Research Report

Its R&D Mission Completed, EGADS Evolves
Mark R. Vagins
Kavli IPMU Principal Investigator

Fifteen years ago, theorist John Beacom and I
first proposed introducing 100 tons of a watersoluble gadolinium [Gd] compound, gadolinium
chloride, GdCl3, or the less reactive though alsoless soluble gadolinium sulfate, Gd2(SO4)3, into theSuper-Kamiokande (Super-K , SK) detector. CalledGADZOOKS! (Gadolinium Antineutrino Detector
Zealously Outperforming Old Kamiokande, Super!),the basics of this load-SK-with-Gd idea have beendetailed in a Physical Review Letters article [1], as
well as in this very publication [2].
In a nutshell: neutron capture on gadolinium

produces an energetic gamma cascade, so thene + p → e+ + n,
inverse beta decay (IBD) reaction, −in such a Gd-enriched Super-K will yield coincident
positron and neutron capture signals. Looking forthese coincidences (or the lack thereof) will allow alarge reduction in backgrounds and greatly enhancethe detectorʼs response to both supernova neutrinos(galactic and diffuse) and reactor antineutrinos. Itwill also improve the detectorʼs sensitivity to proton
decays by cutting backgrounds; genuine nucleon
decay events should usually not have free neutrons
in the final state. In addition, solar neutrino studies

Figure 1: EGADS, the large-scale gadolinium test facility in the Kamioka mine.

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Kavli IPMU News No. 43 September 2018

Figure 2: Primary EGADS results.

will benefit from reduced spallation backgrounds.
While the physics potential of the GADZOOKS!
concept was readily apparent, there were technicalissues to address if it were to become a reality.
Naturally, the compatibility of gadolinium withdetector materials needed to be demonstrated, but
the primary challenge was that in detectors such as
Super-Kamiokande the long mean free path of light(~100 meters) is maintained by constant recirculationof the water through a water purification system.
The existing SK purification system would rapidly
eliminate any added gadolinium along with all of
the unwanted contaminants that are removed tomaintain optical clarity.
To solve this crucial problem, I developed afundamentally new type of filtration system: myMolecular Band-pass Filter was designed to
selectively extract Gd2(SO4)3 from the water andreturn it to the tank, while simultaneously allowing
all other impurities to be removed.
Starting in September of 2009 a newexperimental chamber was excavated in theKamioka mine, located close to Super-Kamiokande.
There, a dedicated, large-scale gadolinium test
facility and water Cherenkov detector (essentiallya 200-ton scale model of Super-K , along with afew prototype Hyper-K phototubes) was built [3] asdepicted in Figure 1. Known as EGADS (EvaluatingGadoliniumʼs Action on Detector Systems), it
was designed to make absolutely sure that theintroduction of Gd would not interact adversely withthe detector materials and to certify the viability of
the Gd-loading technique on a large scale, closely
matched to the final Super-K requirements. Thosestudies have now been completed.
Figure 2 shows the main EGADS results: watertransparency – characterized as the light remaining
after 15 meters of travel through the water, theaverage distance traversed by light in Super-K – andthe amount of gadolinium sulfate in the 200-tontank are plotted as a function of time. They are bothmeasured at three points in the tank: the bottom,11Research
Reportthe middle, and the top. Transparency (percentageof light remaining, left hand scale) is indicated by
the upper set of three lines, and concentration(parts per million, right hand scale) by the lower setof lines. The transparency range of the ultrapure
water in Super-K, the most transparent large volumeof water ever made, is shown as a blue band near
the top of the figure.
Once full Gd-loading was achieved in April of2015, the key EGADS findings were:1) Whenever the Gd-capable water systems wereallowed to operate normally (i.e., excluding thegray hashed areas), the transparency of the Gdloaded water became comparable to that of SKʼsultrapure water.
and2) During 650 passes of the entire water volume
through the Gd-capable water filtration systems
there were no detectable losses of gadolinium
sulfate whatsoever.
With these results in hand it was time to open
the tank and look inside. Figure 3 shows us getting
ready to open the big square hatch on the top of

the 200-ton EGADS tank. Figure 4 is a view throughthis hatch at the Gd-loaded water inside, and Figure
5 is an image taken in the bottom wall region
looking up the side of the drained tank. Everything
was shiny and beautiful: there were no changes
after years of exposure to the 0. 2% Gd2(SO4)3.
The EGADS findings were sufficient for us toreceive official approval from both the SuperKamiokande and T2K Collaborations to move
forward with plans to load gadolinium into Super-K .
Meanwhile, with its R&D role finished, EGADS
has had its data acquisition hardware and online
computing power significantly upgraded and now
lives on as the worldʼs most advanced water-based
supernova neutrino detector. Refilled and building
upon the confidence provided by Gd neutron
tagging of IBD events, its ambitious goal is to makea fully automated, standalone announcement ofa galactic supernova explosion within one secondof the first neutrinoʼs arrival in the detector [4].
This reborn EGADS is now associated with aJapanese network of optical, X-ray, gamma-ray,infrared, and gravitational wave observatories,
for true multimessenger astronomy. Even theEGADS acronym has been repurposed: Employing
Gadolinium to Autonomously Detect Supernovas.

Figure 3: Getting ready to open the EGADS tank.

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Kavli IPMU News No. 43 September 2018

Figure 4: Looking down into the full EGADS tankfollowing two years of exposure to Gd.

Figure 5: Looking up the side wall of the empty
EGADS tank after 2.5 years of soaking in Gd.

References[1] J. F. Beacom and M. R . Vagins, GADZOOKS! Antineutrino spectroscopy with large water Cherenkov
detectors, Phys. Rev. Lett. 93, 171101 (2004) [hepph/0309300].
[2] M . R . Vagins, Kavli IPMUʼs Neutrino Forecast : MostlySunny, with a Good Chance of Supernovas, Kavli
IPMU News, No. 19, September 2012, pages 4-9.

[3] M . R . Vagins, EGADS Gets Going, IPMU News, No.
15, September 2011, page 15.
[4] S. M. Adams, C. S. Kochanek, J. F. Beacom, M. R . Vagins,
K . Z . Stanek, Observing the Next Galactic Supernova,Astrophys. J. 778 (2013) 164 [arXiv:1306.0559 [astroph.HE]].

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