M. Selvi – SN detection with LVD – NNN‘06 Supernova detection with LVD Marco Selvi – INFN Bologna, Large Volume Detector @ LNGS.

M. Selvi – SN detection with LVD – NNN‘06

Supernova detection with LVD

Marco Selvi – INFN Bologna,

Large Volume Detector

@ LNGS


LVD detector

• 3 identical towers in the detector

• 35 active modules in a tower

• 8 counters in one module


Construction and data acquisition

Start construction and installation: 1990

•First tower - Start data acquisition: june, 11th 1992

•Second tower – Start data acquisition : june, 1st 1994

•Third tower – Start data acquisition: december, 13th 2000

1 32

LVD is huge: 1000 tons of liquid scintillator and 2000 m2 of limited

streamer tubes.

LVD is highly modular: 840 independent

counters


The detector: basic elementsThe scintillation counter:

External dimensions: 1.5 x 1 x 1 m3

Scint. composition: CnH2n+2 <n>=9.6 +1 g/l PPO + 0.03 g/l POPOP

Scint. density: ~ 0.8 g/cm3

Attenuation lenght: > 15m @ =420 nm

Flash point at: ~39oC

PMT: FEU-49B Photocathode diameter: d=15 cmQuantum efficiency: 10-15%


SN generalitiesand the role of oscillations


Helium-burning starHelium-burning star

HeliumHeliumBurningBurning

HydrogenHydrogenBurningBurning

Main-sequence starMain-sequence star

Hydrogen BurningHydrogen Burning

Onion structureOnion structure

Degenerate iron core:Degenerate iron core:

101099 g cm g cm33

T T 10 1010 10 K K

MMFeFe 1.5 M 1.5 Msunsun

RRFeFe 8000 km 8000 km

Collapse (implosion)Collapse (implosion)

Stellar Collapse and Supernova Explosion


Collapse (implosion)Collapse (implosion)ExplosionExplosionNewborn Neutron StarNewborn Neutron Star

~ 50 km~ 50 km

Proto-Neutron StarProto-Neutron Star

nucnuc 3 3 10101414 g cm g cm33

T T 30 MeV 30 MeV

NeutrinoNeutrinoCoolingCooling



Newborn Neutron StarNewborn Neutron Star

~ 50 km~ 50 km

Proto-Neutron StarProto-Neutron Star

nucnuc 3 3 10101414 g cm g cm33

T T 30 MeV 30 MeV

NeutrinoNeutrinoCoolingCooling

Gravitational binding energyGravitational binding energy

EEbb 3 3 10 105353 erg erg 17% M 17% MSUN SUN cc22

This shows up as This shows up as 99% Neutrinos99% Neutrinos 1% Kinetic energy of explosion1% Kinetic energy of explosion (1% of this into cosmic rays) (1% of this into cosmic rays) 0.01% Photons, outshine host galaxy0.01% Photons, outshine host galaxy

Neutrino luminosityNeutrino luminosity

LL 3 3 10 105353 erg / 3 sec erg / 3 sec

3 3 10 101919 L LSUNSUN

While it lasts, outshines the entireWhile it lasts, outshines the entire visible universevisible universe



Main goal of the experiment

One SN each 30-50 years is expected in our galaxy.

Typical energy 0 - 100 MeV

Detection of neutrinos from a gravitational core collapse SN-II.

99% of the available energy (EB ~ 1053 erg) is released through the emission of neutrinos of all flavours

e e


SN fluxesThe main features of the flux produced in the star are:

1. Neutrinos have a Fermi-Dirac energy spectrum,

2. Hierarchy of the temperatures: Te< Te< Tx.

3. Approximate equipartition of energy among flavors: Le Le Lx EB/6.

Typical parameters:

• distance of D=10 kpc,

• binding energy EB= 3 x 1053 erg,

• perfect energy equipartition Le = Le = Lx= EB/6.

• assume identical fluxes ( x),

•fix the ratio Tx/Te =1.5 , Te/Te =0.8 and Te =5 MeV.

Warning! Large uncertainties in the astrophysical parameters !!!

Second warning ! It’s very difficult to consider rotation, magnetic fields

and non spherical geometry in the MC simulations

Third warning ! These models are not able to explain the SN explosion


Neutrino oscillations in SNWe consider the system of 3 active neutrinos f=(e, ), mixed in vacuum such that f=U m where m=(1, ) is the vector of mass eigenstates and U is the mixing matrix.

If neutrinos have mass they could oscillate between flavors.

The oscillation is resonantly enhanced if a flavor-asymmetric medium is present (MSW matter effect).

The medium density res for the resonance tooccur depends on the oscillation parameters.

The wide range of density values in the SN matter allows for 2 resonance levels. (g/cc) Medium Osc. parameters involved

H 103–104 He “ATM” (m2atm , Ue3

2).

L 10–30 H “MSW LMA” m2sol, Ue2

2)

The resonance is expected for

or depending on the mass hierarchy (=sign of m2

atm)

sign of m2atm Resonance in

+ (normal hierarchy)

- (inverted hierarchy)


interactions in LS


Neutrino interactions in LS interactions in LVD(mass = 1000 tons)

Energy threshold (MeV)

Detection Efficiency above

threshold (%)

e+ p n + e+ 1.8 95

x + e- x + e- / /

e+ 12C 12N + e- 17.8 85

e+ 12C 12B + e+ 13.9 70

x +12C x + + 12C 15.11 55

Target Contained in Mass Number of targets

Free protons Liquid Scintillator 1000 t 9.34 x 1031

Electrons Liquid Scintillator 1000 t 3.47 x 1032

C Nuclei Liquid Scintillator 1000 t 4.23 x 1031

(-) (-)

(-) (-)


Inverse beta decay


CC interactions on 12C nuclei

e 12C, 12N e-, observed through two signals: the prompt one due to the e-

above h (detectable energy Ed Ee - 17.8 MeV) followed by the signal, above h , from the decay of 12N (mean life time = 15.9 ms).

8

=85%

e 12C, 12B e+, observed through two signals: the prompt one due to the e+ above h

(detectable energy Ed Ene - 13.9 MeV + 2 me c2), followed by the signal, above h , from the - decay of 12B (mean life time t = 29.4 ms).

Eth=17.8 MeV

Eth=13.9 MeV =70%

Detector modularity allows precise event tagging

Elastic scattering


NC interactions on 12C nuclei

15.11 MeV energy deposit

P. Antonioli et al., NIM A309 (1991) 569

The NC carbon reaction allowsa bolometric flux measurement,“oscillation” independent.

An energy window is selected to look forexcess of events due to this reaction

=55%8

Elastic scattering


interactions in Fe


Neutrino interactions in iron

Fe

p

Vissani-Strumiaastro-ph/0302055

nucl-th/0003060

nucl-th/0003060

The considered interaction is:

e 56Fe, 56Co e-


LVD support structure


Results

-Fe

• the nb of interaction in iron is 15% of the total number interactions


Search for SN in LVD


Since: To:LiveTi

me [days]

Duty cycle

Mass [ton]

Published in:

RUN 1

Jun 6th ‘92May 31st

‘93285 60% 310 23rd ICRC 1993

RUN 2

Aug 4th ‘93Mar 11th

‘95397 74% 390 24th ICRC 1995

RUN 3

Mar 11th

‘95Apr 30th

‘97627 90% 400 25th ICRC 1997

RUN 4

Apr 30th ‘97

Mar 15th ‘99

685 94% 415 26th ICRC 1999

RUN 5

Mar 16th ‘99

Dec 11th ‘00

592 95% 580 27th ICRC 2001

RUN 6

Dec 12th ‘00

Mar 24th

’03821 98% 842 28th ICRC 2003

RUN 7

Mar 25th ‘03

Feb 4th ’05 666 >99% 881 29th ICRC 2005

RUN 8

Feb 4th ‘05Apr 13th

‘06430

99.98%

940 NU 2006

Jun 6th ’92 Apr 13th ’06 4503 89% 623

LVD data history


How can the neutrino burstHow can the neutrino burstbe identifiedbe identified ? ?

T

Detection of a burst of N pulses in a short time interval T

i Ethr

iiiMdEEEI

RN )()(

4

1~

2

А

t


Most recently analyzed data set: 4.2.2005 - 13.4.2006

Effective time: 430.5 days Average trigger mass: 940 t

Duty cycle: 99.98 %

Search for SN burst: detector performances


Search for SN burst: the method

SN selected pulses:

•Filter noisy counters

•Energy in the 7-100 MeV range

Then we perform an analysis of the time sequence.

We define a cluster as a set of m subsequent events in the time window of duration t .

For each cluster (m, t) we compute the probability that it is due to poissonian fluctuations of the flat background.

We have an alarm if the probability that the event is given by background is below one per century


Search for SN burst: results

LVD Data since 1992Upper Limit to SN event in the Milky

Way 0.18/year (90% c.l.)

m>3m>6m>10m>16m>30

Galactic SN signal


Neutrino burst detectionExpected Fermi-Dirac -spectrum from core collapse

<E>10 MeV for e ,e (i) and <E>15 MeV for (x)

Te = 3 MeV

a pessimistic assumption


SNEWS


The SNEWS systemSuperNova Early Warning System: working group between experiments looking for SN burst (currently LVD, SK, SNO, Amanda;

Borexino, MiniBoone and KamLAND expected to join)

Give prompt information to astronomical comunity.Doing online twofold coincidence allows to send a prompt alarm and to reduce to zero fake alarm!

SK LVD

SNO

BROOKHAVENserver

Scientificcommunity

Every experiment looks for SN burst and send alarm at average rate of 1/monthNetwork as much as possible fault tolerant

Inte

rval (y

r)

Nb of active experiments

1012

106

103

109

100

http://snews.bnl.gov/alert.html

AMANDA

Since July ‘05


Gd-doped LVD scintillator


Inverse beta decay (double signature)

Delay (ms)Energy (MeV)

E = 2.2 MeV = 200 s

Neutron capture efficiency = 60% (from 252Cf measurement)

n + p d +

e+ p e+ + n

1. Positron detection followed by ...

2. Gamma (2.2 MeV) from neutron capture ( = 200 s)


Gd in LVD scintillatorTwo counters (1.5 m3 each) have been Gd-doped up to 0.1% in weight (work done

at LNGS together with C.Cattadori, Bezrukov et al.):

• 11stst tank tank doped in May 2005May 2005, placed in the Mounting Hall @ the external LNGS laboratories;

• 22ndnd tank tank doped in Oct. 2005Oct. 2005, inside the LVD experiment in the LNGS Hall A.

Gd carboxylate (Gd-CBX):


Background at low energy

• The counting rate in the low energy region is related to the position of the counter inside the apparatus=> the bkg sources are external to the detector

• In the low energy region (E <2 MeV) the main bkg source is natural radioactivity (222Rn).

• The average rate over the low-threshold is <flth>~230Hz.

In order to detect the 2.2 MeV from neutron capture, the value of the low-energy threshold is ~0.8 MeV.

0

100

200

300

400

500

0 1 2 3 4 5 6COLUMN

Low

Thr

cou

ntin

g ra

te (

Hz)

0

200

400

600

800

0 2 4 6 8LEVEL

Low

Thr

cou

ntin

g ra

te (

Hz)

TOWER 1 TOWER 2 TOWER 3


Gd in scintillator

• three main improvements:

• increase of the neutron capture cross section(from 0.33 barn to 250000 barn)

• increase of the gamma energy(from 2.2 MeV to about 8 MeV)

• decrease of the capture mean time (from 180 s to about 30 s)

e+ p e+ + n


Results with Gd

Neutron Capture TimeEnergy spectrum

= 202.3 ± 1%

= 24.7 ± 1%

(Neutron source: 252Cf)

Black: before Gd doping

Red: with Gd inside


Detector performances with Gd

Change the value of the low-energy threshold (0.5÷4.5 MeV) and look at the resulting

• neutron detection efficiency (signal)

• Background rate

For comparison, requiring the same efficiency of the non-doped case (60%) the bkg rate is about 12 Hz, instead of 230 Hz.


Scintillator Stability•The stability of the scintillator has been monitored for 170 days measuring

• neutron capture efficiency

• mean time

•Consistent with a flat behaviour

•Other measurements are done on smaller samples by directly measuring transmittance, light yield, and fluor concentration … result still preliminary

•The monitoring is going on ...

Neutron capture efficiency

Average capture time

M. Selvi – SN detection with LVD – NNN‘06 Supernova detection with LVD Marco Selvi – INFN Bologna, Large Volume Detector @ LNGS.

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