Argon purity measurement of the D0 calorimeter

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Argon purity measurement of the D0 calorimeter
10th International Conference on Calorimetry in
High Energy Physics Calor 2002.

• Auguste Besson
• (ISN - Grenoble)
• for the D0 collaboration

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Argon purity measurement of the calorimeter
A. Besson, Y. Carcagno, G. Mondin, G. Sajot with
help from Solveig ALBRAND, Germain BOSSON,
Philippe MARTIN

• Argon Test Cell (A.T.C.)
• measures the equivalent O2 pollution with 2
  radioactive sources ? ?.
• general setup, cryostat, electronics
• Calibration
• ? Source
• ? Source
• Measurements and results
• Conclusion

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The D0 calorimeter

• 1 Barrel 2 End Caps
• Liquid Argon
• U (EM) / Cu-Stainless steel (Had.)
• 5000 towers
• ?? x ?? 0.1 x 0.1
• 4 EM read out layers
• 4/5 Had read out layers
• ?(EM) / E 16 / ?E
• ?(Had) / E 50 / ?E

Central Cal.
South End Cap
North End Cap
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Why do we have to monitor the Argon purity ?

• Shower
• liquid argon ionization.
• Liquid Argon purity
• Any electronegative molecule (O2 )
• absorbs electrons and decreases
• the signal.

(collected signal) / (ideal signal)

• Need purity better
• than 0.5 ppm
• Monitor precisely the pollution.

Pollution (ppm)
E10kV/cm, gap2mm (ATLAS LARG-NO-53)
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Principles 2 radioactive sources ? and ?

• ALPHA
• 5.5 MeV, T 430 ans
• BETA
• 3.5 MeV, T 1 an, 40kBq

Gap d 2.15 mm

• Sources electrodeposited on a stainless steel
  electrode.
• Sources immerged in liquid Argon (_at_ 85 K).
• Ionisation, drift of the charges in an
  adjustable electric field E through a gap between
  electrodes d 2.15 mm.
• The collected charge depends on the pollution
  p and on the field E.

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A.T.C. history

• System used in Run I (1992-96)
• LAr stocked in a dewar for 5
• years in a dewar ( 20 000 gallons)
• Upgrade for Run II
• New Beta source
• New electronics (preAmplis, Pulsers, etc.)
• Data acquisition soft in LabWindows/CVI
• Added a O2 pollution system for calibration
• complete check of the cryostat (leak detection,
  checks of valves, etc.)
• Calibration of system (2000)
• Dewar purity measurement
• July 2000 and october 2000 (before filling the
  Calorimeter)
• Calorimeter measurements
• December 2000 and Dec. 2001

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ATC General setup
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ATC operating condition
Pressure (bar absolute)
Temperature (K)
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LN2 exchange
Signal cables
sources
High Voltage
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Alpha source measurements

• ? particle highly ionizing particle
• Energy deposited over 20 ?m constant
  currant
• Ramping on the electric field E (20 values)
• Collected charge f (E,p)
• 40 000 evts / value
• normalized signal

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Alpha principles of the measurement

• Collected charge

• Recombination

with
(a,b,c constants)

• Absorption

With absorption length
Trapping constant ? 0.142 ? 0.014 cm2.kV-1.ppm
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Alpha absorption
Theoritical expression of the absorption
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Alpha absorption an example

• Fit of Abs(E,p) vs E.
• Argon from dewar.
• Black fit 0.37 ppm
• Blue fit - 0.1 ppm
• Red fit 0.1ppm

Absorption
E (kV/cm)
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Alpha errors estimates

• Statistic errors
• Statistics and fit error ? 0.07 ppm
• Systematic errors
• High Voltage 2
• gap between electrodes d 2.15 ? 0.05 mm
• Error on parameters
• a 474 ? 1.4 kV/cm
• b 0.143 ? 0.006 cm/kV
• c 0.403 ? 0.010
• trapping constant ? 0.142 ? 0.014
• Other systematics
• electronics and non linearity of the preamp.
• temperature effects, etc.

Need a calibration.
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Alpha calibration

• Recipe
• Start from a high purity Argon sample lt 0.1 ppm
• Pollute with a well known amount of O2
• (for instance 0.5 ppm)
• mix well, wait for 1 or 2 hours
• Measure

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Calibration (2)
? Source

• Errors on the pollution
• Volume of liquid Argon
• 8-10 liters ? 5
• Volume of O2
• 8.3 ? 0.1 cm3
• Pressure of O2
• 15 ? 0.5 P.S.I.

Absorption
Measured / nominal
E (kV/cm)
Error on the nominal pollution 10
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Alpha calibration results
Nominal (ppm)
Measured (ppm)
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Alpha errors
Error vs nominal pollution (ppm)
Linear Fit ?gives the final errors.
Error measurement (ppm)
Nominal Pollution (ppm)
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Alpha C.C. and calibration, example
C.C.
Measured / nominal
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Trapping constant ? measurement

• Trapping constant ?
• ? Relates absorption length ?,
• field E and pollution p
• main error for absolute measurements.
• Its value is not very well known
• With our calibration

? parameter
? 0.142 ? 0.014 cm2.ppm/kV
Nominal Pollution (ppm)
(Andrieux et al. NIM A 427, 568 -1999)
average
? 0.141 ? 0.011 cm2.ppm/kV
? 0.138 ? 0.019 cm2.ppm/kV
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Beta Source

• Characteristics
• Complete spectrum.
• Low ionizing particle
• the tracks cross the gap
• Use a trigger gap in
• Coincidence to decrease the noise
• No theoritical formula empirical fit

With a, b, c, d, g parameters of the fit.
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Beta parameters vs pol.
A et B given by the calibration.
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Beta errors
Error measurement (ppm)
Pollution (ppm)
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Beta example of C.C. measurement
C.C.
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Beta example of N.E.C. measurement
N.E.C.
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Beta example of S.E.C. measurement
S.E.C.
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Summary of measurements
(ppm)
Measurements compatible and stable
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Conclusion and outlook

• Errors on measurements
• ? Absolute measurements and errors.
• ? better than ? 0.15 ppm
• We checked the stability of purity compared to
  last year measurements.
• ? Purity OK for the 3 calorimeters lt 0.5 ppm
• ? No need to apply correction for calorimeter
  response
• ? Give a calibration for the alpha internal cells
  of the calorimeter (Purity monitoring by Mainz
  Univ.)
• Trapping constant measurement
• Experiments must be reproduceable, they should
  fail the same way each time.

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