GADZOOKS! How to Detect SN
Neutrinos in SK... Next Year!

Mark Vagins
University of California, Irvine

ICRR - Kashiwa
June 29, 2005

A long time ago, in a (neighbor) galaxy far,
far away…

A long time ago, in a (neighbor) galaxy far,
far away…

oSanduleak -69 202 was gone, but not forgotten.

In fact, based on the handful of supernova
neutrinos which were detected that day,approximately one theory paper has been
published every ten days…

…for the last eighteen years!

Now, I shouldn’t need to tell this crowd (at ICRRheadquarters!) why neutrinos, particularly supernova
neutrinos, are interesting.

However, one little-noted fact is that at 170,000 light years, SN1987A
remains the most distant source of neutrinos ever detected.
It’s still the only recorded neutrino source more distant(by an easily-remembered factor of 1010) than our own sun.

４１．４ｍAn array of impressive experiments
stand ready to observe the neutrinosof the next galactic supernova.

４０ｍ

More SN neutrino-capable detectors, somequite fantastic, are planned or being built.

On the other hand, we’ve recently
observed a big anniversary!

As of last October 9th, it has been exactly 400
years since a supernova was last definitely
observed within our own galaxy.

So, what’s a supernova neutrinoloving scientist to do?

Well, galactic supernovas may be somewhat rare
on a human timescale, but supernovas are not.

On average, there is one supernova
explosion somewhere in our universe
every second!

These constitute the diffuse supernova neutrino
background [DSNB], also known as the “relic”
supernova neutrinos.

After traveling an average distance of
six billion light years, about 100,000 of thesegenuine supernova neutrinos pass through ourbodies every second.

As of 2002, here’s what the various
predictions of this flux looked like:In 2003, Super-Kamiokande published the world’s best limits on this sofar unseen flux [M.Malek et al., Phys. Rev. Lett. 90 061101 (2003)].

Unfortunately, the search was strongly limited bybackgrounds, and no clear event excess was seen.

Here’s how the SK result
constrained the existing models:

So, experimental DSNB limits are approachingtheoretical predictions. Clearly, reducing theremaining backgrounds and going lower in energy
would extremely valuable. But how?
Well, all of the events in the present SK
analysis are singles in time and space.

And this rate is actually very low… justthree events per cubic meter per year.

With this in mind, John Beacom and I undertook totackle the DSNB problem once and for all, with theambitious goal of detecting a clear, positive signal
within the next four years.

This partnership of theory and experiment has proven quite productive.

“Wouldn’t it be great,” we thought, “if there was a
way to tag every DSNB event in Super-K?”

Well, the reaction we are looking for ise + pe+ + n

What if we could reliably identify theneutron and look for coincident signals?

But we’re going to have to compete withhydrogen in capturing the neutrons!

Also, plain old NaCl simply won’t work…
We’d need to add 3 kilotons of salt to SK just to
get 50% of the neutrons to capture on the chlorine!

Seawater

So, we eventually turned to the best neutron
capture nucleus known – gadolinium.

• GdCl3 , unlike metallic Gd, is highly water soluble• Neutron capture on Gd emits a 8.0 MeV cascade• 100 tons of GdCl3 in SK (0.2% by mass) would yield>90% neutron captures on Gd
Man, that’s
one tasty• Plus, it’s not even toxic!
lanthanide!

But, um, didn’t you just say 100 tons?
What’s that going to cost?

In 1984: $4000/kg
In 1993: $485/kg
In 1999: $115/kg
In 2005: $4/kg$400,000,000$48,500,000$11,500,000$400,000

So, perhaps Super-K can be turned into a great bigantineutrino detector… it would then steadilycollect a handful of DSNB events every year withgreatly reduced backgrounds and threshold.
Also, imagine a next generation, megaton-scalewater Cherenkov detector collecting 100+ per year!

This is the only neutron detection
technique which is extensible to such
scales, and at minimal expense, too:
~1% of the detector construction costs

If we can do relics, we can do agreat job with a galactic supernova:• The copious inverse betas get individuallytagged and can be subtracted away from• the directional elastic scatter events,
doubling SK’s SN pointing accuracy.
• The 16O NC events no longer sit on a large
background and are hence individually
identifiable, as are• the backwards-peaked 16O CC events.

Here’s our proposed name for this water Cherenkov upgrade:

G adolinium
A ntineutrino
Detector
Z ealously
O utperforming
O ld
K amiokande,
S uper !

Oh, and as long as we’re collecting

e’s…

KamLAND’s
first 22months ofdata
GADZOOKS!

GADZOOKS! will collect this much reactor neutrino data in two weeks.
Hyper-K with GdCl3 will collect six KamLAND years of data in one day!

Here’s what the coincident signals in Super-K withGdCl3 will look like (energy resolution is applied):As I mentioned earlier, for the last three years John Beacom and I
have been working on this issue with an eye towards enhancing
the (soon to be) existing Super-Kamiokande-III detector.
We finally got our first GADZOOKS! (Gadolinium Antineutrino
Detector Zealously Outperforming Old Kamiokande, Super!)paper written up as hep-ph/0309300 and sent it off to PhysicalReview Letters.

After a long wait due largely to oneof the world’s slowest referees,

our paper was finallypublished in Physical Review Letters asPhys. Rev. Lett., 93:171101, 2004

While waiting for PRL, we have been quite pleased by the
many excellent phenomenological papers which have beenin some way inspired by (and cite) our preprint.
Here’s a brief selection ordered by topic:• Relic supernova neutrinos –Strigari, Kaplinghat, Steigman, and Walker, JCAP 0403:007, 2004
S. Ando, Astrophys.J 607, 20-31, 2004
Fogli, Lisi, Mirizzi, and Montanino, hep-ph/0401227• Galactic supernova neutrinos –Tomas, Semikoz, Raffelt, Kachelriess, and Dighe, Phys.Rev. D68:093013, 2003
Odrzywodek, Misiaszek, and Kutschera, Astropart.Phys. 21:303-313, 2004

• Reactor antineutrinos Choubey and Petcov, hep-ph/0404103

Relics, Now With More Flavor:

Inspired by GADZOOKS! and the current SK
relic limit, Strigari et al. have updated theircalculations using the latest Sloan Digital Sky
Survey data and solar oscillation results…
They conclude that the present SK limit is justabove their best fit prediction of the DSNB rate!

Odrzywodek et al. note that late-stage Si burning invery large, very close stars could provide
a two day early warning of a core collapse supernova
if neutron detection is possible.

In SK with GdCl3 this would mean an increase inthe low energy singles rate… a factor of 10
increase in the case of Betelgeuse.
[SK with Gd is the only detector which can do this]Choubey and Petcov consider the reactor signal of GADZOOKS!

From hep-ph/0404103:•••••“SK-Gd can rule out the ‘wrong’ [LMA] solution with about 0.4 to 0.5
years of data. This is in sharp contrast to what we have obtained for
KamLAND, where it is hard to resolve the low-LMA – high-LMA
ambiguity even with 2.6 kTy of data once the Shika-2 reactor isswitched on.”“The upper limit on m221 is determined solely by the SK-Gd data.”“The lower limit on sin2 is determined by the SK-Gd data.”“The results of our analysis show that the SK-Gd experiment has aremarkable potential in reducing the uncertainties in the values ofm221 and sin2 .”“We find that the SK-Gd experiment could provide one of the mostprecise (if not the most precise) determinations of the solar neutrino
oscillation parameters m221 and sin2 .”Here are their main results…

Choubey and Petcov consider the reactor signal of GADZOOKS!

Therefore, we could very well be just a few years
away from the world’s first true precision
measurement (<1 % uncertainty) of afundamental neutrino parameter!

So, our GADZOOKS! proposal has definitely
been getting a lot of attention recently:
At NNN05, before I had evengiven my talk, John Ellis suddenly
stood up and demanded of
the SK people in attendance:

Why haven’t
you guys put
gadolinium inSuper-K yet?

As I told him, studies
are under way…

You see, Beacom and I never wanted to merely propose
a new technique – we wanted to make it work!

Now, suggesting a major modification of one of theworld’s leading neutrino detectors may not be theeasiest route…

…and so to avoid wiping out, some careful
hardware studies would be needed.

••••

What does GdCl3 do the Super-K tank materials?
Will the resulting water transparency be acceptable?
Any strange Gd chemistry we need to know about?
How will we filter the SK water but retain GdCl3?

As a matter of fact, I very rapidly made two
discoveries regarding GdCl3 while carrying asample from Los Angeles to Tokyo:

1) GdCl3 is quite opaque to X-rays2) Airport personnel get very upset when they finda kilogram of white powder in your luggage

While hep-ph/0309300 was being assembled, I wasawarded a U.S. DoE Advanced Detector Research Program
grant to support the study of key gadolinium R&D issues.

Work immediately got under way at UC Irvine building asmall version of the SK water filtration system, while at
Louisiana State University a materials aging study was
put together under the supervision of Bob Svoboda.

Example of Soak Sample

Tank Weld Joint:
Room temperaturesoak in 2% GdCl3
We will inspect surface
every three months via
SEM, optical, and XRD

Now at 20 years ofequivalent exposure!

With the help of a crew and equipment from theUniversity of Hawaii, very preliminary measurementswhich we made in Japan last March indicate anabsolute lower bound on the attenuation of 0.2% GdCl3(plus contaminants) which is> 58 +/- 1 meters.

These bench tests have been very encouraging.
In a single pass through our UCI water filtering system
we can remove ~99.99% of the Gd and returnit to our 500 liter holding tank.
But can this be scaled
up to kilotons of
Gd-enrichedwater?
With the help of this big
detector at KEK we areabout to find out!

This summer I’ll employ some excellent large-scale
hardware to find out if the GdCl3 technique will work:

K2K’s 1 kiloton tank will be used for “real world” studies of•••

Gd Water Filtering – UCI built and maintains this water system
Gd Light Attenuation – using real 20” PMTs
Gd Materials Effects – many similar detector elements as in Super-K

We are nearly ready for this effort…

Here’s our design for therebuilt 1 kton water system:~0.00002% GdCl3

0.2% GdCl3

0.6% GdCl3

For SK, “Gd-unsafe” components
like vacuum degas would go here.

The entire one kiloton volume is recirculated every two days.

A Busy Summer
In August/September we will rebuild theone kiloton’s water system at KEK.
We expect to introduce 2,000 kg of GdCl3 into
the tank sometime in September!

A GdCl3 Timeline:
2003 2004 2005 2006 2007 2008

Bench Tests @UCI & LSU
1 kton trial run @ KEK

GADZOOKS! @ Super-K

Soon we will betaking ourGADZOOKS!
studies toTsukuba…

Gd

Let’s go put thein gama oil!

That’s it for now…
time for me to get back to seeing if thisbeautiful dream can be made a reality!