# Presentation: 271264

## Systematic Errors in Reactor Neutrino ExperimentsThe challenge of measuring 13 

**Systematic Errors in Reactor Neutrino Experiments**The challenge of measuring 13** **

- Karsten M. Heeger
*Lawrence Berkeley National Laboratory*

## Outline

- Past and  Present Experiments			**absolute measurements**
- 	KamLAND				1 fixed detector
- 	CHOOZ				1 fixed detector
- Overview of Backgrounds and Systematics
- 	Detector Systematics
- 	Backgrounds
- Proposals for Next Generation Experiments	**relative measurements**
- 	Kashiwazaki				3 fixed detectors
- 	CHOOZ II? 				2 fixed detectors
- 	Diablo Canyon, Wolf Creek		2+  detectors with variable 						baseline
- 	rate vs shape measurements of 13

## KamLAND - Kamioka Liquid Scintillator Antineutrino Detector

- Uses reactor neutrinos to study  oscillation with a baseline of L ~ 140-210 km

- time correlation	   200s
- space correlation	< 1m

## Systematics in an Absolute Measurement

- kinetic energy spectrum  	    x  	detector response function

- • thermal power
- • fuel composition and time dependence
-  number of fissions ()

- • target volume/mass
- • reaction cross-section
- • candidate event selection
- 	- detection efficiency
- • energy response and threshold
- • time variation of detector response
- • backgrounds
- 	- physics events
- 	- radioactivity
- 	- cosmic rays
- 		correlated
- 		accidental

## KamLAND - Systematic Uncertainties

- 				 %
- Total liquid scintillator mass	2.1
- Fiducial mass ratio		4.1
- Energy threshold		2.1
- Tagging efficiency		2.1
- Live time			0.07
- Reactor power			2.0
- Fuel composition		1.0
- e spectra			2.5
- cross section			0.2
- Total uncertainty		6.4 %

**E > 2.6 MeV**

- FV

*given by reactor company, difficult to improve on*

*theoretical, model-dependent*

- • volume calibration
- • energy calibration or
- analysis w/out threshold
- • detection efficiency

-      			162 ton•yr,
- 			*E**prompt** > 2.6** **MeV*
**Expected**		**86.8 ± 5.6 events**
**Background**		1 ± 1 events
-     	*accidental**	**0.0086 ± 0.0005*
-    	*9**Li/**8**He **	**	**0.94 ± 0.85*
*     **	**fast neutron**	**< 0.5*
**     **

- Event Rates at KamLAND

- Note:  error from background << total systematic error

## Evidence for  Reactor e Disappearance

## Chooz

## Chooz

## Chooz Systematics

*theor.*

*Absolute measurements are difficult!*

- Partial cancellation of systematic errors in relative measurement

## Reactor Neutrino Measurement of 13 - Basic Idea

- 13=?

- Reactor Neutrino Measurement of 13 - Basic Idea

- atmospheric frequency dominant
- last term negligible for 	             and

- Pee, (4 MeV)

- e flux* *

## Concept of a Reactor Neutrino Measurement of 13 

- Concept of a Reactor Neutrino Measurement of 13

- • No degeneracies
- • No matter effects
- • Practically no correlations
- E = Ee + mn-mp
- Eprompt = Ekin + 2me

## Reactor Neutrino Measurement of 13 

**Present Reactor Experiments**

**Absolute Flux**
**and Spectrum**

## Systematics in a Relative Measurement

- kinetic energy spectrum  	    x  	detector response function

- • thermal power  number of fissions ()
- • fuel composition and time dependence

- • target volume/mass
- • reaction cross-section
- • candidate event selection
- 	- detection efficiency
- • energy response and threshold
- • time variation of detector response
- • backgrounds
- 	- physics events
- 	- radioactivity
- 	- cosmic rays
- 		correlated
- 		accidental

- Detectors are never “identical”. Only partial cancellation of systematic errors.

## Event Identification: Positron Detection

**E****th ****> E****min****:** systematics due to threshold
**E****th ****< E****min****:**  no systematics on energy threshold
-  start of the spectrum may provide calibration point between near and far detector
-  will allow measurement of background at low energy (<1 MeV)
- 	  lower threshold  requires  lower backgrounds(accidental + correlated)

- Fiducial Volume Boundary** ****	****	****	**(1% in CHOOZ based on MC )
**non-scintillating buffer:** 	’s can escape without being detected
**scintillating buffer:****	**full e+ energy detected within the target, 						but e+ efficiency non zero outside the target volume

- Fiducial Volume** ****	****	****	****	****	**** **(0.8% in CHOOZ )
- 	FV cut requires position reconstruction (cannot be controlled at the O(%) level), 	or accurate knowledge of target mass.

- effects will largely cancel in ratio of 2 detectors

## Event Identification: Neutron Detection

- Energy Window**	****	**
**	**Gd (8 MeV ’s)  vs 	H2 (2.2 MeV),
- 	loaded vs unloaded liquid scintillator
- 	ratio of Gd/H2 capture (~80% on Gd)
- 	error will depend on detector geometry

- Fiducial Volume Boundary** ****	****	****	**
**non-scintillating buffer**: 	’s can escape without being detected
**scintillating buffer:****	**full e+ energy detected within the target, 						but e+ efficiency non zero outside the target volume

- effects will largely cancel in ratio of 2 detectors

## Event Identification: (e+-n) tag

- Event Identification: (e+-n) tag

- Distance Cut** ****	****(***d*(e+-n) < 100 cm)			(0.1% in Chooz)
- distance cut requires position reconstruction, cannot be controlled at the O(%) level
- not required if accidental background very low
- Time Cut	(neutron capture time) 			 (0.4% in Chooz)
-  (non) exponential behavior of neutron time capture on Gd and H2
- 				 Gd may increase systematics
- no need for Gd in case of lower accidental backgrounds

- effects will largely cancel in ratio of 2 detectors

##  Backgrounds in a Reactor Neutrino Experiment

**Geophysical anti-********e****’s**
**Background from radioactivity**** **
-  rocks, detector material, water shielding, scintillator
**Background induced by cosmic rays**
-  radioactive nuclei produced in the detector
-  neutrons induced by muons in detector & rocks

- depends on depth

- Goal:  error from background  <<  total systematic error

## Backgrounds from Radioactivity

- Correlated
- 	U-chain: Radon daughters 214Bi-214Po decays (  237 s)   mimick  tag
- 	E=0.8 MeV (quenched)  >10 from Ed=2.2 MeV ( = 5% @ 1 MeV)
- Accidental
- 	**Liquid ****Scintillator****	****	**40K   	<< 10-5  	(ng/g) for KamLAND
- 					238U  	<< 10-5
- 					232Th 	<< 10-5
- 	**PMTs + Concentrators****	**** **40K  	80
- 					238U  	480
- 					232Th 	470
- 	**Acrylic ****	****	****	**** **40K	0.008
- 					238U	0.008
- 					232Th	0.05
- 	**Passive Shield (water)****	**40K	~1
- 					238U	~1
- 					232Th	~1
**	****Rock**			 40K	1500
- 					238U	1600
- 					232Th	3800

- Backgrounds from Radioactivity

## Muon Flux Underground

- Diablo Canyon   plant boundary* *

- depth 	muon rate
- (mwe)	for 100-t detector
- 	*mountain         flat*
- 300	31.1	    17.3
- 500	9.1	     4.8
- 800	2.6	     1.3

## Muon-Induced Production of Radioactive Isotopes in LS

|  | Isotope | T1/2 | Emax (MeV) | Type |
| --- | --- | --- | --- | --- |
| - | 12B | 0.02 s | 13.4 | Uncorrelated |
|  | 11Be | 13.80 s | 11.5 | Uncorrelated |
|  | 11Li | 0.09 s | 20.8 | Correlated |
|  | 9Li | 0.18 s | 13.6 | Correlated |
|  | 8Li | 0.84 s | 16.0 | Uncorrelated |
|  | 8He | 0.12 s | 10.6 | Correlated |
|  | 6He | 0.81 s | 3.5 | Uncorrelated |
| +, EC | 11C | 20.38 m | 0.96 | Uncorrelated |
|  | 10C | 19.30 s | 1.9 | Uncorrelated |
|  | 9C | 0.13 s | 16.0 | Uncorrelated |
|  | 8B | 0.77 s | 13.7 | Uncorrelated |
|  | 7Be | 53.3 d | 0.48 | Uncorrelated |

**uncorrelated:**
- single rate dominated by 11C

**correlated:**
- -n cascade, ~few 100ms. Only 8He, 9Li, 11Li (instable isotopes).

- rejection through muon tracking

## Muon-Induced Neutrons

- Muons in passive (water) shielding
- 	Muon preceding a neutron  (need to have less than 0.1% of this background)
- 		Muon track used for discrimination
- Muons in surrounding rocks
- 	From high energetic shower developing in the rocks (up to GeV) From (,n) reactions (few MeV)
- 		Use muon track in outer veto for estimate but no complete discrimination
- 		Passive and active shielding to be optimized

## Neutron Production in Rock

- A. da Silva
- PhD thesis, UCB 1996

- go deep (> 300 mwe) and build muon veto

*Proposals for Relative Measurements*

## Kashiwazaki: Proposal for Reactor 13 Experiment in Japan

**Kashiwazaki**	- 7 nuclear power stations, World’s most powerful reactors
-  		- requires construction of underground shaft for detectors

**near**

**near**

**far**

**Kashiwazaki-Kariwa**
**Nuclear Power Station**

## Kashiwazaki: Proposal for Reactor 13 Experiment in Japan

**near**

**near**

**far**

**70 m*** *

**70 m*** *

**200-300 m*** *

- 6 m shaft hole, 200-300 m depth* *

***Kashiwazaki:*** Proposal for Reactor 13 Experiment in Japan

- • Chooz-style detector
- • Gd concentration 1.5 x CHOOZ to increase neutron absorption efficiency
- • background rate < 2%:
-  major component from fast n

## Kashiwazaki: Systematics

- multiple neutrino sources
-  ~ 0.2%

- 					**CHOOZ****	****	****Kashiwazaki**
- Detector systematics 			1.7%  		1.1%
- Far/near ratio of detector systematics	N/A		0.5%-1%
- Far/near ratio of flux systematics		N/A		0.2%
**Total****	****	****	****	****	****	****	****< 1%**

- Kashiwazaki

- rate-based measurement

## CHOOZ II (?): Systematics

**CHOOZ II** (?): Systematics

| Systematics | Error | CHOOZ | CHOOZ II
(2 det, lower bkgd) | 
 |
| --- | --- | --- | --- | --- |
| Reactor Complex | cross section/fission | 1.9% | - |  |
|  | power | 0.7% | - |  |
|  | E/fission | 0.6% | - |  |
|  |  | 2.1% | << 1% |  |
| Detector  | scintillator density | 0.1% | - | +difficult with Gd |
|  | target volume | 0.3% | - (V) |  |
|  | % H | 1.2% | - | +difficult with Gd |
|  | Fiducial volume boundary | 1.0% | - | Scint. buffer |
|  |  | 2.5% | << 1% |  |
| Analysis Cuts

Ee+ < 8 Mev
6 < En (MeV) < 12
d(e+-geode) < 30cm
d(n-geode) < 30cm
d(e+-n) < 100 cm
2 < n delay < 100 s
n multiplicity = 1 |  e+ energy | 0.8% | No threshold:  0%  | scint. buffer |
|  |  e+ pos. cut / vessel (30cm) | 0.1% | - |  |
|  |  n capture | 1.0% | - | scint buffer |
|  |  n energy | 0.4% | - | Gd 8 MeV ’s  |
|  |  n pos. cut / vessel (30 cm) | 0.1% | - |  |
|  | (e+-n) distance | 0.3% |  No distance cut:  0%  |  |
|  | (e+-n) time delay | 0.4% | - | No Gd  ~0% |
|  |  n multiplicity | 0.5% | better |  |
|  |  | 1.5% | << 1% ? |  |

**rate-based measurement**

## Reactor Experiments with Variable Baseline

**Reactor Experiments with Variable Baseline**

**Option I: Shafts + Tunnel **

## Flux Systematics with Multiple Reactor Cores

- Neutrino flux at detector *I * from reactors *A* and *B*

- Indivual reactor flux contributions and systematics cancel *exactly* if
**Condition I:** 			1*/r**2** fall-off of reactor  flux the same for all detectors.*
**Condition 2:**  			*Survival probabilities are approximately the same*

- 				Relative Error Between Detector 1 and 2
- 				**rate ****	****shape**
- Relative flux error (1%)		< 0.6%	< 0.01%
- Reactor core separation (100 m)	< 0.14%	< 0.1%
- Finite detector length  (10 m)		< 0.2%	< 0.1%

-  Shape analysis largely insensitive to flux systematics.
-  Distortions are robust signature of oscillations.

-  Approximate flux cancellation possible at other locations

## A Next Generation Detector Concept(spherical or cylindrical)

- liquid scintillator

- buffer oil

- muon veto

- passive shield

**A Next Generation Detector Concept**(spherical or cylindrical**)**

- 5 m

- 1.6 m

**Movable Detector? **	Variable baseline to control systematics and 				demonstrate oscillation effect (if |13| > 0)

- acrylic vessel

**Tunnel with Multiple Detector Rooms and Movable Detectors**

- ~12 m

- • Modular, movable detectors
- • Volume scalable
- • Vfiducial ~ 50-100 t/detector

- 1-2 km

## Experimental Systematics

**Reactor Flux ****	****	**• near/far ratio, choice of detector location** **

- Best experiment to date: CHOOZ

**Target Volume &****	****	**• no fiducial volume cut
**Backgrounds ****	****	**• external active and passive shielding 						for correlated backgrounds

**Detector Efficiency****	**• built near and far detector of same design 					• calibrate *relative*  detector efficiency             					 *variable *baseline may be necessary

- Total    syst   ~ 1-1.5%

*Note: list not comprehensive*

## Past and Present Reactor Neutrino Experiments 

## A Future 2-Detector Experiment

- e.g. Diablo Canyon

## 13 Sensitivity Scaling with Backgrounds and Exp. Errors 

- Ref: Huber, Winter, Linder et al.

- Nominal setup:
- Lnear = 0.2 km
- Lfar = 1.7 km

- shape = 2%

- m232 = 2 x10-3 eV2

## Future Constraints on 13  

| Experiment | sin2(213) | 13 | When? |
| --- | --- | --- | --- |
| CHOOZ | < 0.11 | < 10 |  |
| NUMI Off- Axis (5 yr) | < 0.006-0.015 | < 2.2 | 2012 |
| JPARC-nu (5 yr) | < 0.006-0.0015 | < 2.3 | 2012 |
| MINOS | < 0.06 | < 7.1 | 2008 |
| ICARUS (5 yr) | < 0.04 | < 5.8 | 2011 |
| OPERA (5 yr) | < 0.06 | < 7.1 | 2011 |
| KR2DET (Russia) | < 0.016 | < 3.6 | ? |
| Kashiwazaki (Japan) | < 0.026 | < 4.6 | [2008] |
| Penly/Cruas (France) | < 0.025 | < 4.5 | [2010] |
| Diablo Canyon (US) | < 0.01-0.02 | < 2.9 | [2009] |

- Upper limits correspond to 90% C.L*.*

## Summary: Reactor Measurement of 13

- • Reactor neutrino oscillation experiment is promising option to measure 13 and gives clean measurement of sin2213  (no degeneracies, no matter effects).
- • Sensitivity of sin2213 ~ 0.01 comparable to next-generation accelerator experiments. Complementary to long-baseline program. Allows combined analysis of reactor and superbeam experiments.
- • Measurement will be systematics limited. Do not expect to go beyond sin2213 ~ 0.01.
- • Minimum setup: 2 detectors, overburden, and muon veto.
- • Better to have *2 or 3 detectors **and** variable baseline.*
- • Be careful of detector design and backgrounds.

## Slide 40