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

1. 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.

2. 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.

3. Tracking Detectors • Old Style - Photographic • Cloud Chambers • Bubble Chamber • Modern - Ionization • Multiwire proportional chambers • Time projection chambers • GEM and Silicon

4. 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 ….

5. 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

6. Gargamelle – bubble chamber @ CERN that discovered neutral currents.

7. Drift Chambers – Faster! -HV Invented by Georges Charpak (actually MWPC) and won Nobel prize in 1992 Fill chamber with evenly spaced wires that are raised to a positive potential WRT to cathode planes (see lines of equipotential -> ) Fill chamber with gas Charged particles will ionize the gas. Electrons move toward wires Electrons accelerate and produce avalanche Produces signals in the wires so you can determine where the charged particle traversed! -HV

8. Region III Drift chambers in CLAS detector at Jefferson Lab

9. Time Projection Chambers Invented by Dave Nygren at LBL Needed for high track density experiments where drift chambers are completely saturated Huge Chamber full of gas that is ionized by the charged particles Electric field is set up so electrons go toward endcaps Often magnetic field is used for charge separation Total charge deposited @ ends gives total ionization and therefore dE/dx and PID Endcaps have MWPC on end to collect the signal and determine x,y location. Z location determined by drift time Must know your drift velocity well over time in order to calibrate your TPC

10. STAR TPC

11. 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 CMS ATLAS

12. 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

13. 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 • 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.

14. 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 Super Kamiokande

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