M. R. Vagins / Nuclear Physics B Proceedings Supplement 00 (2011) nately, galactic supernovas are relatively rare, occurring somewhere between once and four times a century. For that matter, even SN1987A wasn t in our own Milky Way, but rather was located out in the boondocks of the Large Magellanic Cloud, some six times more distant from Earth than the galactic center. However, all of the neutrinos which have ever been emitted by every supernova since the onset of stellar formation suffuse the universe. These comprise the so-called diffuse supernova neutrino background [DSNB], also known as the relic supernova neutrinos. They have not yet been seen, but if they proved to be observable they could provide a steady stream of information about not only stellar collapse and nucleosynthesis but also on the evolving size, speed, and nature of the universe itself. What s more, their observation would have far-reaching implications for cosmology, as it would positively determine the stellar evolution rate. In 2003, Super Kamiokande published the results of a search for these supernova relic neutrinos [1]. However, this study was strongly background limited, especially by the many low energy events below 19 MeV which swamped any possible DSNB signal in that most likely energy range, as well as by Michel electrons from sub-cherenkov threshold muons produced by atmospheric neutrino interactions in the detector. Consequently, this previous SK study could see no statistically significant excess of events and therefore was only able to set the world s most stringent upper limits on the relic flux. 2. Doing Something About The (Neutrino) Weather In order to finally see the elusive DSNB signal, theorist John Beacom and I are proposing to introduce a water-soluble gadolinium [Gd] compound, gadolinium chloride, GdCl 3, or the less reactive though also less soluble gadolinium sulfate, Gd 2 (SO 4 ) 3, into the Super Kamiokande detector. As neutron capture on gadolinium produces an 8.0 MeV gamma cascade, the inverse beta decay reaction, ν e + p e + + n (1) in such a Gd-enriched Super K will yield coincident positron and neutron capture signals. This will allow a large reduction in backgrounds and greatly enhance the detector s response to both supernova neutrinos (galactic and relic) and reactor antineutrinos. The gadolinium must compete with the hydrogen in the water for the neutrons, as neutron capture on hydrogen yields a 2.2 MeV gamma, which is essentially invisible in Super K. So, by using 100 tons of gadolinium compound we would have 0.1% Gd by mass in the SK tank, and just over 90% of the inverse beta neutrons would be visibly caught by the gadolinium. Due to recent breakthroughs in the price of gadolinium as a result of new large-scale production facilities operating in Inner Mongolia, adding this much gadolinium to Super K would cost no more than $500,000 today, though it would have cost a staggering $400,000,000 back when SK was first designed. We propose calling this new project GADZOOKS! In addition to being an expression of surprise, here s what it stands for: Gadolinium Antineutrino Detector Zealously Outperforming Old Kamiokande, Super! This proposal is detailed in our Physical Review Letters article [2]. Note that it is the only method of detecting neutrons which can be extended to the tens-ofkilotons scale and beyond, and at reasonable expense adding no more than 1% to the capital cost of detector construction as well. 3. Signals All The ν s That s Fit To Print 3.1. DSNB Signal If adding Gd 2 (SO 4 ) 3 to Super Kamiokande makes it possible to look for coincident signals, i.e., for a positron s Cherenkov light followed shortly within 50 microseconds and in the same spot easily within our best vertex fitter s position resolution by the gamma cascade of a captured neutron, then troublesome spallation singles backgrounds could be eliminated and the analysis threshold lowered far below the old 19 MeV cutoff. This would be made possible by simply applying most of the same techniques used in our solar neutrino analysis [3], the only major difference being that a search for pairs of correlated events would allow us to extract the inverse beta signal. Going lower in energy will not only allow a detection of the so-far unseen DSNB flux, but it will also allow us to extract important and unique, barring a galactic supernova information regarding the neutrino emission parameters of supernovas. The sparse SN1987A data is in disagreement regarding the average luminosity and energy of the supernova ν e s. DSNB models vary, and there is in fact some tension between the models (and their proponents), but according to one widely accepted modern analysis Super K with gadolinium should see about five of these supernova events every year [4]. This rate, if correct, would allow a rather prompt (within one year) discovery

3 M. R. Vagins / Nuclear Physics B Proceedings Supplement 00 (2011) of the DSNB and hence lead to correspondingly rapid solutions to a number of long-standing questions, including the seemingly incompatible SN1987A neutrino data sets, the actual stellar formation rate, the correct heavy metal production model, and the average supernova neutrino emission parameters. Furthermore, with fresh supernova neutrino data in hand for the first time in a generation, such an observation will undoubtedly stimulate new theoretical (and perhaps even experimental) developments in the neutrino and cosmology communities Galactic Supernova Signal If we are fortunate enough to observe a nearby supernova in the coming decades, it would be most beneficial to have Gd 2 (SO 4 ) 3 in the water of the large water Cherenkov detectors which are online when the resulting neutrino wave sweeps across the planet. This is primarily because their most copious supernova neutrino signal by far ( 88%) comes from inverse beta events. These are only produced by one of the six species of neutrinos and antineutrinos which are generated by a stellar collapse, and so if we could tag them individually by their follow-on neutron captures then we could extract the ν e time structure of the burst precisely, gaining valuable insight into the dynamics of the burst. What s more, we could then subtract them away from the more subtle non-ν e signals, uncovering additional information that would otherwise be lost from this once-in-alifetime happening. For example, being able to tag the ν e events would immediately double SK s pointing accuracy back to the progenitor star. This is merely the result of statistics, since our elastic scatter events (about 3% of the total) would no longer be sitting on a large background in angular phase space [5]. Super Kamiokande is the only running detector with useful neutrino pointing capability; reducing the error on this quantity by a factor of two would reduce the amount of sky to be searched by a factor of four. This could prove quite important for the narrow-field astronomical instruments which would be attempting, assuming of course that we can get the word out in time, to see the first light from the new supernova. At the same time, this event-by-event subtraction would allow identification of the initial electron neutrino pulse from the neutronization of the infalling stellar matter, a key input in understanding supernova dynamics. If the exploding star was big and rather close ( two kiloparsecs or less) we would get an early warning of its impending collapse [6]. Approximately a week before exploding, the turn-on of silicon fusion in the core would raise the temperature of the star sufficiently that electron-positron annihilations within its volume would begin to produce ν e just above inverse beta threshold. The sub-cherenkov positrons would be invisible, but in SK the captures of the resulting neutrons on gadolinium would result in a sudden, dramatic, and monotonically increasing singles rate. As early as six days before collapse we would see a five sigma excursion in our low energy singles rate in the case of Betelgeuse nearing the end of its lifetime. The continuing increase in singles rate would clearly indicate a coming explosion, and ensure that we did not intentionally turn off Super K for calibration or maintenance and thereby miss the big event. Only Super K with effective neutron tagging can receive this early warning; no other existing detector can do this. In addition, a gadolinium-enriched Super K would be sensitive to very late black hole formation following a supernova explosion anywhere within our galaxy, since the distinctive coincident inverse beta signals from the cooling phase could be distinguished from the usual singles backgrounds. An abrupt cutoff of these coincident signals occurring even many minutes after the main burst would be the conclusive signature of a singularity being born. Direct observation of such an event would clearly be of great value, especially when correlated with electromagnetic signals from X-ray or gamma-ray observatories Reactor Antineutrino Signal The dominant antineutrino flux at the Kamioka site comes, not surprisingly, from Japan s many nuclear power reactors, a fact most successfully exploited by the nearby KamLAND experiment. Adding gadolinium to Super K would immediately turn our detector into a kind of Super KamLAND, with at least twenty-three times KamLAND s fiducial mass. As a result, after introducing a 0.2% solution of Gd 2 (SO 4 ) 3 into Super Kamiokande, reactor antineutrinos would immediately become our largest single source of neutrino events, similar in rate to that of SK s solar and atmospheric neutrinos. We would collect enough reactor ν e data about 15 events a day (after oscillation) with a 3.0 MeV threshold for coincident events to reproduce KamLAND s first published results [7] in just four days of operation. Although Super K with gadolinium will not be able to extract spectral information with an energy resolution as fine as that of scintillator detectors, the extremely high statistics involved will provide the best measurement to date of spectral distortions in the reactor antineutrino spectrum, and hence will allow sig-

4 M. R. Vagins / Nuclear Physics B Proceedings Supplement 00 (2011) nificantly tighter constraints to be placed on the solar neutrino oscillation parameters than any other method which could conceivably become operational within the next ten years, and possibly far beyond [8] Signal and Background Summary Figure 1 shows the expected spectrum of neutrontagged positrons signal and background in a Gdenriched Super K. The width of the band labeled DSNB reflects the remaining allowed range of theoretical flux predictions for the relic signal. dn/de e [(22.5 kton) yr MeV] Reactor! e Supernova! e (DSNB) GADZOOKS! Atmospheric! µ! e Measured E e (= T e + m e ) [MeV] Figure 1: Expected positron spectrum tagged by neutron captures in a Gd-enriched Super Kamiokande. One year of data is shown, with SK s energy resolution and all known backgrounds taken into account. Note the huge reactor antineutrino signal, and the clear window for observing the relic supernova neutrinos between the reactor events and the tail end of the neutron-liberating atmospheric neutrino events. In summary, we expect to see some 5,000 coincident reactor antineutrino events in a gadolinium-enriched Super K each year, along with about five coincident supernova relic neutrinos events. Remaining coincident backgrounds from atmospheric neutrinos which contribute to the DSNB flux will be small, less than one a year, and their number decreases with falling energy while the flux of supernova neutrinos rises. The remaining spallation background will lie under the huge reactor flux and will therefore be negligible. In addition to these new signals and the exciting prospects for detailed and unique observations in the happy event of a galactic supernova, it is highly likely that adding Gd 2 (SO 4 ) 3 to Super Kamiokande will make possible a wide variety of other observations and improvements to existing analyses, to wit: 1. Background reduction (up to 80%) for proton decay candidates in particular there should be no neutron captures seen after actual decay of unbound protons, while backgrounds to this search produced by GeV-scale atmospheric neutrino interactions will often have ejected neutrons in the final state. 2. The limit on wrong-sign neutrino production in the Sun could be improved. 3. Elastic neutrino-neutron scattering, which will eject neutrons from oxygen nuclei in time with the beam spill, should provide a brand-new total flux normalization for the T2K beam as it arrives at SK, since this neutral current process is unaffected by the flavor of the incident neutrinos. 4. It is possible that matter-enhanced and anti-matterenhanced atmospheric neutrino samples could be collected and their oscillatory behavior compared as a test of CPT violation. 4. Gadolinium R&D Making It Look Easy Is Hard Work 4.1. Selective Water Filtration Since maintaining the excellent light transmission of a water Cherenkov detector is a crucial requirement, the insertion of any chemical compound is a challenging task. In current detectors such as Super Kamiokande, the long mean free path of light ( 100 meters) is maintained by constant recirculation of the water through a water purification system. The existing SK purification system would eliminate the gadolinium as well as the contaminants that are currently removed to maintain optical clarity. To solve this problem, we are trying to do something which has not been done before: create a molecular Band-pass Filter, a system capable of selectively filtering the water to retain the Gd while removing the impurities. To this end, a scaled-down version of the SK water filtration system has been built under my direction at the University of California, Irvine [UCI], where I hold a joint appointment. The essential idea is as follows: there are a variety of commercially produced, membrane-based filters on the market. Rated by the size of pores in the membrane, they reject contaminants larger than these holes, while passing those which are smaller into the product water stream. By using a suitable sequence of filters, and by introducing nanofiltration, a new membrane intermediate in pore size between reverse osmosis (which rejects all gadolinium and everything larger) and ultrafiltration (which passes

5 M. R. Vagins / Nuclear Physics B Proceedings Supplement 00 (2011) all gadolinium and everything smaller), we expect that a system can be assembled which selectively extracts Gd 2 (SO 4 ) 3 from the water stream and returns it to the tank, while allowing all other impurities to be removed via the usual combination of reverse osmosis and deionization. In Figure 2, we schematically show the operational concept of the filtration. The system has already been demonstrated to work on a small scale; chemical analysis shows that a particular two-stage nanofilter separates all Gd and SO 4 ions from the main water stream and allows de-ionizing of that main water stream while maintaining the transparency of the water. It is now necessary to optimize the filters pore sizes to remove the largest amount of impurities while keeping the high separation efficiency for the Gd and SO 4 ions &3%9(:$&4$9(;$&59"<=">:?&6@$:=">A:&7$:?#)& &'()*$+,-./)*(0$ 1 2 $34)$&'()*$ FGH;$I-:6$E'4$E>-79-.:J$!"#$%$!"#$%&'(#)&*+, -./ 0 1 2& ;<0+3$ ;<0+3$ 1,*%'7-8)9$ &-:6/$;./).(-/4$ 3)5/6$ =6-($-($'.$>5:*'9)$,?$'$@A%)/)*$7,.:$$ 9)B-C)$(>CC)((?>774$ >()9$?,*$;DE$ Figure 3: IDEAL, a much-improved version of a similar device which was first used by the Irvine Michigan Brookhaven [IMB] proton decay and neutrino experiment during the 1980 s. silicon photodiodes [PD] measure the intensity before and after the light is injected, thereby allowing fluctuations in input intensity to be properly normalized. Since the variation in received light intensity can quite small, especially in the case of high-quality water, much effort has gone into understanding and reducing systematic uncertainties. The device is now performing very well, and is capable of measuring extremely pure water with (wavelength-dependent) attenuation lengths of well over 100 meters. This will allow us to fine-tune the band-pass filtration system for maximum effect. Figure 2: Selective Water Filtration Conceptual Design 4.2. Water Transparency Measurement In order to characterize the attenuation length of ultrapure water in the presence of various Gd solutes, we have developed an extremely sensitive light attenuation measuring device which we call IDEAL (Irvine Device Evaluating Attenuation Length). A schematic of IDEAL is shown in Figure 3. IDEAL extracts the absolute light attenuation length of water (with and without dissolved gadolinium) from an exponential fit to the light intensity of a pulsed laser beam as a function of the height of the column of water inside a 6.5 meter long vertical pipe. Six semiconductor lasers (whose wavelengths are 375 nm, 405 nm, 445 nm, 473 nm, 532 nm, and 595 nm) and a nitrogen gas laser (337 nm) are mounted at the top of the pipe. A reflective neutral density filter splits the beam and steers it into the pipe. Two integrating spheres [IS] read out by 4.3. EGADS Although a small, sealed, gadolinium-loaded calibration device has already been deployed in Super Kamiokande to verify the detector s predicted response to Gd neutron capture gammas [9], before Gd can be introduced into SK itself we must first demonstrate that the selective water recirculation technique works and that light transmission will only be marginally reduced by the presence of dissolved Gd 2 (SO 4 ) 3. While the filtration and recirculation techniques are being finalized at UCI, a new experimental chamber has been excavated in the Kamioka mine. There, a dedicated, large-scale gadolinium test facility and water Cherenkov detector (essentially a 200 ton scale model of Super K) is being built as depicted in Figure 4. Known as EGADS (Evaluating Gadolinium s Action on Detector Systems), it will be used to make absolutely sure that the introduction of Gd will not interact with the detector materials and to certify the viability of the Gd-loading technique on a large scale, more closely matched to the final Super K requirements.

6 M. R. Vagins / Nuclear Physics B Proceedings Supplement 00 (2011) Comparative studies both with and without dissolved gadolinium in the 200 ton tank will take place during 2011 and If all goes well, we should be ready to introduce gadolinium into Super Kamiokande sometime within the next few years. Gadzooks, indeed! References Figure 4: EGADS, the new gadolinium test facility in the Kamioka mine. Shown (from left to right) are the main selective water system, pre-treatment vessel (with stirrer), 200 ton tank containing cm PMT s, and UDEAL. [1] Malek, M., et al. 2003, Phys. Rev. Lett. 90, [2] Beacom, J.F., & Vagins, M.R. 2004, Phys. Rev. Lett. 93, [3] Hosaka, J., et al. 2006, Phys. Rev. D 73, [4] Strigari, L.E., et al. 2004, JCAP 0403, 007 [5] Tomas, R., et al. 2003, Phys. Rev. D 68, [6] Odrzywolek, A., Misiaszek,M., & Kutschera, M. 2004, Astropart. Phys. 21, 303 [7] Eguchi, K., et al. 2003, Phys. Rev. Lett. 90, [8] Choubey, S., & Petcov, S.T. 2004, Phys. Lett. B594, 333 [9] Watanabe, H., et al. 2009, Astropart. Phys. 31, 320 Funding for the new facility has been obtained in Japan to the tune of 390,000,000 yen (about $4,300,000 at the current exchange rate); construction began in September of Within nine months we had gone from solid rock to an excavated hall with a total volume of about 2.5 kilotons ready for physics occupancy, complete with a 200 ton stainless steel tank. UDEAL, an Underground, improved version of IDEAL has now been completed next to the tank. The various water systems are currently being assembled and tested, and in the summer of 2011 a total of cm photomultiplier tubes, the same design as those being used in Super K, will be installed in the tank. The flow chart of the EGADS selective filtration system can be seen in Figure 5; it has certainly gotten a bit more complicated than the conceptual design shown in Figure 2, but this is what it takes to make things work in the real world. To Drain EGADS Selective Filtration System Ultrafilter #1 Chiller A product of seven years of R&D at UC Irvine 200 ton EGADS Test Tank (>0.6 MPa, >4 ton/hr) 0.2 µm 2 nd Stage Filter UV 5 µm 1 st Stage Filter Intake (>4 ton/hr) Nanofilter #1 (>0.6 MPa, >3 ton/hr) Nanofilter #2 Concentrated Gd NF Reject Lines DI #1 (>0.9 MPa, >2 ton/hr) 0.2 µm Filter TOC Recycles RO Reject Lines RO #1 0.5 ton Buffer Tank (>0.9 MPa, >1.5 ton/hr) DI #2 0.2 µm Filter Conveying (~0.35 MPa, >4 ton/hr) 0.5 ton Collec6on Buffer Tank RO #2 To Drain Ultrafilter #2 Membrane Degas RO Permeate Lines Figure 5: The selective filtration water system for EGADS. It will be capable of processing 100 tons of gadolinium-loaded water each day.