Transcript Document

TG10 status report
L. Pandola
INFN, Gran Sasso National
Laboratories
for the TG10 Task Group
Gerda Collaboration Meeting, Tuebingen
November 9th – 11th, 2005
MaGe: what’s in the common part?
Radioactive isotopes and 2b
Cosmic ray muons
PNNLiso
cosmicrays
RDMiso
musun
decay0
Generators
AmBe
G4gun
wangneutrons
neutronsGS
Neutrons
TUNLFEL
g beam
SPS
General
MaGe:physics list
Low Energy
package: includes
fluorescence and
atomic effects
Default list
Off
EM physics
Hadronic
physics
Optical
physics
Standard
package
QGSP_BIN_ISO
list from M. Bauer.
Optimized for DM
and for m-induced
neutron production*
On
Cut realms
Different cuts selectable according to application (e.g. for
CR they are more relaxed).
Cuts can be different in different regions of the set-up
*
[M.Bauer, Proc. of V Inter. Workshop on the Identification of DM]
MaGe: the Gerda-specific part
top m-veto
water tank
Gerda main
geometry
neck
and test
stands
cryo
vessel
lead
shielding
tank
PMT
Ge array
LArGe set-up at MPIK
reflector
and WLS
crystal
LArGe set-up
at GS
Muons crossing the detector
Phase I: 9 Ge crystals (total mass: 19 kg). Energy threshold: 50 keV
annihilation
peak
Energy spectrum
without and with
the crystals anticoincidence
6.2 years
Energy (MeV)
background reduction
of a factor of 3-4
(1.5  2.5 MeV): 2.1·10-3 counts/keV kg y
No cuts
2.1 · 10-3 (cts/keV kg y)
Crystals anci-coincidence
6.4 · 10-4
Anti-coincidence+ top m-veto (plastic scint)
5.4 · 10-4
Cerenkov m-veto
< 3 · 10-5 (95% CL)
Physics list dependence < 25%. Total systematics ~ 35%
Radioactive contaminations
MaGe is used for detailed Monte Carlo simulation
of contaminations in the inner parts of detector
Components:
Natural decay chains of 238U and 232Th (assuming
broken equilibrium)
Cosmogenically produced isotopes
68Ge
and
60Co
Surface contamination with 210Pb
Analysis: Anti-coincidence between detector and
segments (Phase II detectors), energy cut
Studies performed in Munich
Conclusions
The development of the MaGe framework is ongoing
regularly  mutual benefit (debugging, extensions)
A common dedicated MaGe poster was presented at the
TAUP in Zaragoza: contribution in the Proceedings
A large variety of applications has been succesfully
performed with MaGe: cosmic rays, neutrons, radioactive
contaminants, optical photons (including WLS)
First consistency check of hadronic physics with FLUKA
(16N in water), in addition to the existing work! Validation
of EM physics with radioactive sources (60Co and 133Ba
with the existing detectors) at GS and Hd
Draft of a TG10 paper concerning m-induced background
Muons crossing the detector
Phase I: 9 Ge crystals (total mass: 19 kg). Energy threshold: 50 keV
annihilation
peak
Energy spectrum
without and with
the crystals anticoincidence
6.2 years
Energy (MeV)
background reduction
of a factor of 3-4
(1.5  2.5 MeV): 2.1·10-3 counts/keV kg y
No cuts
2.1 · 10-3 (cts/keV kg y)
Crystals anci-coincidence
6.4 · 10-4
Anti-coincidence+ top m-veto (plastic scint)
5.4 · 10-4
Cerenkov m-veto
< 3 · 10-5 (95% CL)
Physics list dependence < 25%. Total systematics ~ 35%
Muons crossing the detector (2)
The contribution coming from neutrons and hadronic
showers is < 0.1 %. Due to the specific Gerda set-up:
crystals surrounded by low-Z material (low n yield from m)
Events/bin/5.4 y
water and nitrogen are effective neutron moderators
Integral:
1.4 n/kg y
Above Qbb:
0.6 n/kg y
Qbb
(n,n’)
thr
Spectrum of neutrons in the
crystals from QGSP_BIC_ISO
physics list (good for m-induced
neutrons). Agreement with
FLUKA within a factor of 2
[Araujo et al. NIM A 545 (2005) 398]
Thermal:
0.02 n/kg y
Log(Energy/keV)
In the assumptions that all
neutrons above threshold give
(n,n’) interaction, neutron signal
is conservatively < 10% of the
EM signal (without any cut)
Muons interacting in the rock
Estimate the contribution from high-energy neutrons
produced in the surrounding rock by cosmic ray m’s
Spectrum and total flux (~ 300 n/m2y) from Wulandari et al.,
hep-ph/0401032 (2004)  agrees with LDV measurements
Background: ~ 4 ·
10-5
cts/keV kg y
(without any cut: can be further
reduced by anti-coincidence)
Water and nitrogen are effective
neutron moderators
Conservative estimate: the distance m-n
is <R> = 0.6 m (from LVD)  good
chances that neutrons in the crystal are
accompained by the primary m in the
water (veto is effective!)
dN n
 E 1.19
dE
LVD, hep-ex/9905047
Gerda
water
tank
radius
distance from track (m)
Mu-induced activation
Muon-induced interactions can create long-lived (> ms)
unstable isotopes in the set-up materials with Q > Qbb
cannot be vetoed or shielded against
74Ga
8.1 m
<0.08 ev/kg y
69Ge
39 h
<0.05 ev/kg y
75Ga
2m
0.09 ev/kg y
77Ge
11 h
<0.02 ev/kg y
76Ga
33 s
0.06 ev/kg y
71Ge
and
75Ge
not dangerous
n capture,
g inelastic
m- and pcapture
Isotopes in the crystals are relevant (detected with highefficiency). From the MC  6· 10-5 cts/keV kg y
Isotopes in LN2 (12B, 13N, 16N), copper (60Co, 62Cu) and water (16N,
14O, 12B, 6He, 13B) give contributions below 10-6 cts/keV kg y
Notice: 16N production rate in water is in good agreement with
FLUKA (& data from SK) [hep-ph/0504227]  good MC cross-check
Neutrons from fission and (a,n)
Neutrons from rock radioactivity: flux: ~ 3.8 10-6 n/cm2 s
In water, flux reduced exponentially with <R> ~ 5 cm
Then, 2 m of nitrogen  suppress of a factor of 150
(2 m of LAr: only a factor of 2!)
If a neutron enters in the water, it does not get out!
Concern: neutrons channeling through the neck
No extensive simulation, only rough estimate
surface
flux
LN2
vertical
suppression neutrons (?)
FGe ~ (40 cm)2 p  3.8 10-6 n/cm2s  0.0065  0.02
 ~ 0.2 neutrons/day
3 times higher than m-induced flux
thermal component ( 77Ge)
Specific MC needed
Materials & masses in Phase II set-up
Part
Material
Mass [g]
Crystal
Germanium
2400 (per detector)
Holder
Copper
Teflon
31 (per holder)
7 (per holder)
Cable
Copper
Kapton
Copper
Nickel
Gold
Aluminum
1.3 (per cable)
0.8 (per cable)
0.04 (per detector)
0.04 (per detector)
5.6·10-4 (per detector)
8.2·10-4 (per detector)
Support strings
Copper
10 (per string)
Electronics
misc (mixture)
100 (per set)
Measured activities in materials
Material
Contamination
Copper (from LENS)
≤ 16 μBq/kg
≤ 19 μBq/kg
≤ 10 μBq/kg
Ra-226
Th-228
Co-60
Teflon
≤ 160 μBq/kg
≤ 160 μBq/kg
Ra-226
Th-228
Kapton
~ 2 mBq/kg [1]
Ra-226, Th-228, Co-60
Enr. Germanium
≤ 0.1 μBq/kg
≤ 0.1 μBq/kg
145 atoms/kg
40 atoms/kg
0.63 μBq/surface
0.13 μBq/surface
Ra-226
Th-228
Ge-68
Co-60
Pb-210
Th-232
Electronics
10 mBq/kg [1]
Ra-226, Th-228
[1] estimate!
Background estimation
Part
Bkg
[10-3
Rate
cnts/kg/keV/y]
Events
[cnts/y] [1]
Crystal
U-238
Th-232
Co-60
Ge-68
Pb-210 (s)
Th-232 (s)
0.25
0.05
0.03
1.53
0.13
0.17
0.25
0.05
0.03
1.55
0.13
0.17
Holder
all (copper)
all (Teflon)
0.12
0.17
0.12
0.17
Cables
all (copper)
all (Kapton)
0.02
1.31
0.02
1.34
23.65
23.84
~27
~27
Electronics all
Sum
[1] 20 keV window
Plan for further studies in Munich
Pulse shape simulation and analysis first results show
suppression ~4 at 90% signal efficiency for photons
Ensemble tests based on MaGe and statistical analysis
Implementation of test-stands geometries into MaGe
(for local MPI set-ups)
Neutron studies (m-produced and from radioactivity)
Simulation of LArGe setup at MPIK
Simulation of LArGe integrated in the MaGe framework
Simplified toy-geometry
tank
Goal: complete simulation
of the scintillation photons
understand better shadowing
effects and optimize the
detector packing
PMT
reflector
and WLS
crystal
LAr scintillation: large yield (40,000
ph/MeV) but in the UV (128 nm)
Possibly, understand and derive
optical properties of interest
(e.g. reflectivity of Ge crystals),
that are poorly known in the UV
Output from the simulation
Ar
peak
VM2000
emission
Cerenkov
spectrum
The ratio between the LAr peak
and the optical part depends on
the WLS QE: critical parameter
Scintillation yield  40,000 ph/MeV
Frequency
spectrum of
photons at the PM
(to be convoluted
with QE!)
The MaGe framework
Idea: collaboration of the two MC groups for the
development of a common framework based on Geant4

avoid the work duplication for the common parts
(generators, physics, materials, management)



provide the complete simulation chain
more extensive validation with experimental data
runnable by script; flexible for experiment-specific
implementation of geometry and output;
mjgeometry
gerdageometry
Generator, physics
processes, material,
management, etc.
mjio
gerdaio
Measurement with collimated
LArGe setup irradiated with external collimated
Measurement:
Drawback: the simulation is
very slow (a few seconds per
122-keV event)
57Co
source
Simulation of 122 keV line:
(PMT QE included)
46 p.e.
(80%
WLS QE)
From measurement: 122 keV
correspond to 24.5 p.e.
57Co
34 p.e.
(60%
WLS QE)
Optimization of Cerenkov veto
Assumptions on Cerekov veto
threshold: 120 MeV (~60 cm)
40 p.e. (0.5% cov + VM2000)  80 PMTs
Input
angular
spectrum
Detailed Monte Carlo studies
(Tuebingen and Dubna) with optical
photons to optimize the placement
of the PMTs
neck
cosq
minimum GS
coverage
Pb plate
shadow
Optimization of Cerenkov veto (2)
top m-veto
PMT
Water tank
“Ring”
“Pillbox”
VM2000
Configurations
with 72 and
78 PMTs are
being
explored.
Crytical
regions: neck
and bottom of
cryovessel
Optical photons tracked within the MaGe framework.
CPU-intensive but works ok. It also works with LAr
scintillation and WLS