Transcript Slide 1

Detectors
• First the physical processes of how the
particles interact with matter and then the
details of how we get electrical signals
from the process. We start by asking how
can particles penetrate solid matter and
what happens when they do?
What is a detector?
• Anything which gives us information about the
particles we wish to study.
• Electroscopes such as Wolf and Kohlhorster
took up the Eifel Tower or up in balloons
• The multitude of devices which make up CMS
surrounding an intersection point of the LHC
• The huge vats of cleaning fluid, along with a
system for measuring a few atoms of radioactive
Argon, used to detect neutrinos from the sun.
• Lots of ice and photomultipliers as used in Ice
Cube. The ice is part of the detector too.
A common thread: How do particles
get through matter and what do
they do to it as they pass
• What does a film only one atomic layer thick look like to
an incident particle?
• Hitting an electron is generally no big deal but hitting a
nucleus may be serious. (Demo, different size balls)
• The electrons contribute a sort of friction and gradually
slow the particle, but also make side effects revealing
the passage of the particle.
• Nuclear collisions can be dramatic as in the cloud
chamber pictures yesterday.
• So is a particle incident on our single atomic layer likely
to hit an electron or a nucleus? The radius of an atom is
larger by a factor of 2 to 5 x10^4 than that of the nucleus.
Let’s see what that means
Pretend atoms and nuclei are
cubes so we can make simple
pictures
• The projected area of an Al atom is (2.4e-8)^2 =
6.2 e-16 sq cm. The projected area of the
nucleus is (6.92e-13)^2 = 4.8 e-25 sq cm.
Fraction of area covered by nuclei is 7.7e-10,
and a stack of 1.3 e9 layers = 31 cm of
aluminum, arranged with no overlap, would
make the nuclei just cover the slide so that a
particle perpendicular to this plane would have
to hit a nucleus to get through. If layers are
arranged at random, 1/e is NOT covered. Is
this consistent with results from our counters?
Not hit a nucleus is dotted line
Extending our straight line gives
So even our simple counter shows
that muons can pass right through
a nucleus!
• The elements in a brick have about the same
atomic and nuclear sizes as Al, so our 5 bricks
should let only 1/e = 0.37 pass through without
interacting. 295/314 = 0.94
• Mines or deep water provide much more than 5
bricks of cover. Are there cosmic rays down 4
km into the ocean? I worked on one such
measurement and as preparation we needed to
measure bioluminescence at depth.
Picture from deck of Ship
Second DUMAND Ship
DUMAND Beamline
Lowering Phototube String
Conclusion: The chance that a
muon can penetrate to 4 km
WITHOUT passing through a
nucleus is 10^-3500
We measured at 2 km and 4 km depth. 4 km is
8000 of those 1/e distances, so the fraction
which could get down there without a collision is
e^-8000 or about 10^-3500
Our measurements are consistent with others and
compared to sea level:
Sea level 1 km 2 km 4 km 10 km
7e-3
1e-6 1e-7 5e-9 5e-13
-- 10^-6 observed --vs 10^-3500
Why are there fewer muons deep
in the ocean than at the surface?
• Those electrons we brushed aside are to
blame: “Friction” loss of energy is 2 MeV
per cm. A kilometer of water is 10^5 cm
and so muons with energies of less than
2x10^5 MeV = 200 GeV will slow down
and stop in a kilometer of water. The fact
that muons are observed at 10 km means
that there are 2 TeV muons at the surface!
A piece of history left out yesterday.
• The remarkable penetrating power of cosmic rays and rough mass
measurements were known early on in our saga.
• The need for a strong, short range (strong) force was obvious.
• The quantum theory of the electromagnetic field (QED) involved
photons and was a phenomenal success.
• Yukawa generalized QED to the nuclear force and predicted a mass
about the same as the mass from cosmic rays – but it interacted
stongly with the nucleus.
• A non-interacting particle should not be made in nuclear collisions.
• The pion was discovered a bit later . It is made in nuclear reactions
and it decays into a muon and neutrino!
• Most cosmic rays at sea level and an even larger fraction under a
kilometer of water are decay products of the pions made in the
nuclear showers such as you saw in the cloud chamber pictures
yesterday.
An idea for a detector!
• Put a few meters of water in the particles
path. If it gets through, it can pass right
through the nucleus – It does not “feel” the
nuclear force. (1 meter of steel is
equivalent to about 5 m of water. That
steel can often do double duty also as part
of a magnet, as in CMS.)
All charged particles lose energy to
electrons as they pass - friction
• A singly charged particle with speed
greater than 0.95 the speed of light looses
about 2 MeV energy to the electrons in
passing through a cm of water. Slower
particles lose energy more rapidly. This
energy given to the electrons is the basis
for our detectors. Most of the electrons
are bound in atoms: excitation or
ionization.
What about electrons?
• All charged particles lose energy gradually
by this “friction” mechanism. Strongly
interacting particles also lose energy
dramatically by hitting nuclei.
• Electrons and photons have an additional
way to lose energy: radiation and pair
production. Lets explore these.
How does a radio (or cell phone)
antenna work?
• A steady (DC) current makes a constant
magnetic field.
• An alternating current (AC) also produces radio
waves. Accelerated charge radiates.
• When a high energy electron goes past another
electron or a nuleus, the electric force
accelerates the electron and it radiates. At high
energy the radiation is likely to be a single
photon – which often has a significant fraction of
the electron’s initial energy.
• *****Heavier particles are dramatically less likely
to radiate.********
What does a high energy photon
do?
• Pair production dominates above a few MeV. (Compton
below a few MeV)
• A high energy photon passing near an electron or a
nucleus turns into an electron and a positron both going
very nearly along the direction of the photon. The
probability this will happen is proportional to the square
of the charge of the target particle. Much more likely
near a uranium nucleus than near a proton – by a factor
of 82^2. A relatively thin shell of uranium will make most
electrons produce pairs while strongly interacting
particles produce only a few nuclear interactions.
Summary of physical properties of
detector materials
• All charged particles lose energy by “friction”.
• Muons don’t lose energy (significantly) any other way.
• Strongly interaction particles (hadrons - particles made
of quarks) lose energy by nuclear interaction, often
making more hadrons which interact to make a shower
of hadrons.
• Electrons turn into photons which turn into pairs. Each
electron or positron radiates photons – a shower is born.
• In material of high atomic number, the electron showers
develop and die out very much more quickly than hadron
showers.
What have I meant by “friction”?
• The interaction with atomic electrons
causes a particle to lose energy – which is
transferred to the electrons, and this
disturbance of the atomic electrons is the
source of the electrical signals we record
and feed to our computers to analyze the
event and to make the pictures you have
seen.
Kick an electron and it may go to a
higher shell or may escape entirely
• Kicking an electron clear out of the atom is
ionization. We have seen the droplets and
bubbles which form on the ions. The ions can
be “collected” as in Geiger tubes, proportional
tubes, or silicon (transistor) detectors.
• An excited atom emits a photon and that is the
basic process in the scintillator paddles of our
muon counters.
• A third process is Cherenkov radiation, the
production of a shock wave of light analogous to
the sonic boom sound wave of a jet.
Ionization, pixels, prop tubes, our
visual detector
• Pixel – a transistor which has an input signal of
electrons produced by the passage of a charged
particle in the silicon a few hundred electrons
are produced and that can be turned into a
useful signal by an amplifier of manageable cost
– even for millions of pixels. But too expensive
to use for large areas.
• Prop tube – filled with gas. A few tens of
electrons. Much cheaper than layers of silicon,
but too little charge to detect with a reasonable
amplifier – gas amplification needed.
Excitation – The scintillator paddles
in our muon counters
• The disturbed atom radiates low energy (a few
eV) photons. Unfortunately most of these
photons are ultraviolet (like those that make
sunburn, skin cancer) and these are absorbed in
a few mm of the plastic.
• Addition of a substance with large organic
molecules captures the uv and reradiates it as
visual photons, where the plastic is much more
transparent. These second generation photons
bounce around in the plastic and some hit the
photomultiplier at the end.
What does the photomultiplier do?
• It is misnamed – it first converts photons to
electrons and then multiplies the number of
electrons.
• A photon hits the photocathode and simple
Einstein photoeffect ejects an electron (about
30% of photons make photo electrons). A series
of carefully placed and shaped plates and
successively higher voltages direct that first
electron to hit the first plate where it ejects 3 or 4
electrons which are directed to the next plate
where each of these 3 makes 3 or 4, etc.
• A phototube typically has 10 or 12 such
stages and turns each initial photoelectron
into a cascade of a few million electrons.
That is a manageable electrical signal
which is fed into logic cards. These cards
look for instances when several of the
attached scintillator paddles gave a signal
at the same time, and those are the events
of interest.