Particle production

1
Lecture 2

• Particle production
• Passage through material
• Targets, converters and absorbers
• Electron, hadron and muon beams
• Instrumentation specific to areas
• Detectors and their underlying physics
• Some operational aspects
• Timing

2
WHAT HAPPENS TO PARTICLES IN MATTER ?
Hadronic showers (p, n, K, p, L, ) Typical
length scale Lint
po
m
p, p
Electromagnetic showers (g, e, e-) Typical
length scale Xo
Muons are produced mainly via pion decay. They
traverse many metres of material with minimum
energy loss 2 GeV / m Iron)
3
Primary targets
Primary beam
Secondary beam Typically p, e
400 GeV protons

• Typically you want to produce
• Protons (target serves as attenuator)
• Pions, produced in hadronic interactions Need
  about 1 Lint
• Electrons, produced in electromagnetic
  processes The more Xo, the lower the e energy
  coming out Need about 1 Xo
• As few muons as possible Put shielding (TAX)
  before pions decay into muons

• Longer target
• More production
• More re-absorption

Optimum around 40-50 cm
etarget
0.4
0.2
Ltgt
Beryllium
Target material with large Xo/Lint
0
0 20 40 60 80 100 cm
4
SECONDARY TARGETS
Example X7 beam
Secondary beam
X7 beam 5 100 GeV/c, e or p
-120 GeV 90 p-, 10 e-
? 1 Xo , ? 0 Lint

• 4 mm thick Lead target
• Almost all pions fly through at -120 GeV/c,
  Electrons loose energy due to
  Bremsstrahlung Many low-energy electrons are
  produced
• 40 cm Copper target
• Electrons are essentially absorbed Pions have
  time to interact and produce low-energy pions
• 3) 40 cm Beryllium target
• Produces both pions and electrons

Pure electrons
? 30 Xo , ? 3 Lint
Hadrons
? 1 Xo , ? 1 Lint
Mixed beam
5
OTHER WAYS TO PRODUCE ELECTRONS
1. Electron Wobbling
e-
In the target, charged and neutrals are
produced. Sweep away all the charged by a strong
field. The photons fly though and convert in a
lead sheet. The produce electron-positron
pairs. Either electrons or positrons can be
transported by the beam line.
g
p
B3T Large I
Target
Lead converter
E-loss in H3 (GeV)
2. Use synchrotron radiation
30
At high energy, electrons loose energy along the
beam (like in LEP), whereas pions do not due to
their higher mass. Therefore they follow
increasingly differenttrajectories, until the
pions (or electrons)can be stopped by well
chosen collimation.The currents must take the
energy loss into account.
20
10
Eo
100
200
300
6
Muons from pion decay

• Pion decay in p center of mass

m
(p, E)
mp2 mm2
q
p 30 MeV/c
2 mp
n
mp2 mm2
E 110 MeV/c
2 mp
m

• Boost to laboratory frame

Em gp (E bp p cos q) with bp ? 1

• Limiting cases

cos q 1 ? Emax 1.0 Ep
0.57 lt Em / Ep lt 1
cos q -1 ? Emin 0.57 Ep
7
Particle production formula
Hadron beam intensity calculations are based on a
parametrisation of data taken by the NA20
experiment, many years ago. A simple formula
gives absolute intensities for protons, pions and
kaons. These are expressed as particles per
interacting proton and per steradian. Note that
normally the beam acceptance is of the order of
microsteradians Acceptance p qx qy Where
qx,y are the half openings of the beam acceptance
in radians.
A web interface (applet) is available to
calculate the rates
http//cern.ch/gatignon/partprod.html
These calculations are quite precise for 60 GeV/c
and above.
8
SCINTILLATORS
Scintillating material (some plastics) produce
light when traversed by charged particles. Light
is transmitted to photomultiplier by light
guide. In the photomultiplier the light is
converted into an electrical pulse. After
discrimination these pulses are counted by
scalers and the count rates are transmitted to
the control system.
Individual particles are counted as a function of
beam conditions. Useful for monitoring, beam
tuning and as a timing signal (T0) for more
complicated detectors (XCET, Cedar, XDWC).
Strobing of complicated detectors
Limited to 107 particles per second. Examples X
TRI,XTRS Big scintillators to count full
beam FISCS Narrow, mobile scintillators to
scan through beam
9
WIRE CHAMBERS
Charged particles ionise the gas. The electrons
drift to the anode wire, where the field
increases, due the extremely small radius ? Gas
amplification. An electrical pulse is produced,
discriminated and sent to DAQ. The positive ions
drift slowly to the cathode plane ? slow
detectors.
Due to well chosen geometry each wire corresponds
to a cell, electrically insulated from its
neighbour. The wire hit gives an indication
about the position of the particle, resolution
0.5 d.
Examples Wire chamber Each hit gives xd/2 for
the particle measured, limited to 107 particles
per burst. XWCA Integrate charge deposited on
each wire over the burst. Depends on HV! No
information about individual particles, but
profiles for 104 to 1010 ppp. XWCD The time
between the signal on the wire and the time of
particle passage (XTRI, XTRS) measures the
distance between particle and wire. Improves
the resolution to about 100 mm. Rates 107
ppp. XDWC Idem but use a simple delay line for
readout. Easy to use, but 104 - 105
ppp SPECTRO Measure 2 positions before and 2
after a bend. Obtain q for the particle. As the
BL of the bend is known, the momentum of each
individual particle can be measured to a few
permille
10
Threshold Cerenkov counters
In a medium (e.g. He or N2 gas)       particle
v/c p/v(p2m2)       light v/c 1/n If a
charged particle goes faster than light in a
medium, it emits Cerenkov light in a cone with
half-opening angle f              f2 2kP - m2
/p2 where k depends on the gas, Ppressure.
Light is thus only emitted when Ø2 0 !!! The
gs Ø2 and increases from 0 at threshold to
100 at very high pressures.
By selecting the right operating pressure, one
type of particle has good efficiency and the
other gives no signal. By making a coincidence
with scintillator signals, particle
identification can be made. XCET counters are
better at low momenta, CEDARS allow good
separation at high momenta (300 GeV/c), but are
more complicated and need careful tuning.
XCETs are usually operated with Helium or
Nitrogen at pressures between 20 mbar and 3 bar.
11
Experimental scalers
Rather than reading our instruments, the NIM
signal from any detector in an experiment can be
connected to a BI scaler. This allows to count
the rate in that detector as a function of beam
settings. This is very useful for beam tuning, as
it is the end user who counts!

• The final beam definition for the experiment must
  comefrom the experiment
• Often experiments take rates that are too high
  for a single counter. They can make logical
  combinationsof several counters locally (and
  fast) and send a prescaled signal to our EXPT
  scaler.

For small test areas, there are 4 scalers per
barrack For big experiments, there are up to 20
scalers (NA48, NA60)
12
CALORIMETER
HV
Principle
Computer
Beam
Particles shower in the leadglass block. At the
end of the shower, the small energy quanta
remaining deposit their energy in the form of
light. The light is captured by a photomultiplier
that transforms it into an electrical pulse. The
amount of light (thus the electrical signal) is
proportional to the deposited energy. As the
energy is deposited in N quanta, the relative
precision of the measurementis limited by
statistical fluctuations on N, i.e.
s(E)/E 1/?E
Normally a calorimeter is used for energy
measurements, But in our case its main use is for
particle identification.
13
Electron shower
Regular Fully contained
Hadron shower
Irregular, Partly contained
Muon shower
Only dE/dx Constant, small
dE/dx
14
Particle identification via
Ebeam
Electrons
Hadrons
Muons
15
OTHER INSTRUMENTATION

• IONISATION CHAMBERSIonisation is accumulated
  over the spill and subsequently digitisedby a
  frequency divider.The integrated charge is a
  measure for the beam intensity.The number of
  real particles per hit must be calibrated, e.g.
  against a TRIGGER or an EXPT scaler.
• COLLIMATORSCollimators can be scanned through
  the beam and the particles passingthrough the
  gap counted later in the beam line (often at the
  experiment). This gives a profile of the good
  particles at the collimator.

16
Operational aspects of instrumentation
Triggers
They have an in/out movement and a
high-voltage The USE option in TUNE / MEAS / TRIG
puts them IN and HV on The exact value of the
high-voltage is under BDI control Some Triggers
in H2 and H4 have lead on them (absorber
function) Be careful they would kill an electron
beam and degrade pion beams !
Analog chambers
There are two mechanically different
types XWCA 10x10 cm, motorised XWCM 20x20
cm, not motorised Some XWCA motorisations were in
common with triggers fixed in 2003 There are
exceptions some XWCM in M2 (MWPC 1-8) are
motorised The HV is under user control and must
be adjusting depending on beam intensity and spot
size. It is run down automatically in case of
saturation.
17
Delay wire chambers
These chambers are kept in operation by BDI
experts. We have only a status program and a very
nice profile display. These chambers are not
motorised. The experiments can attach signal
cablesTDCs to them for their own DAQ
Threshold Cerenkovs
When not in use, they should be emptied (to
minimize beam degradation). A pressure scan
program allows to verify the threshold pressure
The pressure can be set by the user to the
wanted value. The data are acquired by the
experiment via their own signal cable. For our
reading, the two triggers surrounding the counter
should be in the beam and HV on. There is no
piquet service outside working hours for XCET and
CEDARs
18
Effective Spill
Calculated from EXPT scalers
Spill G . S2 / C
(G S) / (C/S)
GS time over which the gate is
open C/S fraction in overlap As S contains
overlaps already, G S2/C is a goodestimator.
19
Timing
The timing in the zones is different from the
machine TG3.
Only 4 events are used WWE - 1 second before
flat top SFLAT - Start of flat top EFLAT -
End of flat top SPULSE - Start of magnet pulse
Available in user barracks
The timing distribution in the zones is separate
for BDI and for the magnets
RUN lt364gtSLEA/TIMING/READ/building to check PO
timing RUN lt145gtPIQ/STATUS/TIMING/BARRACK to
check BDI timing
SPULSE
EFLAT Read out detectors, ramp down magnets
SFLAT check current of magnets
WWE reset scalers to zero