Experimental study of a positron source using channelingExperimental study at CERN ofa positron source using channelingR. Chehaba , R. Cizeron, C. Sylvia

Laboratoire de l'Accelerateur Lineaire,
IN2P3-CNRS et Universite de Paris-Sud, BP 34, 91898 Orsay Cedex, France

V. Baier, K. Beloborodov, A. Bukin, S. Burdin, T. Dimova,
A. Drozdetsky, V. Druzhinin, M. Dubrovinb , V. Golubev,
S. Serednyakov, V. Shary, V. Strakhovenko
BINP, Av. Lavrentyeva, 630090 Novosibirsk, Russia
X. Artru, M. Chevallier, D. Dauvergne, R. Kirsch,
Ph. Lautesse, J-C. Poizat, J. Remillieux
IPN-Lyon, Universite Claude Bernard, 69622 Villeurbanne, France
A. Jejcic
LMD, Universite Paris, Jussieu 75005-Paris, France
P. Keppler, J. Major
Max-Planck Institute, Heisenberg Str. 70569 Stuttgart, Germany
L. Gatignon
CERN, CH-1211 Geneve 23, Switzerland
G. Bochek, V. Kulibaba, N. Maslov
KIPT, Academisheskaya Str. 310108 Kharkov, Ukraine
A. Bogdanov, A. Potylitsin, I. Vnukov
NPI-TPU, Prospekt Lenina, 634050 Tomsk, Russia

a chehab@lal.in2p3.fr
Research under INTAS contract 97-562
b presently at Wayne State University - Cornell, Ithaca NY, USA

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Abstract
Axially oriented crystals, penetrated by high energy
electrons, are powerful photon sources and, henceforth,intense positron sources. Such kinds of positron sources
have been studied experimentally at CERN, during thesummers 2000 and 2001, with the tertiary electron beamof the SPS having an energy of 6 and 10 GeV. Four andeight mm thick tungsten crystals and a compound target
made of a 4 mm crystal followed by a 4 mm amorphous
disk, were used with an orientation along the < 111 >axis. A 20 mm tungsten amorphous target was also used.
The positrons were detected by a Drift Chamber,partially immersed in a magnetic field, of 0.1 or 0.4
Tesla, working as a magnetic spectrometer in thepositron energy range up to 150 MeV. The magnetic field
was parallel to the wires. The positron trajectories werereconstructed in the Drift Chamber and allowed thedetermination of the energy spectrum and of the angular
distribution for all the targets. Significant enhancements
were observed for the crystal source when compared tothe amorphous one. The gain was larger than 4 for the 4mm target and larger than 2 for the 8 mm and thecompound target when the crystal was aligned with thebeam direction, compared to the non-aligned casecorresponding to an amorphous target. These results aredescribed after short presentations of the experimental
set-up and of the method of track reconstruction.

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Experimental study of a positron source using channeling

INTRODUCTIONThe objective of the experiment is to measure

the enhancement in positron production when the incident
electron beam is in channeling conditions- at glancing angles to the rows- in a crystal.
The main physical parameters, which can be measured,
are the emission angle and the momentum of the positrons.
These quantities are needed to determine the phasespace of such positron source for a comparison withthe conventional one in the framework of a possible use
in the future e+ e; linear colliders.

The interest

 a large number of soft photons giving correspondingly

a large number of soft positrons easily capturedby the known optical matching devices
 a thinner thickness for the targets leading
to less energy deposition
Moreover, the diculties encountered by theconventional devices where excessive heating andmechanical stresses are met brings some additional interest.

We present here: some preliminary resultsof our experiment
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Experimental study of a positron source using channeling

The experimental conditions
An experiment has been carried out during two weeks in
July|August 2000, after a calibration run at the end of May2000. The experiment was resumed in July 2001. The
experiment installed on the X5A transfer line of the SPS
provided data on the positrons and photons created in thetungsten targets: a 4 mm thick crystal, a 8 mm thick crystal, a compound target made of a 4 mm crystal followed by a

4 mm thick amorphous disk,
 an amorphous tungsten target 20 mm thick

The electron incident energy was taken in the range 5 to 40
GeV mainly at 6 and 10 GeV. The positrons were measured ina magnetic spectrometer made of a Drift Chamber partiallyimmersed in a magnetic eld two values of the magnetic eld
were used: 1 and 4 kGauss. These eld values allowed toinvestigate the energy domain up to 80|100 MeV. Data has
been taken on:
 the Drift Chamber (DC), the positron counters installed on two lateral walls of theDC, with a specic domain of acceptance for each ofthem, with respect to the positron energy level,
 the preshower and the calorimeter for the photons.

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The experimental set-up
CrystalCounters1000
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Discriminator
Amplifier
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300 channels

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Pentium II
Windows NT

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GPIB

Buffer
LeCroy 4299

TDC
controller
LeCroy 4298

CAMAC

CAMAC
controller
LeCroy 8901A
HYTEC
LP 1341

CAMAC

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Experimental study of a positron source using channeling

The results obtained with reconstructed
positrontracksA reconstruction process, taking into account the actual

magnetic eld distribution in the Drift Chamber, has been
operated.
 Magnetic eld = 1 or 4 kGs as maximum value with fringeeld at the entrance of the Drift Chamber
 Method of templates (trajectories of determined energy andangle)
 inuence of interaction processes in the medium isstudied
 geometrical track  average track for tracks aectedby multiple scattering angular domain: ;5 to 35 (steps of 0:5 ) energy domain: 5 to 150 MeV (steps of 1 MeV ) Parametrisation with 9-degree polynomial y(x)]Coecients are stored in the databasexy

Set of positron tracks with Sketch of the DC with theenergy
20 MeV and angle positron counters and the10 simulated by GEANT. cells near to the lateral walls.
Magnetic eld is 1 kGs.

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Experimental study of a positron source using channeling
The reconstruction programme makes 2 maps ofthe functions:

L0(Ei  k) =(1)X

N1 +N2j d ; rn j
1
exp
(; n
N1 + N2 n )
L2(Ei  k) =(2)N21exp (; j dn ; rn j ) with  = 1cm

X

N2 n

representing the distance between the simulated tracks ofknown initial angle and energy and the hitted wires of the DC.
Indexes 1 and 2 mean DC1 (part of the DC outside of themagnetic pole) or DC2 (part of the DC inside of the magneticpole), N | number of hitted wires,  = 1 cm | coecientcharacterising the dimension of the DC cells, dn cm] |distance between the track (Ei  k ) and the n-th wire,rn cm] | the radius calculated from TDC value of the n-thwire.
k > lim = 5 ! L0
k <= lim = 5 ! L2

Peaks on the map L0=2 (Ei  k) ! realtracks.
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Experimental study of a positron source using channeling0.70.6
0.5
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160 140120 100

580 6040 200
Energy(MeV)
Angle(degrees)

1015202530

Distribution L0=2 (Ei  k ) for two simulated positron tracks:(57 MeV, 8 ), (71 MeV, 10 ).

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Example of the simulated event for 8 mm crystal,energy 10 Gev, DC magnetic eld 1 kGs

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Example of the experimental event for 8 mmcrystal, energy 10 Gev, DC magnetic eld 1 kGs

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Rotation scan
Aver ampl in DC counter 1, MIP

The 8mm crystal target, 10 GeV, 4kGs. Amplitude of the signal
normalised per energy deposition from minimal ionised particle.
Average amplitude on DC counters 1.
0.9Entries

8889

0.8
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00.5

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1.5 2 2.5 3Rotation, degree

Aver ampl in preshower, MIP

Average amplitude on preshower.
8889

43210-0.5RREPS'01

Entries

500.51

1.5 2 2.5 3Rotation, degree

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Density of the reconstructed tracks (e+)
with chamber
Energy, MeV

the 8mm crystal target, 10 GeV, 4kGs
110ENTRIES

4931

10090
80
70
60
50
403020-5051015 20 25Angle, degree

Energy, MeV

the 8mm crystal target, 10 GeV, 1kGs
50ENTRIES

2037

454035302520
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Page 12Experimental study of a positron source using channelingResults on 8 mm crystal, 10 Gev

For the \good" events selection the following
cuts were supplied:
1. reconstructed energy < 45MeV
reconstructed angle > 0 degree
2. the \side" positron counters should have asignal if the particle horizontal angle < 7degree one of the positron counters shouldhave a signal if this angle > 7 degree.
All results presented for DC magnetic
eld 1 kGs.

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Simulation and experiment
Events / number of electronsBlue points | simulation. Black one |experiment. 8 mm crystal, 10 GeV.
Energy spectrum.
Entries

4728

0.035
0.03

0.025
0.02

0.015
0.01

0.005
05

10 15 20 25 30 35 40 45
Energy, MeV

Events / number of electrons

Angular spectrum.
Entries

4728

0.05
0.04
0.03
0.02
0.01
00

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2025Angle, Degree

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Experimental study of a positron source using channeling

Simulation and experiment
Corrected by eciency.

Blue points | simulation. Black one |experiment. 8 mm crystal, 10 GeV.
Positrons / number of electrons

Energy spectrum.
Entries

8787

0.05
0.04
0.03
0.02
0.01
0510 15 20 25 30 35 40 45
Energy, MeV

Positrons / number of electrons

Angular spectrum.
Entries

8787

0.1

0.08
0.06
0.04
0.02
00

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Experimental study of a positron source using channelingComparison data from 2000, 2001
Corrected by eciency. Red points | year
2001. Black one | 2000. 8 mm crystal, 10 GeV.
Positrons / number of electrons

Energy spectrum.
Entries

10580

0.06
0.050.04
0.03
0.02
0.01
0510 15 20 25 30 35 40 45
Energy, MeV

Positrons / number of electrons

Angular spectrum.
Entries

10580

0.12
0.1

0.08
0.06
0.04
0.02
00

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Amorphous 8mm target
Events / number of electrons

Blue points | simulation. Black one |experiment. Energy 10 GeV.
Energy spectrum.
Entries

0.014

2164

0.012
0.01

0.008
0.006
0.004
0.002
0510 15 20 25 30 35 40 45
Energy, MeV

Angular spectrum.
Events / number of electrons

0.0225

Entries

2164

0.02

0.01750.015

0.01250.01

0.00750.005

0.002500RREPS'01

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Experimental study of a positron source using channeling

Amorphous 8mm target

Corrected by eciency. Blue points |simulation. Black one | experiment. E = 10 GeV .
Positrons / number of electrons

Energy spectrum.
Entries

0.03

3818

0.025
0.02

0.015
0.01

0.005
0510 15 20 25 30 35 40 45
Energy, MeV

Positrons / number of electronsAngular spectrum.
Entries

3818

0.06
0.05
0.04
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0.01
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Crystal and amorphous targets
Corrected by eciency. Red points |Entries

8787

0.05
0.04

Entries

8787

0.1

0.08

0.03

0.06

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Positrons / number of electrons

Positrons / number of electrons

crystal. Black one | amorphous. 8 mm, 10 GeV

0.020010 15 20 25 30 35 40 45
Energy, MeV

Energy spectrum

510152025Angle, Degree

Angular spectrum.

3.5Entries

12605

3RatioRatio

Positrons ratio between crystal and amorphous targets
5Entries

12605

42.5
321.5

2110.505

10 15 20 25 30 35 40 45
Energy, MeV

Energy spectrum

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10152025Angle, Degree

Angular spectrum.
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Crystal 8 mm & 20 mm amorphous

Events / number of electrons

Blue points | 8mm crystal. Black | 20 mm
amorphous. Energy 10 GeV.
Energy spectrum.
Entries

52630.120.1

0.08
0.06
0.04
0.02
05

10 15 20 25 30 35 40 45
Energy, MeV

Angular spectrum.
Events / number of electrons

0.225

Entries

5263

0.2

0.175
0.15

0.125
0.1

0.075
0.05

0.025
00

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4 mm targets, 10 Gev, 1 kGs
Corrected by eciency. Blue points |0.025

Entries

11896

Positrons / number of electrons

Positrons / number of electrons

simulation. Black one | experiment.
Crystal target
Entries

57290.06

0.02

0.05
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0.015

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0.01

0.02

0.005
050.010010 15 20 25 30 35 40 45
Energy, MeV

Energy spectrum

510152025Angle, Degree

Angular spectrum.

0.008

Entries

2308

0.006

2308

0.02

0.005

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0.01

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10 15 20 25 30 35 40 45
Energy, MeV

Energy spectrum

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Entries

0.025

0.007

050.03

Positrons / number of electrons

Positrons / number of electrons

Amorphous target

00510152025Angle, Degree

Angular spectrum.
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Crystal / amorphous 4mm targets
Corrected by eciency. Red points |Positrons / number of electrons

0.0225

Entries

5729

0.02

Entries

5729

0.06

0.0175

0.05

0.015

0.04

0.01250.01

0.03

0.0075

0.02

0.005

0.01

0.002505

Positrons / number of electrons

crystal. Black one | amorphous. Energy 10 GeV

0010 15 20 25 30 35 40 45
Energy, MeV

Energy spectrum

510152025Angle, Degree

Angular spectrum.

6Entries

8037

5RatioRatioPositrons ratio between crystal and amorphous targets
Entries

8037

65443322

105110 15 20 25 30 35 40 45
Energy, MeV

Energy spectrum

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005

1015
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Angular spectrum.
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8 mm targets, 6 GeV, 1 kGs
Corrected by eciency. Corrected byeciency. Red points | crystal. Black one |
0.045
0.04
0.035
0.03
0.025
0.02
0.015
0.01
0.005
05Entries

5016

Positrons / number of electronsPositrons / number of electrons

amorphous.

Entries

0.12

5016

0.1

0.08
0.06
0.04
0.02
00

10 15 20 25 30 35 40 45
Energy, MeV

Energy spectrum

51015
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Angular spectrum.

4.5
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Entries

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Energy spectrum

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Ratio

Ratio

Positrons ratio between crystal and amorphous targets
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Entries7808

10 15 20 25 30 35 40 45
Energy, MeV

Angular spectrum.
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SUMMARY / CONCLUSIONS
 the reconstruction programme works

 the experimental results are in good agreementwith the simulations
 enhancements in e+ production for the 4 mm asfor the 8 mm crystal, for 10 and 6 GeV electron
beam, are noticeable when comparing crystal andamorphous targets of the same thickness
 enhancement somewhat larger than 3, for the 4mm target enhancement somewhat larger than 2, for the 8 mm target in the regions of interest (in energy and angle)

the enhancements show good perspectives for low
energy and small angle positrons. This is more
pronounced for the 4 mm target
 Estimation of systematic error (dierence
between simulation and experiment, betweendierent experiment) is 20 %. Further analysis isnecessary for decreasing the systematic error
 photon detectors used mainly for crystal
orientation gave also indications on theenhancement in photon production on the axis
 The preliminary results represent an analysis ofpart of the gathered data. The full analysis will becompleted in nearest future
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