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 ) water and nitrogen are effective neutron moderators In the assumptions that all neutrons above threshold give (n,n’) interaction, neutron signal is conservatively < 10% of the EM signal (without any cut) Spectrum of neutrons in the crystals from QGSP_BIC_ISO physics list (good for -induced neutrons). Agreement with FLUKA within a factor of 2 [Araujo et al. NIM A 545 (2005) 398] Log(Energy/ keV) Events/bin/ 5.4 y Q (n, n’) thr Integral : 1.4 n/kg y Above Q : 0.6 n/kg y Thermal: 0.02 n/kg y
crystals surrounded by low-Z material ( low n yield from m ). water and nitrogen are effective neutron moderators. 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. Q bb. Integral: 1.4 n/kg y. - PowerPoint PPT Presentation
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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 )water and nitrogen are effective neutron moderators
In the assumptions that all neutrons above threshold
give (n,n’) interaction, neutron signal is
conservatively < 10% of the EM signal (without any cut)
Spectrum of neutrons in the crystals from QGSP_BIC_ISO
physics list (good for -induced neutrons). Agreement with FLUKA within a factor
of 2 [Araujo et al. NIM A 545 (2005) 398]
Log(Energy/keV)
Events
/bin
/5.4
y
Q
(n,n’) thr
Integral: 1.4 n/kg
yAbove Q: 0.6 n/kg y
Thermal: 0.02 n/kg
y
distance from track (m)
Gerda water tank
radius
Muons interacting in the rock
Water and nitrogen are effective neutron moderators
Conservative estimate: the distance -n is <R> = 0.6 m (from LVD) good chances that neutrons in the
crystal are accompained by the primary in the water (veto is
effective!)
Estimate the contribution from high-energy neutrons produced in the surrounding rock by
cosmic ray ’sSpectrum and total flux (~ 300 n/m2y) from Wulandari et al., hep-ph/0401032 (2004) agrees with LDV
measurementsBackground: ~ 4 · 10-5 cts/keV
kg y(without any cut: can be further reduced by anti-coincidence)
19.1 EdE
dNn
LVD, hep-ex/9905047
Mu-induced activationMuon-induced interactions can create long-lived (> ms) unstable isotopes in the set-up materials with Q
> Q
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
cannot be vetoed or shielded against
Isotopes in the crystals are relevant (detected with high-efficiency). From the MC 6· 10-5 cts/keV kg y
74Ga 8.1 m <0.08 ev/kg y
69Ge 39 h <0.05 ev/kg y
75Ga 2 m 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
- a
nd
- ca
ptu
ren
captu
re,
in
ela
stic
Neutrons from fission and (,n)Neutrons from rock
radioactivity:flux: ~ 3.8 10-6 n/cm2
s
Ge ~ (40 cm)2 3.8 10-6 n/cm2s 0.0065 0.02
surface fluxLN2
suppression
vertical neutrons (?)
~ 0.2 neutrons/day 3 times higher than -induced flux
thermal component ( 77Ge)
If a neutron enters in the water, it does not get out!
Concern: neutrons channeling through the neck
No extensive simulation, only rough estimate
In water, flux reduced exponentially with <R> ~ 5 cmThen, 2 m of nitrogen suppress of a factor of 150(2 m of LAr: only a factor of