Transcript Misura di CP dall'esperimento NA48

Mini-Workshop on Crystal Collimation
CARE
CERN – 8 March 2005
Interaction of particles with strong
crystalline fields – Some application in
high energy beam lines at CERN
Cristina Biino
INFN Torino
Introduction
The application of coherent phenomena of high energy
particles in strong crystalline fields, to NA48, an
experiment to measure the direct CP violation in the
decay of neutral kaons, lead to a series of detailed
studies at CERN beamlines.
(NA43 collaboration - Aarhus, CERN, Firenze, Grenoble, MPI,
Johannesburg, Strasbourg, Torino)
In particular we studied :
• channeling of high energy protons in bent crystals
• pair production enhancement in aligned crystals.
I present some of the results obtained.
NA48 used successfully channeling in a Si crystal to
split and deflect the proton beam and an Ir crystal
to efficiently convert photons (minimizing multiple scattering).
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Talk Summary
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Channeling efficiency in Si crystals
Channeling measurements in Ge crystals
Radiation damage
Pair Production in W and Ir crystals
33 TeV/c Pb ions deflection
NA48 applications: channeling and
pair production
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The experiment configuration for Si
Schematic view of the experimental arrangement in the H8 microbeam at the
CERN SPS. DC1 and DC2 are drift chambers to detect incoming and outgoing
particle positions, SC1 and SC2 are trigger scintillators, SC3 is an anticounter
installed to reduce background in DC2. The bent crystal is mounted on a two
stage goniometer with 1.7 mrad stepsize in turn (horizontal) and tilt (vertical).
Primary 450 GeV/c proton;
Secondary mixed pion/proton
of 200 GeV/c
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SPS - H8 microbeam
The SPS-H8 microbeam is a unique beam for
channeling studies.
It has an extremely small emittance, obtained by
successive collimation of the primary beam.
At the position of the crystal the beam size is
around 2x1 mm2 FWHM to be compared to the
typical crystal size of 1x10x50 mm3.
Furthermore the horizontal divergence is around
3mrad rms to be compared to p = 7 mrad for (110) Si
at 450 GeV.
The beam intensity is up to a few 106 protons per
burst.
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A classical 3-point bender
four solid state
detectors on the
crystal measured
dE/dx for
channeled particles
Channeled positive particles
experience less energy loss
than non channeled ones.
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The crystal alignment
In order to align the crystalline
planes with the proton beam, an
angular scan using the goniometer
turntable is performed. The countrate in a window around the low
dE/dx region is used as a sensitive
signal for channeling. A typical scan
is shown in this figure.The width of
the curve is an indication of the
critical angle for channeling (in this
scan +/- 9.4 mrad) while the slopes
are indicating that the beam
divergence is small.
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dE/dx spectra
random
Channeled positive particles experience
less energy loss than non channeled ones.
The figure shows the dE/dx spectra as
measured by a surface barrier detector on
the straight upstream end of the crystal:
aligned
200 GeV/c
450 GeV/c
aligned
• for a non aligned crystal the well-known
Landau distribution is observed.
• for the aligned crystal we observe two
peaks : the higher dE/dx corresponds to
particles which are not within the critical
angle for channeling, while the lower dE/dx
peak represents particles channeled.
(The ratio of particles in the peaks indicates
that the 450 GeV beam was more parallel.)
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Results for Si (111) at 450 GeV
The measured deflection efficiencies for 450 GeV protons are shown
for deflection angles ranging from 1.4 to 11.5 mrad. The
experimental values are compared to calculations for a crystal with
uniform curvature. Deflection efficiencies of up to 50% were
observed for (111) planar channeling in agreement with theoretical
calculations.
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High-Z crystals: Ge(110)


The equivalent magnetic field of a bent crystal
increases linearly with Z of the crystal and in
principle this should lead to:
a larger critical angle
smaller dechanneling losses
An experiment with 450 GeV/c and with 200 GeV/c
protons was performed in 1995 at CERN to investigate the deflection efficiency obtainable with a
germanium crystal.
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The experiment configuration for Ge
Schematic view of the experimental arrangement in the H8 microbeam at the
CERN SPS. DC1, DC2 and DC3 are drift chambers to observe beam profile.
SC1 and SC2 (motorized), SC3 SC4 and SC5 used to define the beam and veto
interactions with origin in the bending device; the scintillator hodoscope H1,H2
and H3 to detect the deflected beam, are scintillators installed about 5 m
downstream of the crystal to measure the protons deflected by the crystal.
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The crystal alignment
In order to align the crystalline
planes with the proton beam, an
angular scan using the goniometer
turntable is performed. The countrate in the 3 H-counters is recorded and the bent beam is detected as a function of the goniometer angle. A typical scan is shown
in this figure.The width of the
curve is an indication of the critical angle for channeling (+/- 9.4
mrad in this scan) while the slopes
are indicating that the beam
divergence is small.
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The horizontal beam profile in DC3
8.1 mrad
4.6 mrad
2.4 mrad
Straight and bent beam peaks as observed in the horizontal profile of
drift-chamber DC3, 4.1 m downstream of the crystal. Results for the
three different bending angles are shown. The region between the
peaks contains protons which were initially channeled and later lost
duringWorkshop
the passage of the crystal
(dechanneling).
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Results for the bent Ge crystal
450 GeV/c
The two data sets show different
trends: the high energy result show
a higher efficiency at low angles and
a fast drop, while the lower energy
data starts out at a not very high
efficiency but drops slowly.
200 GeV/c
The reason is probably related
to the interplay of dechanneling
lenght and crytical curvature.
At low energy the dechanneling
is comparatively low and the
crytical curvature high;
viceversa for the higher energy.
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Radiation Damage


A crucial question for most applications is how
sensitive is high energy proton channeling to
proton irradiation.
The irradiation must affect a significant fraction of the
atoms encountered in one oscillation in the channel in
order to reduce the deflection efficiency.
In 1996 an experiment was performed at CERN SPS
where a (111) Si crystal
(52x10x1mm3), used
successfully as bent crystal to deflect 450 GeV
protons in 1991,was irradiated in the T6 target
station of the SPS during a full year (1992)
Total dose received : 2.4 1020 protons/cm2
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The experiment configuration
for irradiated Si
Schematic view of the experimental arrangement in the H8 microbeam at the
CERN SPS. DC1 and DC2 are drift chambers to detect incoming and outgoing
particle positions, SC1 (hor. motorized) and SC2, SC3 are trigger scintillators,
SC4 is an anti-counter installed to reduce background in DC2. The bent
crystal is mounted on a goniometer with 1.7 mrad stepsize. H1,H2 and H3 are
scintillators installed about 5 m downstream of the crystal to measure the
protons deflected by the crystal.
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Radiation Damage

In order to localize the beam impact on the
irradiated crystal, it was exposed to a film
about two years after irradiation in T6.
Unfortunately, during the process the crystal
broke into two pieces of 29 and 23 mm length,
resulting in a strong anticlastic bending
(a curvature perpendicular to the applied curvature)

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Picture of contact radiography
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Radiation Damage
The reduction in deflection efficiency, assuming
linear dependence,corresponds to a deterioration
coefficient of about :
6% / 1020 p/cm2
This means that NA48
could run up to 100
years in the intense
proton beam before the
crystal needs replacement.
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Deflection of 33TeV Pb82+ Ions
A 33 TeV beam of lead ions (beam div 50 mrad, 400 GeV
per charge) is steered through 4 mrad using a bent crystal.
About 15% of the beam particles are channeled and deflected, as
in the case of protons
an equivalent momentum. 19
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NA48 – principle of simultaneous, nearly collinear
KL and KS beams
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NA48 – Schematic layout of
KL and KS beams
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NA48 – Layout of KL target
and bent crystal zone
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NA48 bending Silicon crystal
Advantages of using the
crystal:
•Deflects cleanly the proton
beam in a very short length
(equivalent to 14.4 TM)
•Upstream muon sweeping
action is not affected
•Splits the desired beam
fraction (about 5 10-5)
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•Garanties a sharply defined
emittance of the outgoing
beam in both hor. and vert.
Planes.
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NA48 bending Si crystal
The NA48 beam design aim is to obtain two simultaneous Kaons
beams, KL and KS, as collinear as possible. At the same time the
intensity of the proton beam creating the KS beam has to be
reduced substantially, while maintaining a low emittance. For
this a a bent crystal has been implemented to deflect a small
fraction of the incident particles upwards. Before this
application, bent crystal channeling had always been done by
using the straight part at the end of the crystal as entrance.
In order to be able to vary the deflection angle in a well
defined way without changing the curvature of the crystal, a
novel deflection scheme using a fraction of the curved crystal
by aiming at the side was chosen.
In fact, using the side of the crystal actually turns out to be a
virtue since in that case there is a coupling between the
horizontal position and the vertical angle, such that the
channeled beam that exits is well defined in both planes.
The measured background of muons in the experiment is about
a factor 10 lower that what would be expected from deflection
in a magnet.
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NA48 – detailed layout of the
KS target station
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Pair Production by a Photon in
an oriented single crystal
In crystalline materials, the electric field experienced by an
incident photon are strongly dependent on the direction of the
photon with respect to the crystal axis.
This leads to a change in the pair production cross section, and the
effective radiation lenght, if the crystal is aligned with the incident
beam direction.
This effect, coherent pair production, find an experimental application in the NA48 AKS converter. Photons should be efficiently
detected by inducing pair production in the converter, but neutral
kaons passing through the converter should be disturbed as little as
possible. The aligned crystal used as a converter could be made with
reduced thickness and still maintain the same conversion efficiency,
whilst reducing the average scattering angle of the kaons.
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The NA48 AKS counter
•
Defines the beginning of
the decay region for p+pand p0p0 KS decays and
determines the energy
scale.
• Is made of plastic scintillation counters following a
photon converter:
- aligned iridium crystal
3mm thick (0.98 X0 of
amorphous iridium but
1.79 X0 for the aligned
crystal and 30% less
scattering if compared
to a 1.33 X0 lead
converter).
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Measurements of Pair Production by a
Photon in an oriented single crystal
The pair production enhancement in W and Ir have been measured in
dedicated experiments in the H2 secondary beam line at CERN SPS for
photon energies between 5 and 150 GeV and a variety of incident angles
up to 10 mrad from the crystal axis.
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The tagged photon experiment
The electron beam, whose energy can be varied by up to a maximum of
150 GeV/c is incident onto a converter, consisting of a 1 mm thick
sheet of copper (7% X0). This radiator induces bremsstrahlung. The
parent electron is deflected away from the axis by a bending magnet.
Its position before and after the magnet is measured in both
coordinates by a set of drift chambers (DC3 and DC4). These
measurements allow to calculate the energy of the bremsstrahlung
photon, for energies up to 100 GeV. On either sides of DC4, helium bags are
used to reduce the probability of the photon interacting before reaching the
crystal. The W (Ir) crystal is mounted on the AKS goniometer with a
scintillator veto counter immediately upstream, to veto photons which produced
showers before reaching the crystal.
Downstream of the crystal there are the AKS scintillator counters to measure
the shower multeplicity. Finally the photon shower energy is again measured in a
leadglass block.
Events were recorded with the photon beam with the crystal axis set to a
variety of angles with respect to the beam axis and along different planes.
Events with multeplicity greater or equal to one in the AKS counter were
flagged as conversions. Since the photons are tagged you can compute the
probability of pair production dividing by the number of incident photons
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The crystal orientation
Orientation of the (100) and (110)
planes of the W crystal with
respect to the crystal axis (100).
The octant between the strong and
the weak plane has been mapped by
the photon conversion experiment.
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In order to align the crystal with the electron
beam, an angular scan using the AKS goniometer
is performed. The increase in the count rate in
a scintillator, normalized by the number of
incident electrons, is used as an indication of
enhanced Bremsstrahlung and Pair Production
and therefore of photon beam alignment. A
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typical
scan is shown in the figure.
Pair Production Enhancement in
the W crystal
enhancement definition:
= -9/7 X0(amorphous) /X ln(1-pcrystal)
X0(amorphous) is the radiation length
X is the crystal thickness
pcrystal is the probability of conversion
in the crystal
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Pair Production Enhancement in
the Iridium crystal
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Conclusions
NA48 used successfully channeling in a Si
crystal to split and deflect the proton
beam and an Ir crystal to efficiently
convert photons minimizing multiple
scattering.
The crystal setting were very reliable and
stable in time.
NA48 achived the measurement of direct
CP Violation with the precision of 2 10-4
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