DETECTION OF ELEMENTARY PARTICLES WITH FAST ELECTRONICS AND MEASUREMENT OF THEIR SPEED

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DETECTION OF ELEMENTARY PARTICLESWITH FAST
ELECTRONICS AND MEASUREMENT OF THEIR SPEED
Luigi Benussi, Daniele PierluigiLuciano
Passamonti
Incontri di Fisica 2004 Laboratori Nazionali di
Frascati dellINFN
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What we want to do and how we will do it

• Our goal is to measure high energy cosmic rays
  speed
• Our tools are
• Relativistic formulas
• Plastic scintillators phototubes
• Electronics for discrimination and logic
• Data Acquisition System (DAQ)
• Analysis program

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What are cosmic rays ?

• Cosmic rays are particles, of different energies,
  that reach the earth from the space.
• These particles are produced by different sources
  like stars (also our sun), black holes, neutron
  stars, quasar. Cosmic rays are composed by
  several kind of particles, but must of them are
  hadrons, i.e. protrons.
• At see level large part of cosmic ray are muons
  which are produced by the interaction of the
  hadrons with the molecules of the atmoshepere.
• The energy of cosmic rays ranges form few hundred
  of MeV/c2 to several hundred of GeV/c2. However
  the most probable energy for muons reaching the
  sea level is around 2 GeV/c2.

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Elementary relativistic formulae

• The energy E of relativistic particles is given
  by the relationship
• E2 p2c2 m2c4 (1)
• In which p is the quantity of motion or momentum,
  m the mass of the particle, and c the speed of
  light in vacuum. for computational simplicity it
  is customary to set c 1, and so
• E2 p2 m2. (2)
• The energy is measured in eV (electronVolt),
  which is the energy acquired by the electron
  crossing a potential difference of 1V. For the
  greatest values are used theprefixes Kilo (KV
  103 eV), Mega (MeV 106 eV), Giga (GeV 109 eV)
  and Tera (TeV 1012 eV) while for the smallest
  values are used the prefixes milli (m 10-3 ),
  micro (? 10-6), nano (n 10-9 ) and pico (p
  10-12 ).
• For very fast particles, Galilei transformations
  are replaced by Lorentz transformations.The
  fundamental relationships are
• b v/c (3)
• in which v is the speed of the particle and
• g 1 / ?(1-?2) (4)
• because of Eq. 2 we also have
• ? p/E (5)? E/m (6)
• in which E is the energy of the particle in
  motion and m the energy of the particle at rest.
  

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Propagation of errors
Statistical errors are defined as the fluctuation
in a measurement, when the fluctuations are
larger than the sensitivity of the
instrument. Systematical errors are due to
biases in either the instrument or the procedure.
They produce a result always larger or smaller
than the true value. Be a physical quantity y a
function of n quantities xi y
f(x1,x2,...xn) (7) The error ?y to assign to
y because of errors ?xi is y x1x2 ?y
?x1?x2 (8) y x1?x2 ?y x2?x1
x1?x2 (9) y x1/x2 ?y (x2?x1
x1?x2 )/x22 (10)
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Computing the speed of cosmic rays

• We know that the most part of the particles
  present in the cosmic rays are muons so we
  canrelativistically calculate their speed.
• E2 p2 m2 (11) ß p /
  E (12)? E / m (13) Legend E
  energy
• p momentum
  
  m mass And, according to Lorentz
  transformation, we can also say that
• b v/c (14)
• g 1 /?1-?2 (15)
• Knowing the muon momentum pµ
  and mass mµ pµ 2 GeV 2000 MeV
  (16)mµ 105.698389 0.000034 MeV (17)
• we can obtain the muon energy and from this, the
  value of ßE (4000000 11163.69517) MeV2
  4011163.695 MeV2 2002.788979 MeV (18)ß 2000
  MeV / 2002.788979 MeV 0.998607452
  (19)
• This value is very near to 1, as we wanted to
  demonstrate.

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Scintillators

• Scintillators are materials capable to point out
  the passage of a particle or of a radiation
  bundle which cross them. The phenomenon on which
  they are based is the fluorescence and it has its
  origin in the energy exchange which happens when
  the particle interacts with the scintillator
  material. The whole instrument is wrapped by
  black adhesive tape, so that it is made
  insensible to environment light.
• The light produced by the scintillator is
  conveyed on the amplifier through an optical
  guide, whose function principle is the total
  reflection of the light inside itself. The guides
  are usually in transparent plexiglas with
  polished ending surfaces, and mirrored lateral
  surfaces, to avoid light loss.

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Photomultipliers

• The signal is amplified through the
  photomultiplier or phototube (PM), a device that
  converts a light pulse into an electric signal,
  basing its operation on the photoelectric effect.
  It is well-suited to work with the scintillators
  because it is able to convert the light signals,
  which usually consist of some hundreds of
  photons, in an acceptable electric impulse
  without the introduction of big noise quantity.
  
• The two more important parts of a phototube are a
  photosensible slab, called photocathode, in which
  the photoelectric effect happens, and a sequence
  (usually ten) of dinodes which are used to
  multiply the number of the electrons produced by
  the photocathode. These are some electrodes
  placed at a distance of 1 cm about, and with a
  voltage increasing from a dinode to another. The
  phototube ends with some connectors which are
  inserted in an apposite base of the power supply.
  
• When a primary electron, extracted by the light,
  which has stroke the photocathode, is led towards
  the first dinode by the potential difference
  between the photocathode and the dinode, gives up
  its energy to some of the atomic electrons of the
  dinode these latter acquire energy sufficient to
  escape to the second dinode, which has a higher
  potential, and this process is repeated until the
  last dinode.
• In this way the few electrons gone out from the
  photocathode, after being passed through all the
  dinodes, have become many more.
• The whole process lasted few nanoseconds, and
  this feature gives the possibility to know the
  time in which the particles passage in the
  detector has happened, also with a good
  precision. So with a scintillator counter it is
  possible to determine not only the number of the
  particles which pass, but also the energy
  deposited by each single event because there is a
  relationship between the quantity of light
  produced by the originary process and the
  quantity of electrons which arrive to the anode.

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Particles counter
Particles counters are very important in nuclear
physics. They consist of three main elements a
detector, which generates observable signals when
it interacts (through energetic exchange) with a
particle or with a radiation bundle an
amplifier, which increases the intensity of the
signal produced by the detector and an analyser,
which selects and counts the number of signals
made by the detector.
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Discriminators

• The discriminator is a device which is able to
  make a selection between the analogical pulse
  (the pulse which comes from phototube), rejecting
  the impulses with a voltage amplitude inferior to
  a certain threshold, which is chosen arbitrarily.
  Instead when the impulse overgoes the threshold
  voltage, the discriminator sends a digital signal
  whose features belong to an international
  standard called NIM, with a width regulated by
  the front panel, and a fixed voltage amplitude of
  . 0.8 V.
• The function of the discriminator is double to
  eliminate the noise and to make the signal
  analysable by logical electronics elements
  (coincidence, scaler, TDC, etc)
• It is very important to choose a threshold
  voltage not too low in order to avoid the noise,
  but, at the same time, it must not be too high to
  avoid a data loss.

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Time to Digital Converter (TDC)

• A TDC (time to Digital converter) is a electronic
  device used to measure the time elapsed between
  two digital signal.
• The basic idea of a TDC is the following
• a digital signal enters inside the TDC circuit
  and gives the start to an internal clock to start
  to measure the time until the second digital
  signal enters the circuit and stops the clock.
  The output of a TDC is a integer number
  corresponding to the number of internal clock
  time units (typically nanoseconds) elapsed
  between start and stop

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Time to Digital Converter (TDC)

• A TDC (time to Digital converter) is a electronic
  device used to measure the time elapsed between
  two digital signal.
• The basic idea of a TDC is the following
• a digital signal enters inside the TDC circuit
  and gives the start to an internal clock to start
  to measure the time until the second digital
  signal enters the circuit and stops the clock.
  The output of a TDC is a integer number
  corresponding to the number of internal clock
  time units (typically nanoseconds) elapsed
  between start and stop

start
Time
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Time to Digital Converter (TDC)

• A TDC (time to Digital converter) is a electronic
  device used to measure the time elapsed between
  two digital signal.
• The basic idea of a TDC is the following
• a digital signal enters inside the TDC circuit
  and gives the start to an internal clock to start
  to measure the time until the second digital
  signal enters the circuit and stops the clock.
  The output of a TDC is a integer number
  corresponding to the number of internal clock
  time units (typically nanoseconds) elapsed
  between start and stop

start
stop
Time
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Time to Digital Converter (TDC)

• A TDC (time to Digital converter) is a electronic
  device used to measure the time elapsed between
  two digital signal.
• The basic idea of a TDC is the following
• a digital signal enters inside the TDC circuit
  and gives the start to an internal clock to start
  to measure the time until the second digital
  signal enters the circuit and stops the clock.
  The output of a TDC is a integer number
  corresponding to the number of internal clock
  time units (typically nanoseconds) elapsed
  between start and stop

start
The TDC output is read by the Data Acquisition
(DAQ) program and stored in a PC for further
analysis
stop
Time
Dt
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The basic idea
L
Scintillators
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The basic idea
start
L
Scintillators
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The basic idea
start
stop
L
Scintillators
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The basic idea
start
stop
L
Dt
If we measure the time that the cosmic ray takes
to pass both scintillators, knowing the the
distance L, we can calculate the particle speed
using the well know relation V s/t where
sL and t Dt
Scintillators
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Experimental setup

• The experimental set up consists of the following
  components
• 2 scintillation detectors, composed by a
  scintillator (30x30x0.5 cm3) Philips NE110, an
  optical guide (22cm), a photomultiplier
  Philips 56AVP and a voltage divider.
• Crate VME, equipped with Low threshold
  discriminator (mod.CAEN 417), Quad coincidence
  logic unit ( mod.CAEN 455), Quad scaler and
  present counter time ( mod.CAEN 145), 4ch
  programmable HV power supply (mod.CAEN 470) and
  Dual delay (mod.CAEN 108).
• Camac crate DDC, equipped with Status A
  (mod.CAEN 236), TDC (mod.LeCroy 2228) and SCSI
  interface, connected to PC
• PC software Microsoft Word, for the texts
  editing LabVIEW 6.1, for the acquisition of
  data Origin, for the elaboration of graphics
  Internet explorer.
• The detectors are vertically aligned at a
  distance L, which can be adjusted. When a cosmic
  ray particle crosses the scintillator detector,
  it produces a light pulse which is converted to
  an electrical signal at the photomultiplier exit.
  
• The signal goes to a low threshold discriminator
  which transforms it into a digital signal.
• The digital signal is delayed of 200 ns for the
  top detector (PM1)and 100 ns for the bottom
  one(PM2), and the two delayed signals arrive to
  an coincidence logic unit that performs an AND
  operation. We need an AND operation because we
  must select only cosmic ray particles which cross
  both detectors, otherwise we would have gather
  also signals from particles which pass from every
  direction.
• The logic unit has three outputs one of these
  goes to a scaler for the count of the
  coincidences, another goes to a Status A
  producing a signal which is fed to the
  coincidence VETO and stops the unit for a short
  time after each coincidence the last one goes to
  the TDC common start.
• The same delayed signal of the PM2 detector goes
  to TDC common stop.
• The TDC and the Status A device, contained in the
  Camac Crate, exchange data with the PC via the
  SCSI interface.

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Experimental setup
Diagram
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Experimental setup