EXPERIMENTALSTUDIESOFPOSITRON
SOURCES USING MULTI-GEV CHANNELLED
ELECTRONS AT CERN(a) AND KEK(b)Robert Chehab
LAL-Orsay and IPN-Lyon (IN2P3/CNRS); on behalf of WA103 (CERN) and KEK groups
LAL - Bât. 200, Université Paris-Sud – 91898 Orsay Cedex (France)
Abstract: Enhancements of photon production by multi-GeV electrons moving along theaxes of aligned crystals are used to generate high yields of positrons in the same
crystals or in amorphous disks put after the crystals. The experiment WA103
used tungsten crystals oriented along their <111> axis, alone or followed bytungsten amorphous disks. The incident electron energy was chosen between 5
and 40 GeV and mainly used at 6 and 10 GeV. Photon as positron production
enhanced by channeling and coherent bremsstrahlung have been measured withphoton and positron detectors. The positron detector consisted in a drift chamber
immersed in a magnetic field and presenting rather large momentum andhorizontal angle acceptances of 150 MeV/c and 30°, respectively.The KEK
experiment on the KEKB injector used 8 GeV incident electron beam anddifferent kinds of crystals. Tungsten crystals and more recently Silicon andDiamond crystals have been used. The measurements concerned thepositrons.The results gathered on both experiments are presented andcommented.
Key words: Channeling, Coherent Bremsstrahlung, Crystals, Photons, Positrons,
Drift Chamber

1.

INTRODUCTION+ –Achievement of high luminosities at the interaction point of a e elinear collider requires high intensity and low emittance beams. Concerning
the positrons, conventional sources use high intensity electron beams
impinging on thick amorphous targets with high atomic number. As aconsequence, a large energy deposition induces important heating problems
leading to mechanical stresses and target failure. Moreover, multiple
Coulomb scattering occurring in the thick target leads to large emittancepositron beams.

Experimental Studies of positron sources2

The reduction of the overall target thickness is obtained through thegeneration of a high number of photons- much higher than with ordinary
bremsstrahlung- which are, then, materialized in thin targets. These photon
generators –hereafter called radiators- could be magnetic undulators, as first
proposed by V.Balakin and A.Mikhailichenko [1] or “atomic” undulators
like oriented crystals [2]. The latter has been studied theoretically andexperimentally since many years [3, 4, 5, 6]. Proof of principle andexperiments were ran out in Europe and Japan [7, 8, 9, 10, 11, 12] withdifferent crystals and various incident electron energies.
We report here, on the experiment WA103 carried out at CERN on the
X-5 transfer line of the SPS. The crystal chosen was tungsten and theincident energies were between 5 and 40 GeV with the main measurements
at 6 and 10 GeV, which correspond to the chosen values for the incident
energies of NLC (Next Linear Collider) and JLC (Japanese Linear Collider),respectively. The results gathered at KEK on the KEKB linac are also
reported. They concern Tungsten, Silicon and Diamond crystals. Someemphasis will be put on the latters.

2.

WA103 EXPERIMENT

2-1

Experimental set-up

The experiment WA103 is using tertiary electron beams of the SPS
having energies between 5 and 40 GeV. The electrons, after passing throughprofile monitors (delay chambers) and trigger counters, impinge on targets
installed on a goniometer. Photons as well as e+e- pairs are produced in thetargets, come mainly in the forward direction and travel across the magnetic
spectrometer made of a drift chamber and positron counters (seefigure1).The photons and high energy electrons and positrons come out inthe forward direction. The charged particles are swept by a second magnet(MBPL) while the photons reach the photon detector made of a preshower
and a calorimeter.
The beam: the electron pulses of 3.2/5.2 seconds duration and withperiods of 14.4/16.8 seconds have intensities of ~ 104 electrons/pulse. Twoenergies have been particularly used: 6 and 10 GeV. The channelling
condition requires that the incident electron angle be smaller thanΨ =(2U/E), where U represents the depth of the potential well of an
atomic row and E, the electron energy. At 10 GeV and for the <111>orientation of the tungsten crystal, this angle is 0.45 mrad. As crystal effects
are lasting slightly above this critical angle, we gave to the trigger system,
made of scintillation counters (see figure 1), an angular aperture of
0.75 mrad. The trigger selection is improved by the informations provided
by the proportional delay chamber (off line selection).

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3

Figure 1 : WA 103 set-up

The targets: four kinds of targets have been used:- a 4 mm thick crystal,- a 8 mm thick crystal,- a compound target made of 4 mm crystal followed by 4 mm thickamorphous disk,- an amorphous disk 20 mm thick
The mosaic spread of the crystals was measured by γ-diffractometry andwas less than 500 µrad.
Photon detector: the photon multiplicity is rather high ~ 200 γ/event at
10 GeV for a 8 mm crystal oriented on its <111> axis. The detector is madeof:
- a preshower (0.2 Xo thick Copper disk and a scintillator) gives
information on photon multiplicity. It is used for crystalalignment.
- a spaghetti calorimeter [13] with thin scintillation fibers gives
the amount of radiated energy. Preshower and calorimeter are inclose contact.
Positron detector: it consists in a drift chamber with hexagonal cells, filledwith a gas mixture {He(90%);CH4(10%)} and positron scintillation
counters. It presents two parts:- the first part (DC1) with a cell radius of 0.9 cm, located mainly
outside the magnetic field. It allows the measurement of the exit
angle.
- The second part (DC2) with a cell radius of 1.6 cm is immersedin the magnetic field. It allows the measurement of the positron(electron) momentum. Two values of the magnetic field areused: 1 and 4 kGauss in order to investigate the whole
momentum region of interest, up to 150 MeV/c.
The wires are parallel to the magnetic field. The available space in thebending magnet (MBPS) is restricted vertically to 6 cm. In order to avoid
border effects of the metallic walls of the chamber, the useful part of thewires is limited to a central part of 3 cm defined by the dimensions ofpositron counters put on lateral and back sides of the chamber (see figureExperimental Studies of positron sources

41).That sets the angular acceptances, vertically to ± 1.5° and horizontallyto 30°. The overall acceptance represents 6 % of 2π solid angle. Achieved
coordinate resolution is about 500 µm leading to momentum and angle
resolutions of 0.6 MeV/c and 0.25°, respectively.
Reconstruction; positron (electron) tracks are reconstructed in the drift
chamber. Signals are coming from hitted wires ; each hit is represented by acircle centered on the hitted wire and which radius is proportional to thedrift time. The reconstruction procedure assigns 3 parameters to each track :the e+ or e- trajectory, projected on the horizontal plane is parametrised as acircle with the 3 parameters: r,the circle radius; (xc, zc) the coordinates of
the circle centre. A track is determined by a minimum of 3 hitted wires. To
find a good compromise between the number of fake tracks (reconstructed
but not real) and the lost tracks( real tracks not reconstructed) a minimum of
10 as number of hitted wires has been defined for the track selection. In
these conditions a reconstruction efficiency of 80% has been verified for the
worst case (amorphous target 20 mm thick and 10 GeV).
2-2

Results

Reconstructed trajectories: an example of reconstructed trajectory, isgiven on figure 2. It concerns a crystal 8 mm thick and an incident energy of
6 GeV.

Figure 2: Reconstructed trajectories for 8 mm thick crystal and 6 GeV.

Photon measurement: the preshower gives the average photon
multiplicity in a forward cone. On figure 3, we compare the measurementswith the simulations for a 4 mm crystal and an incident energy of 6 GeV.

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Figure 3: Signal from the preshower for 4 mm thick crystal and 6 GeV

Positron measurement: energy spectrum and angular distributions havebeen measured for all the targets and comparisons carried out between
crystal and amorphous targets of the same thickness. In the latter case, thecrystal was in full random orientation.
(a) amorphous target 20 mm thick; this measurement was important for
two reasons:- to test our reconstruction programme in the “worst case”,
i.e. with maximum occupancy in the chamber (at10 GeV incident energy),- to make valuable comparisons between simulations andexperiment, particularly with GEANT and also tocompare the simulations from GEANT to those of an
original programme [14]The results are conclusive as we can see on figure 4 where both kinds ofsimulations are compared to the experiment for the angular distribution in
the “worst case”.

Figure 4: Angular distributions (simulations and experiment) for 20 mm crystal and
10 GeV Points: experiment. Histograms: simulations; blue: SGC, red: GEANT

6

Experimental Studies of positron sources

The agreement is quite good.
(b) Crystal target 4 mm thick: simulation and measurement are compared
for the crystal on axis and in random orientation. A significant
enhancement is observed when the crystal is aligned on its <111> axis;
this enhancement is slightly larger than 4 for an incident beam of
10 GeV, as can be seen on figure 5. For an incident energy of 6 GeV,
the enhancement is slightly higher than 3.

Figure 5: Positron energy and angular distributions for a 4 mm thick crystal and 10 GeV.
Blue: crystal. Black: amorphous. Points: experiment; histogram: simulations(c) Crystal 8 mm thick and compound target; at 10 GeV as incident
energy, the observed enhancement in positron yield is slightly larger
than 2 for an incident energy of 10 GeV.
Comparison of the 8 mm crystal target with a “compound” target
made of a 4 mm crystal followed by a 4 mm amorphous disk shows
very close results at 6 GeV (figure 6). This is showing that after 4 mm
in the crystal, the processes are the same as in an amorphous medium
The same observations can be made at 10 GeV incident electronenergy.

Figure 6: Positron energy distribution for 8 mm thick crystal and compound target at 6 GeV.
Empty circles: compound target; filled circles: 8 mm crystal Histogram: simulations

Experimental Studies of positron sources

7(d) Distribution in transverse momentum: the transverse acceptance of thepositron matching systems being defined by the maximum transverse
momentum, it is interesting to consider the transverse momentum
distribution of the measured positrons. This is illustrated on figure 7
corresponding to 8 mm crystal target with 10 GeV as incident energy.
We can see that we have a peak around 3 MeV/c and a FWHM widthof 6 MeV/c. Existing matching systems (adiabatic devices) arepresenting such acceptances.

Figure 7: Transverse momentum distribution for 8 mm thick crystal and 10 GeV.
Points: experiment; histogram: simulations(e) Positron yields: given the maximum total and transverse momenta
accepted by a matching system put after the positron target, we candetermine the accepted positron yield. On table 1, we give the yields
for the case of 8 mm crystal target and 10 GeV, electron energy.

Table 1: Measured positron yields in (pl, pt )acceptance domains for the WA 103
experiment

Experimental Studies of positron sources

8(f) Enhancement of positron production with energy using oriented
crystals. The possibility to rise the energy, for the incident electrons,
up to 40 GeV (and more) allowed us to make a comparison of thepositron yields - measured on the side positron counters – for the threeincident energies: 10, 20 and 40 GeV. This can be seen on the rocking
curves of figure 8.

Figure 8: Rocking curves measured on the positron counters for 8 mm crystal and two values
Of the magnetic field: 1 and 4 kGauss

3.

KEK EXPERIMENT

3-1

Motivations

Tungsten crystal based positron sources have also been investigated at
KEK [11]. The experimental set-up, with a different conception from that of
CERN, allowed the measurement of positron yields for different kinds ofcrystals using an electron beam rather intense (~109 e-/bunch).
Intense photon generation by channelled electrons can be operated also inlight crystals as shown in the first proposition [2]. Instead of using tungsten,lighter crystals as silicon or diamond may be used. Theoreticalconsiderations by Baier, Katkov and Strakhovenko [3] showed the interest
to use Diamond, for instance, to get a significative enhancement in theenergy radiated and in the photon yield at optimum thickness. If , after theseauthors, we represent the efficiency of radiation as the ratio Lrad/Lch where
Lrad and Lch are the radiation lengths of the amorphous material and of theoriented crystal, respectively, this ratio is of 8.4 for Silicon and 5.4 fortungsten at an energy of 10 GeV, close to the energies used in CERN and
KEK. From the point of view of radiation resistance, Diamond would be
more interesting than tungsten [15] On the other hand, promising results

Experimental Studies of positron sources9

were obtained at Yerevan with diamond crystals using a 4.5 Gev electron
beam [16] Using the same experimental apparatus for all the targets, results
have been obtained on tungsten, Silicon and Diamond crystals. Someemphasis will be put on the latters.

3-2

Experimental set-up

The experimental set-up, represented on figure 9, is comprising:- a target holder in a goniometer,- a magnetic spectrometer,- collimators,- two kinds of positron detectors: a lead-glass calorimeterand an acrylic Cherenkov counter.
The silicon and diamond crystals are oriented on their <110> axis and havedifferent thicknesses. They are used alone or in combination withamorphous tungsten plates, with thicknesses from 3 to 15 mm with 3 mmsteps installed on an horizontal moveable stage behind the crystal targets.
The positrons emitted from the target in the forward direction are analysed
for momenta up to 30 MeV/c by the 60° bending magnet. Collimators are
installed before the detectors. The acceptance of the measuring system(from 1 to 9.10-4 MeV/c.sr, for momenta from 5 to 30 MeV/c) is rather
small. No measurements on photons are foreseen.

Figure 9: Channeling at KEK: the set-up

Experimental Studies of positron sources

10

3-3
3-3-1

Results
Tungsten crystals

Experiments with 2.2, 5.3 and 9 mm thick tungsten crystals oriented on
their <111> axis have been worked out using 4 and 8 GeV incident beams.
The results show:- rocking curves widths (FWHM), much larger than the channelling
critical angle : crystal effects are present beyond the critical angle,- positron yield enhancements (crystal/amorphous) for the same
thickness going from 5 to 3 and 1.8 when the crystal thickness isincreasing from 2.2 to 5.3 and 9 mm, at an incident energy of 8
GeV and for 20 MeV/c positrons. The enhancements are decreasing
when the incident energy does. They decrease about 25 % when theenergy decreases from 8 to 4 GeV.
- the enhancements in positron yield observed on tungsten crystals in
KEK and CERN experiments are coherent between them, taking
into account the scaling laws with incident energy and crystal
thickness. Even if the angular acceptances of the two positrondetectors are very different, the enhancements found remain
coherent.
- the experimental results for tungsten crystals at CERN and KEK
have been compared to simulations worked out by V.Strakhovenko[17]. Good agreement was found.
3-3-2

Silicon and Diamond crystals

Experiments with Silicon crystals having thicknesses of 9.9, 29.9 and48.15 mm and Diamond 4.57 mm thick were carried out at KEK. Angularscanning has been performed for all the crystals. As an example, we showon figure 10, the scanning operated for 20 MeV/c positrons for the Diamond
crystal and the 30 mm thick Silicon crystal. Peaks are indicating the <110>axis for both crystals [12].

Figure 10: 2-D angular scanning for a 4.5 mm Diamond crystal (a) and a 30 mm Si crystal(b).

Experimental Studies of positron sources

11

Mini peaks are corresponding to passage by planar orientation [12].
Rocking curves: examples of rocking curves obtained for 20 MeV/cpositrons at 8 GeV are shown on figure 11. It is noticeable that the width
FWHM is much larger than the channelling critical angle: 1.4 and 2.4 mrad
for diamond and silicon, respectively, compared to Ψ c = 0.13 and0.17 mrad, also respectively.

Figure 11: Rocking curves for the 4.5 mm Diamond and the 30 mm Si crystals

Enhancement in positron yield: the enhancement is measured on the rocking
curve between the peak yield and the yield 50 mrad apart from the crystal
axis. These enhancements are shown on figure 12 for both crystals which
thickness is expressed in terms of Xo (123 mm for diamond, 93.6 mm forsilicon) .

Figure 12: Positron yield enhancement for the Diamond (4.5 mm) and the Si (10, 30 and
50 mm) crystals

Experimental Studies of positron sources

12

For combined targets associating crystals as radiators and amorphoustungsten as converters, we can observe a comparison for the enhancement
between different crystals on figure 13; the total thickness (crystal+amorphous) is put on abscissa.. we can observe that for the light crystals
the enhancement (with respect to Bethe-Heitler value) is larger than 1 but
smaller than for the “heavy” crystal as tungsten.. The probable reason is thatthe positrons created by photons generated in light crystals have lower
energy due to softer photons than in tungsten crystals (due to smaller
potential well) and are more easily scattered and henceforth, less captured in
the small acceptance detection system.

Figure 13: Positron yield enhancements for the combined targets of KEK.

4

SUMMARY AND CONCLUSIONS

4-1

Concerning the CERN experiment:••

A clear enhancement in photon and positron production is observed
for all the crystals aligned on their <111> axis, when they arecompared to amorphous targets of the same thickness. The
enhancements are respectively slightly larger than 4 and 2 for 4 and
8 mm thick W crystal submitted to 10 GeV electron beam. At 6 GeV,
the enhancements are slightly lower.
Good agreement is found between the results of a 8 mm crystal and a“compound” target made of 4 mm crystal followed by 4 mm
amorphous target. This result confirm that crystal effects are
essentially present in the first millimetres of the crystal leading to aExperimental Studies of positron sources

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4-2
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4-3
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13

separation between the radiator and the converter. Such assemblingis interesting to limit the energy dissipation in the radiator, preserving
high values of the potential well.
A large number of soft photons is created due to channelling andcoherent bremsstrahlung; these photons generate soft positrons,
easily accepted by known matching systems which collect preferably
positrons up to 30 MeV/c.
A good agreement is found between simulations and experimentvalidating these simulations based on crystal processes and allowing
further predictions.
The data collected in this experiment allows 2-D reconstruction in thephase space (p L,pT ) and positron yield determination in given
acceptance domain in longitudinal and transverse momenta.

Concerning the KEK experiment
The results on tungsten crystals are in coherence with those of CERN
taking into account scaling laws for target thickness and incident
energy. This is true though the acceptance of the two detection
systems are quite different.
The results on light crystals (Silicon and Diamond) show promising
results. Real phase space of the produced positrons may be reachedthrough modification of the positron detector towards a larger
acceptance.
Experimental results compared to simulations [17] are in good
agreement within the experimental accuracy

Concerning both experiments
Some complementaries are found between different thicknesses anddifferent incident energies, concerning tungsten crystals.
The KEK set-up is relatively simple and provides rapid indications
on yields, enhancements,… The measurements being direct, theanalysis is straightforward.
However, the KEK apparatus, due to the very small acceptance does
not give indications on the positron phase space.
The CERN apparatus is conceived to proceed to track reconstruction
which is a more complicated task and time demanding
The large horizontal acceptance of the CERN detection systemallows full reconstruction of the positron phase space and precise
estimations of the yield in given acceptance conditions.

Experimental Studies of positron sources

14

(a)

(b)

5.

CERN co-authors: X. Artru, V. Baier, K. Beloborodov,
A.Bogdanov, A.Bukin, S.Burdin, R. Chehab, M. Chevallier,
R. Cizeron, D. Dauvergne, T. Dimova, V. Druzhinin,
M. Dubrovin, L. Gatignon, V.Golubev, A. Jejcic, P. Keppler,
R. Kirsch, V. Kulibaba, Ph. Lautesse, J. Major, J-C. Poizat,
A. Potylitsin, J. Remillieux, S. Serednyakov, V. Shary,
V. Strakhovenko, C. Sylvia
KEK co-authors: M. Satoh, T.Suwada, K. Furukawa,T. Kamitani, T. Sugimura, K. Yoshida, H. Okuno,
K. Umemori, R. Hamatsu, J. Hara, A. Potylitsin,
I. Vnukov, R. Chehab

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[2] R. Chehab et al., Proceedings of IEEE PAC, March 1989, Chicago, IL[3] V.N. Baier, V.M. Katkov, V.M. Strakhovenko Phys.Stat.Solid (b) 133 (1986) 583[4] V.N. Baier, V.M. Katkov, V.M. Strakhovenko “Electromagnetic proceses at highenergies in oriented single crystals” World Scientific Ed. Singapore, 1998
[5] J.F. Bak et al., Nucl.Phys.B 302 (1988) 525[6] R. Medenwalt et al., Phys.Letters B 242 (1990) 517[7] X. Artru et al., Nucl.Instr.and Meth. B 119 (1996) 246[8] R. Chehab et al., Phys.Letters B 525 (2002) 41[9] X. Artru et al., Nucl.Instr.and Meth.B 201 (2003) 243[10] K. Yoshida et al., Phys.Rev.Letters 80 (1998) 1437
[11] T. Suwada et al., Phys.Rev. E 67 (2003) 016502[12] M. Satoh et al., “Experimental study of positron production from silicon and diamond
crystals by 8-GeV channeling electrons” To appear in Nucl.Instr.and Meth. B[13] V. Bellini et al., Nucl.Instr.and Meth. A 386 (1997) 254[14] V.M. Strakhovenko SGC (Shower Generation in Crystal).
See V.N. Baier, V.M. Katkov, V.M. Strakhovenko Nucl.Instr.meth. B103 (1995) 147[15] X. Artru, V.Baier, R. Chehab, A. Jejcic Nucl.Instr.and Meth. A 344 (1994) 443[16] R. Avakian et al., Sov.Tech.Phys.Letters 14 (5) 2003[17] V. Baier, V. Strakhovenko Phys.Rev Special Topics –AB 5 (2002) 121001