Download Inside A Particle Physicist`s Toolbox

A Particle Physicist’s Toolbox
Dan Levin
Saturday Morning Physics
University of Michigan
Nov 6, 2004
My Résumé
Age 3 discovered electricity
–
inserted severed lamp cord into wall outlet
–
blew circuit breakers, blew parental complacency
–
observed copious sparks
Age 7 discovered the telegraph
–
hooked up battery, coil of wire, earphone
–
heard static noise
–
observed sparks
Age 10
–
–
–
discovered hydrogen
filled test tubes with gas from reaction of Zn+HCl
ignited gas with match
sparks observed
Age 12 discovered application for Maxwell’s equations
–
built shortwave transmitter
–
obtained “ham” radio license
Particle Physics is reductionist
1. Break open matter
2. See what parts are within
3. Observe their properties & interactions
4. Describe underlying structure & fundamental principles
5. Go to step 1
In HEP (High Energy Particle Physics)
Observation of subatomic particles
we adopt
a verybeyond
sophisticated
method
requires
working
day-to-day
human scales in
1. Ramp up particles to high velocities
time,them
spaceinto
& energy
2. Smack
each other
We
mustwhat
extend
our senses with specialized
3. See
emerges
instruments & methods
These comprise the contents of our toolbox
How do we “naturally” observe our world?
Source
Probe
Target
Scattered Rays Detector
The Particle Physics Scattering Model
Source
Probe
Target
Emerging
Rays
Detector
Particle Physics aims to characterize the target &
study the probe-target interaction
To do this we measure attributes of the emerging particles
1.
2.
3.
4.
5.
mass
electrical charge
momentum and energy
intensity or how many
direction - location
This is the task of the particle detectors
Our toolbox enables all facets of this model
– radiation producing sources
• radioisotopes
• cosmic rays
• particle accelerators
– targets
• “fixed” stationary targets
• colliding beams
– detectors
•
•
•
•
calorimeters
signal processing
triggers
tracking spectrometers
– data acquisition
• computers & software
The Death of Plum Pudding
Rutherford, Geiger and Marsden
Scattering Model- Example I
Probe: Alpha
(He++)
Source:
Radium
Lead
Collimator
Target:
gold foil
Detector: Zinc
Sulfide Viewing
Screen
circa 1909: Prevailing atomic model is Plum Pudding.
The Coloumb electrical repulsion
results only from a fraction of the
total charge
A Plum Pudding
Atom
Positively charged mass
Negatively charged
embedded electrons
They expected to see small deflections of the alphas as they
barreled through the ultra-thin foil
Alpha particles
Ultra thin Gold
foil
Marsden instead observes many large angle scatters or
ricochets.
Alpha particles
Ultra thin Gold
foil
This scattering is contrary to the PP model!
From Marsden’s observed
scattering angles
Rutherford calculates:
Charged mass is not
distributed over whole
atomic volume.
The observed scatters can
occur when the charge is
concentrated at the center
in volume of 0.0001 atomic
diameter
This was the death of plum
pudding & birth of the
nucleus.
Scattering Model 2:
Cosmic Rays
Early 1900’s: omnipresent radiation discharges electroscopes
-
In 1912, Viktor Hess rises
to the occasion (27,000’).
He carries an electroscope
The discharge rate
is measured at altitude
It is found to increase
+ +
+ +
+
+
+
+
+
+
-
-
What Hess was
detecting
+

μ+

Source:
Supernova
Probe:
Nucleus
Target:
Earth Atmosphere
Emerging Rays:
Cosmic Ray Shower
Let’s examine how different high energy
particle species leave their imprint
Then we can construct instruments to detect
their passage !
Here are collision products with longevity
electrons & positrons (e-, e+)
gammas () - high energy photons
muons ( μ+/-)
hadrons
mesons: , k…
baryons: protons, neutrons…
Particles with Electric Charge:
Two Important Things
Thing 1: They interact with atomic
electrons in matter & leave an ionization
trail
Thing 2: Their path in a magnetic field
curves according to their polarity and
momentum (mass, speed and direction)
A charged particle trajectory curves in a perpendicular
magnetic field
L
charge q
momentum
sagitta
p = mv
2
Lorentz says
“The curvature
depends on the
mass  velocity,
charge, magnetic
field and path”
qBL
Sagitta 
8p
Electrons, positrons, gammas:
They make showers in a massive calorimeter
Electrons leave an ion trail in low density media
e-
e- or e+ 

light
detector
material
(gas)
ee+
massive detector material
(eg lead or tungsten)
muons μ+/- leave an ionized trail
Fast muons leave a continuous trail of
ionization, losing a fraction of their
energy as they pass through.
Neutrinos slip by unnoticed.

Protons, pions & kaons initiate hadronic
cascades. These look like electron showers
but are more spread out.
μ
o
P
+
+
μ
Detection Methods
• scintillators
– calorimeters (energy measurement)
– time of flight
– triggering
• gas filled counters
– tracking in magnetic field
– momentum measurement
– particle identification
So what is really recorded with these methods?
A pulse and nothing but a pulse!
1.
The rate at a location: Always
2.
The amplitude: Sometimes
3.
The time : Sometimes
All physics is reconstructed from the above
basic data!
Plastic Scintillator & Phototubes
What are scintillators good for?
– measure deposited energy
– measure time of particle passing
– measure location
How do they work?
– passing charge excite molecules in plastic
– as molecules de-excite, a small fraction
release optical energy
– this light propagates inside the plastic to
the surface of the phototubes
Phototubes: Electronic Retinas
• See light pulse from scintillator
• Very sensitive
– huge amplification
– they can detect a single photon !
• Produce a signal quickly
– important for triggering
– precise timing
photon
photocathode
photoelectron
100 volts
100 volts
100 volts
output pulse 20 ns
100 volts
100 volts
100 volts
evacuated tube
Put the phototube & scintillator together…
particle
Phototube pulse
The pulse amplitude is proportional to the light
intensity (number of photons)
The number of photons is proportional to the
energy lost by the passing particle
Electromagnetic “Sampling” Calorimeter:
A layer cake of scintillator & lead
electron
+
=
Sum the phototube signals to measure energy of the entering particle !
Separating flotsam from jetsam
(signal from noise)
Discriminators
– not all pulses are made by a passing particle.
– there are also “noise” sources
– we use a discriminator to clean up the noise
– If the pulse is larger than the discriminator threshold
 output is TRUE, otherwise FALSE
noise pulse
discriminator
signal pulse
“threshold”
voltage
digital output pulse when
signal crosses threshold
Let’s say we want
1. to observe a specific particle track
2. not a random noise hit
3. passing from a specific direction or
4. with some flight time
We do this by forming an trigger.
A trigger is defined by a series of pulses or hits
in our detectors in a restricted time interval
Phototube + discriminator
these two overlapping hits make a trigger pulse
scintillator 1 pulses
logical
scintillator 2 pulses
Time axis
1 μsec
AND
An Elegant Cosmic Ray Telescope & Trigger
top left
top right
top coincidence
time of flight ~ nanoseconds
bottom left
bottom right
bottom coincidence
muon
top + bottom trigger
μ-
μ
μ-
How to measure time of flight of energetic particles?
Use Galileo’s Solution
1. start pulse opens an electronic gate
2. current flows into capacitor
3. stop pulse closes gate
4. measure voltage on capacitor
Voltage = Current x Elapsed Time
stop
start
gate
steady
current
source
capacitor
Gas Filled Drift Tube Detectors
Sealed tube, pressurized with Argon and CO2
+
-
Ultra-thin, anode stretched along
axis.
High voltage of ~ 3000 Volts
between anode wire and tube wall
Signal picked
off of anode
Passing particle ionizes gas
Electrons accelerate along electric field lines
Fast electrons ionize more atoms
Ensuing electron avalanche produces big signal
We can pack the drift
tubes into multi-tube,
rigid arrays.
When a particle passes,
the hit tubes produce
signals.
As an integrated unit,
these arrays record a
particle track segment.
24 cm
threshold
The time of threshold crossing
corresponds to the electron drift time.
Measure the drift time to get the
distance of the track from tube axis
Many possible tracks are tangent to the
circle defined by the drift radius
From each tube’s drift
time we obtain the drift
radius or drift circle.
Software algorithms find
the one track tangent to
all drift circles.
We now have a well
measured track segment.
24 cm
Put it all togther
Magnetic
field
μ+
Triggers
μ-
The measured curvature
yields the muons
momenta
Drift tube arrays
We have explored
1. scattering experiments
2. counting hit rate vs location
3. cosmic rays
4. scintillator with phototubes
5. pulse height discrimination
6. triggering with timed coincidences
7. a cosmic ray telescope
8. time measurement
9. charged particles in a magnetic field
10.calorimeters
11.drift tubes
12.precision trackers using drift tube arrays
Next Week:
Building a Higgs Trap