Why are low energy neutrons more dangerous than high energy neutrons?

Generally radiation causes damage to cells because it ionizes atoms. This can break chemical bonds (ie mess with DNA) and create radicals.

The relevant quantity for discussing the effects of irradiation is the absorbed dose D.

Units are Gray = 1 J/Kg or rad = 100 erg/g = 0.01 Gray.

The dose doesn’t account for the TYPE of radiation. To account for this type we need a radiation weighting factor. The relative biological effectiveness (RGB) is a number that expresses the relative amount of damage that a fixed amount of ionizing radiation can inflict on biological tissues.

Radiation Type

RGB

Photons 1

Electrons and muons

1

Neutrons

<10 keV 5

10-100 keV 10

100 keV to 2 MeV

20

2 MeV to 20 MeV

10

> 20 Mev 5

Protons > 2 MeV

5

Alpha particles 20

Why are low energy neutrons more dangerous than high energy neutrons?

Neutron has no electric charge so it doesn’t see the e- in matter

Interacts via the strong force with nuclei The strong force is short range so interactions are rare and

neutron penetrates deeply into matter (1st bad sign) While traversing matter they can scatter elastically from

nuclei They can scatter inelastically – leaving an excited nucleus

which decays by gamma-ray or some other radiative emission, but neutron must have at least 1 MeV of energy

The radiative capture cross-section increases with decreasing velocity. Radiative capture produces photons which can also cause damage

Finally neutrons undergo beta decay which produces a proton that can go on to ionize material further.

Tracking Detectors

Old Style - Photographic Cloud Chambers Bubble Chamber

Modern - Ionization Multiwire proportional

chambers Time projection

chambers GEM and Silicon

Cloud Chambers

Charles Wilson (1869-1959)

In Wilson's original chamber the air inside the sealed device was saturated with water vapor, then a diaphragm is used to expand the air inside the chamber (adiabatic expansion). This cools the air and water vapor starts to condense. When an ionizing particle passes through the chamber, water vapor condenses on the resulting ions and the trail of the particle is visible in the vapor cloud

Wilson, along with Arthur Compton, received the Nobel Prize for Physics in 1927 for his work on the cloud chamber.

Cosmotron at BNL utilized a cloud chamber ….

Bubble chambers

Invented by Donald Glaser – won Nobel Prize in 1960

Fill chamber with liquid that serves as nuclear target (Hydrogen)

Overpressure liquid ( heat it up to almost boiling)

Fire up the beamline Superheat the liquid by suddenly

reducing pressure so that Hydrogen T > boiling point.

Remnants of collisions ionize the liquid and form bubbles

Bubbles expand and pictures are taken

Chamber is compressed and ready for new cycle

~1 sec cycle

Gargamelle – bubble chamber @ CERN that discovered neutral currents.

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ν u + e−→ν u + e

−

Drift Chambers – Faster!

1) Invented by Georges Charpak (actually MWPC) and won Nobel prize in 1992

2) Fill chamber with evenly spaced wires that are raised to a positive potential WRT to cathode planes (see lines of equipotential -> )

3) Fill chamber with gas 4) Charged particles will ionize

the gas. Electrons move toward wires

5) Electrons accelerate and produce avalanche

6) Produces signals in the wires so you can determine where the charged particle traversed!

-HV

-HV

Region III Drift chambers in CLAS detector at Jefferson Lab

Time Projection Chambers1) Invented by Dave Nygren at LBL2) Needed for high track density experiments where drift

chambers are completely saturated3) Huge Chamber full of gas that is ionized by the charged

particles4) Electric field is set up so electrons go toward endcaps5) Often magnetic field is used for charge separation6) Total charge deposited @ ends gives total ionization and

therefore dE/dx and PID7) Endcaps have MWPC on end to collect the signal and determine x,y location.

8) Z location determined by drift time

9) Must know your drift velocity well over time in order to calibrate your TPC

STAR TPC

Silicon Detectors

Used for precision tracking Use strips of silicon - each with small amount of mass

so particles don’t deposit much energy. Can use near beamline.

Electrons are knocked out of silicon and collected by metallic strips attached to silicon strips

http://www.atlas.ch/multimedia/#pixel-silicon-tracker

ATLAS CMS

Calorimeters

A calorimeter is a detector that measures “energy” of the particles that pass through. Ideal it stops all particles of interest.

Usually made of an active and a passive layer Passive is usually a high density material that causes a lot of

interactions with the particle of interest (LEAD). It serves to slow down the particle.

The active layers collect light Two types

1) Hadronic - measures energy of all particles made of quarks 2) Electromagnetic – measures energy of electrons, positrons and

photons

Electromagnetic can be much thinner because electrons lose energy faster than hadrons due to radiation.

Calorimeters stop everything except muons and neutrinos

Cerenkov Counters Based on Cerenkov radiation Cerenkov radiation arises when a charged particle in a material

medium moves faster than the speed of light IN THAT MEDIUM

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βC = v =c

n n = index of refraction is unity in vacuum If particle velocity is > βC then it will emit Cerenkov radiation measurement of angle θC gives particle velocity. often used for electron/pion discrimination.

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cosθC =1

βn(ω)

Super Kamiokande

• located 1000 m underground• Holds 50,000 tons of ultra pure water• Neutrinos + neutron scattering• m/e scattering leads to Cerenkov radiation• e- multiple scatter and cause fuzzier rings

Detector Packages

http://public.web.cern.ch/public/en/research/Detector-en.html