Induced radioactivity in the active gas system of ALICE TRD

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Induced radioactivity in the active gas system of
ALICE TRD

• G.Tsiledakis

TRD meeting, 25 feb 04
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Topics

• Introduction
• Activation of Xe (n,g)
• Calculation of activity
• Study for ATLAS-ID-TRT
• Neutron fluences in ALICE TRD based on FLUKA
  simulations
• Comparing Pb-Pb and p-p collisions
• Scaling ATLAS results to ALICE TRD
• Comparison with CERN safety limits
• Conclusions

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Introduction

• High luminosity and particles multiplicities in
  ALICE
• High background of thermal neutrons
• Radioactive isotope production
• Neutron capture process is dominant for TRD gas
  (xenon)

Xe131
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Activation of Xe (n,g)

• 9 stable isotopes
• ZA n gt (ZA1) ? Q
• Q u ( A M Mn A1M )
• 54 54
• uatomic mass unit(931.494MeV)
• Mn 931.494MeV
• Compound nucleus 7-8MeV

ZA1 radioactive isotopes
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Calculation of activity
(n,g) reactions
Activity decays/s Specific activity A A F
. S . N . M . C . (build up) . (decay)

• F n-flux (KHz/cm2)
• S n-capture cross-section (1024 barn)
• N parent atoms per unit mass (atoms/g)
• M mass of the element (gr)
• X particles emmited per decay
• (build up) 1 e (l . T)
• (decay) e (l . t)

l(ln2)/T ½ T irradiation time
100 d/y at LHC t decay time 250 d T 1/2
half-life

• 1 Bq (part/s)
• 1 Ci 3.71010 Bq
• 1 Sv 1 J/kg (dose)

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Study for ATLAS-ID-TRT

• S.A.Cetin,A.Romaniouk Induced radioactivity in
  the active gas
• system of ATLAS-ID-TRT ATL-INDET-2002-001
• I.Dawson Review of the radiation environment in
  the inner
• detectorATL-INDET-2000-006
• A.Mpszczynski,S.Gadomski On radiation damage by
  thermal
• neutrons to the ATLAS SCT detectors
  ATL-INDET-99-009
• ATLAS Inner Detector Technical Desigh Report,
• CERN/LHCC/97-17, page 395, figure 11-3b by
  A.Ferrari

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Activation properties of Xe

• Isotopes with slt100 b and lifetimelt10 min are
  ignored
• b activity of Xe is not taken into account
  (complete attenuation)

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• Calculation of the g activity of each Xe isotope
• Summed activity of all isotopes

g-activity of Xe during 2 LHC years
Only 6 g with Egt250 keV and 0.5 g with Egt550
keV
For 1 kg of Xe activated by slow neutrons, the
maximum activity is 0.4 mCi
ATLAS
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Neutron fluences in ALICE TRD based on FLUKA
simulations
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Particle fluxes and fluences
"Flux" counts the rate of arrivals per unit area
indepedent of particle direction and its real
physical meaning is that of path density, whereas
"current" counts the rate crossing through a
given plane, referred to area elements in the
surface of the plane. In FLUKA, flux is defined
as the track-length of a particle per unit of
volume and its unit can be expressed as (cm-2
s-1) and fluence is the time integral of flux
expressed in units of (cm-2). The results of
the FLUKA tracklength estimator are always given
as differential distributions of fluence in
energy (cm-2 GeV 1 ) per primary.
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Geometry description
FLUKA uses the Combinatorial Geometry. About 3200
volumes and 1500 regions are needed to describe
the full ALICE experimental area, including the
cavern, tunnels, vertical shafts, rooms,
shielding, inner triplet and separation dipoles,
the surrounding hall, beam elements, detectors
and racks. Particle backscattering from concrete
walls of caverns and shafts is taken into
account by approximating the walls by a 30 cm
layer of concrete. Regions behind this layer
are treated as black-holes.
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Neutrons in ALICE TRD
1 FLUKA event 80000 pions and kaons (HIJING) are
propagated in the ALICE geometry at ?lt5 (1
central event).

• The FLUKA MC simulation indicates that,
  for the geometry
• and materials present in the experiment,
  thermalization time (neutron energy1/40 eV)
• is on the order of a few msec.

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Comparing Pb-Pb and p-p collisions
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Neutron flux in ATLAS and ALICE TRD
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Particles/cm2/GeV/primary
Particles/cm2/GeV/interaction
1 central event
dF/dE
dF/dE
107(4.1410-10 10-14)234010610741013
th.n/cm2 during 1 year
thermal bin 2340MHz sec/year
E (GeV)
E (GeV)

• Similar plots
• No straightforward comparison
• Work with cumulative fluences

ATLAS
ALICE TRD
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Neutron fluence in the Xe layers of the TRD
visual integration of fluence having the areas
under the curves proportional to the fluence.
dF/dE E
Particles/cm2/primary
Total n-fluence (27 /- 3)10-6 n/cm2/primary
Thermal n-fluence (8 /-1)10-6 n/cm2/primary
Total n-fluence/year 8.6109 n/cm2
Thermal n-fluence/year 2.6109 n/cm2
Multiply with 3.21014 to get the cumulative
fluences for each layer in 1 ALICE years of
PbPb. (80000 primaries) (4109)
1 central event
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Scaling ATLAS results to ALICE TRD
Since AFCum.Fluence We can compare annual
fluences and scale.
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1 kg of Xe has maximum activity of 0.4mCi in
ATLAS TRT and 0.04-0.1 mCi in ALICE TRD
TRD
maximum radiation dose2.3 mSv/h
Specific g constant G/D2
0.02 mCi
Worst case
VTRD27 m3 dxe 5.5 kg/m3
Assuming 150 kg of Xe stored in a
container. Total activity(0.006-0.02) mCi
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Comparison with CERN safety limits
2.3 mSv/h is below the supervised area limit and
no special shield is required
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Conclusions

• 1 kg of Xe has maximum activity of 0.1mCi
• The dose equivalent for 150 kg of activated
• Xe lt 2.3 mSv/h
• The resulting activity and dose rate of xenon
  appear
• to be rather low, especially if compared with
  the
• expected general radiation environment.
• All parts of the TRD gas system have rates below
• supervised limit and not special shielding is
  required.