Double Beta Decay With 20-ton Metal Loaded

ScintillatorsA Detector for DUSEL?

Frank CalapricePrinceton University

Aldo IanniLNGS

The Borexino Detector

• 14C ~ 3 x 10-18

• U/Th <10-17

• 85Kr ~ 0.35 cpd/ton• Cleanliness level 25

MIL-STD 1246C

General Idea (Similar to SNO+)

• Use radiopurity MEASURED in the Borexino detector

• Use preliminary data on Nd-loaded LS based on PC

Goal:1. Search for double beta decay with a 20ton-

scale high purity LS detector2. SNO+ proposal is for 1000 ton LS.

Basic working assumptions:

1. T1/2 = 7 x 1018 yr

2. 50 meV neutrino mass (inverted hierarchy)• T

1/2 = 1.2 x 1025 yr

3. Photon yield = 12000 / MeV (PC liquid scintillator)4. QE = 0.35 (new high QE PMTs)5. Coverage = 0.856. Low radioactivity PMTs: ~ 30 mBq/PMT

The Detector: basic idea

FM

Backgrounds:

• PMTs + Steel Sphere ~ 40 Bq• 208Tl from nylon (<1ppt in Th) :

– 0.861 MeV + 2.614 MeV ’s 12% BR– need to define FM– Radiopurity of nylon can be measured in CTF at ppt

level or below.

• Intrinsic Th in Scintillator:– Borexino 10-17 g/g gives ~ 1 c/yr around peak

• Th coming with Nd ??? – Required ~ 10-15 gTh/gNd

1-kton SNO+ vs 20-ton scale detector:

1kton 20ton

Nd concentration 0.1%* 1%*

Nd mass nat. [kg] 1000 195

150Nd [kg] 56 12

Light yield [pe/MeV]

~400** ~700**

* Light absorption by Nd limits concentration**For comparison both detectors are assumed to have 50% PMT coverage.

Nd concentration 10 g/l 20 g/l 32 g/l

Relative light yield to pure LS

0.7 0.6 0.5

150Nd mass

[kg]

10.9 21.9 35.0

Resolution

FWHM [%]

2.54 2.77 3.05

Rate 2 [Bq] 0.14 0.28 0.45

Rate 0 [c/yr] 2.5 5.1 8.1

S / B in

[Q -1,Q+1]

1.4 2.5 4.0

Natural abundance, 50 meV neutrino, 20-ton FM:

i) 1 c/yr from nylon, ii) 1 c/yr from internal Th, iii) << 1 c/yr externalii) Muon-induced activities above 3 MeV are short-lived and vetoed

Nd concentration 10 g/l 20 g/l 32 g/l

150Nd mass

[kg]

98 195 313

Rate 0 [c/yr] 1 2 3

S / B in

[Q -1,Q+1]

0.4 0.5 1.2*

50% 150Nd enrichment, 10 meV neutrino, 20-ton:

•In 5 years the uncertainty in the measurement is 35%

Breakdown of costs:

Item Cost [$M]*

Nd 0.2

WT + SSS 2.0

PMTs 5.0

Nylon Vessel 1.0

Electronics 5.0

LS (solvent) 0.1

solute 0.1

Enrichment 1.0

Total ~15•Add $3M for new purification system if LNGS system is not used.

Roadmap:• Measure radiopurity of nylon with present CTF• Preliminary tests of light propagation and energy

resolution in small test chamber (~5 m3).• Rebuild CTF with Stainless Steel Sphere

– Study Nd-loaded LS• Light propagation• Radiopurity• Energy resolution

• Search for double beta decay at 50 meV sensitivity with CTF system.

• Build larger (~50 ton LS?) system with optimum resolution/background for 10 meV at DUSEL?