MARKUS KLUTE: Welcome back to 8.701.
So in this section, we talk about calorimetry.
In contrast to the discussion of tracking detectors,
here what we're trying to do is measure
the energy of the particles.
And we do this by basically destroying them.
The underlying content is rather straightforward.
We have a particle.
And we put a piece of material in front of it,
such that it slams into it, and the energy
deposited by the particle is the energy, the measurement
we try to undertake.
So in nuclear and particle physics,
that is exactly what we refer to as calorimetrics.
So the detection of particles for measuring the properties
through the total absorption in the block of matter.
The common feature, or the central feature,
is that the measurement is destructive.
So again, in tracking detectors, we
try to minimally disturb the particle
and in calorimetrics, we try to destroy them.
The exception to this might be a muon which
might at high energies deposit only a small fraction
of its energy in the calorimeter,
or a neutrino, which just flies through
without having any interaction.
But the purpose is really to measure energies
by destroying the particle.
And it's widely used in all kinds of areas of particle
and nuclear physics.
Neutrino experiments, proton decay experiments, cosmic ray
detectors, collider experiments, and so on.
And in collider experiments specifically, the idea
is to build the detector such that it completely surrounds
the interaction region, such that you don't lose energy
from the collision just passing through an uninstrumented
region.
The detection mechanism can vary quite a bit.
We use scintillators a lot.
We use silicon in some modern detectors.
With ionization, we use Cherenkov detection.
We use sometimes cryogenic detectors,
which are very sensitive to very small energy depositions,
and they can be quite useful.
They are used in dark matter experiments
or on neutrino [INAUDIBLE] decay experiments like so.
Again, conceptually, you can differentiate
between homogeneous calorimetrics and sampling
calorimeters.
The homogeneous calorimeters, basically the entire absorbable
material is equal or is the same as the detector material.
So an example for this is lead glass, which is often used.
So what you do then in the calorimeter
is you induce electromagnetic and nuclear showers
and then the energy of the incoming party
is converted into photons.
And then what you need is a photodetector,
which then measures the number of photons coming out
of your detector material.
For this to work, the detector needs to be transparent.
Alternatively, one can use sampling experiment,
sampling detectors, where you have the heavy material being
used in order to induce a shower and then the detection material
in order to count, again, the number of photons.
So homogeneous calorimeter have typically very good energy
resolution.
And the reason for this is that nothing gets lost.
Everything is being measured in the absorber, which
is the detector.
But that leads then to some limitations.
For example, that the granularity of the detector
is typically limited.
And then there's no longitudinal information
about the shower development.
You basically have one block.
For example, a left blockage is shown here,
the tungsten block, which is shown here,
are used for the measurement.
So you produce photons, and so then the photons
need to be measured.
And then that's done with photodetectors.
Here, the requirements, the range of requirements,
is quite big.
Sometimes you want to be able to measure every single photon
so the quantum efficiency needs to be quite high.
In other detectors, you need to be
able to put this detector in a radiation hot environment.
And so that then changes.
The main types available are the old-fashioned photomultiplier
tubes, which actually become quite sophisticated, PMTs.
There's gas-based photodetectors.
There's solid-state detectors, which are quite popular,
so-called SIPM silicon photodetectors
or some hybrid modules of those.
So the energy resolution in a calorimeter
depends on a number of things.
As I was saying, one measures the number of particles
being produced in a shower.
And so that's just a counting experiment.
And the uncertainty of that scales
to the square root of the number of particles
produced or measured.
And so here, we have a square root n term.
So the relative energy measurement
has an arrow, which is with 1 over square root of the energy.
And then there's more contributions.
There's constant terms, which come from inhomogeneities.
Those are elements where there's just
no detector, no equipment in the direction of the party.
But those can be overlapped regions or regions
where you have two detector modules being glued together.
You don't measure there.
And that leads to them, a constant term in the energy
resolution.
And then when you translate the signal,
the electromagnetic signals into an electronic signal,
there can be noise induced, and that noise then
leads to a typical turn, which goes with one over the energy.
So very classical, you have those three components
to the energy--
three components to the energy resolution.
One is this 1 over the square root of the energy.
One is constant, and one is depending on 1 over the energy.
And so when you design a detector,
you want to place it such or design it such
that the most important physics you
want to do with this detector is optimize
towards those components.