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MARKUS KLUTE: Welcome
back to 8.701.

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So in this section, we
talk about calorimetry.

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In contrast to the discussion
of tracking detectors,

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here what we're trying
to do is measure

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the energy of the particles.

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And we do this by
basically destroying them.

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The underlying content is
rather straightforward.

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We have a particle.

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And we put a piece of
material in front of it,

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such that it slams
into it, and the energy

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deposited by the particle is
the energy, the measurement

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we try to undertake.

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So in nuclear and
particle physics,

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that is exactly what we
refer to as calorimetrics.

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So the detection of particles
for measuring the properties

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through the total absorption
in the block of matter.

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The common feature, or
the central feature,

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is that the measurement
is destructive.

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So again, in tracking
detectors, we

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try to minimally
disturb the particle

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and in calorimetrics,
we try to destroy them.

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The exception to this
might be a muon which

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might at high energies
deposit only a small fraction

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of its energy in
the calorimeter,

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or a neutrino, which
just flies through

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without having any interaction.

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But the purpose is really
to measure energies

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by destroying the particle.

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And it's widely used in all
kinds of areas of particle

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and nuclear physics.

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Neutrino experiments, proton
decay experiments, cosmic ray

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detectors, collider
experiments, and so on.

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And in collider experiments
specifically, the idea

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is to build the detector such
that it completely surrounds

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the interaction region, such
that you don't lose energy

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from the collision just passing
through an uninstrumented

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region.

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The detection mechanism
can vary quite a bit.

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We use scintillators a lot.

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We use silicon in
some modern detectors.

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With ionization, we use
Cherenkov detection.

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We use sometimes
cryogenic detectors,

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which are very sensitive to
very small energy depositions,

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and they can be quite useful.

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They are used in dark
matter experiments

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or on neutrino [INAUDIBLE]
decay experiments like so.

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Again, conceptually,
you can differentiate

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between homogeneous
calorimetrics and sampling

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calorimeters.

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The homogeneous calorimeters,
basically the entire absorbable

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material is equal or is the
same as the detector material.

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So an example for this is lead
glass, which is often used.

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So what you do then
in the calorimeter

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is you induce electromagnetic
and nuclear showers

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and then the energy
of the incoming party

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is converted into photons.

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And then what you need
is a photodetector,

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which then measures the
number of photons coming out

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of your detector material.

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For this to work, the detector
needs to be transparent.

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Alternatively, one can
use sampling experiment,

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sampling detectors, where you
have the heavy material being

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used in order to induce a shower
and then the detection material

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in order to count, again,
the number of photons.

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So homogeneous calorimeter
have typically very good energy

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resolution.

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And the reason for this
is that nothing gets lost.

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Everything is being measured
in the absorber, which

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is the detector.

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But that leads then
to some limitations.

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For example, that the
granularity of the detector

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is typically limited.

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And then there's no
longitudinal information

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about the shower development.

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You basically have one block.

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For example, a left
blockage is shown here,

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the tungsten block,
which is shown here,

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are used for the measurement.

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So you produce photons,
and so then the photons

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need to be measured.

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And then that's done
with photodetectors.

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Here, the requirements,
the range of requirements,

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is quite big.

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Sometimes you want to be able
to measure every single photon

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so the quantum efficiency
needs to be quite high.

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In other detectors,
you need to be

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able to put this detector in
a radiation hot environment.

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And so that then changes.

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The main types available are the
old-fashioned photomultiplier

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tubes, which actually become
quite sophisticated, PMTs.

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There's gas-based
photodetectors.

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There's solid-state detectors,
which are quite popular,

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so-called SIPM
silicon photodetectors

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or some hybrid modules of those.

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So the energy resolution
in a calorimeter

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depends on a number of things.

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As I was saying, one measures
the number of particles

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being produced in a shower.

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And so that's just a
counting experiment.

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And the uncertainty
of that scales

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to the square root of
the number of particles

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produced or measured.

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And so here, we have
a square root n term.

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So the relative
energy measurement

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has an arrow, which is with 1
over square root of the energy.

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And then there's
more contributions.

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There's constant terms, which
come from inhomogeneities.

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Those are elements
where there's just

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no detector, no equipment in
the direction of the party.

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But those can be overlapped
regions or regions

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where you have two detector
modules being glued together.

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You don't measure there.

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And that leads to them, a
constant term in the energy

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resolution.

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And then when you
translate the signal,

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the electromagnetic signals
into an electronic signal,

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there can be noise induced,
and that noise then

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leads to a typical turn, which
goes with one over the energy.

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So very classical, you
have those three components

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to the energy--

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three components to
the energy resolution.

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One is this 1 over the
square root of the energy.

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One is constant, and one is
depending on 1 over the energy.

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And so when you
design a detector,

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you want to place it
such or design it such

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that the most
important physics you

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want to do with this
detector is optimize

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towards those components.