1. 21 Jan 2005, Lecture 12 Nuclear Physics Lectures, Dr. Armin Reichold 1 Lectures 12-14 Particle interactions with matter

2. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 2 12.0 Overview 12.1 Introduction 12.2 Charged particles in matter Classification of interactions Non-radiating interactions (ionisation) Radiating interactions Ionisation and the Bethe-Bloch formula Radiating interactions Cherenkov-radiation Transition-radiation Bremsstrahlung Synchrotronradiation The em-shower 13.1 Photons in matter Photoelectric effect Raleigh scattering Compton scattering Pair production  -Nucleus interactions 13.2 Detectors For photons only Photomultiplier and APD For charged particles and  ’s Photography Scintillators Gas-counters Semi-conductors (GeLi, Si) em-calorimeters (see particle physics course) 13.3 Radiation units

3. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 3 12.1 Introduction (why do we need to know this) Measure properties of nuclei through decay product particles Measure energy, momentum, mass & charge of particles with M  [0 (  ) ; few 100 GeV (fission fragment)] E kin  [keV (Radioactiviy) ; few GeV (accelerator experiments)] Q/e  [0 ( ,n); O(100) (fission fragments)] Need to translate microscopic particle properties into quantitatively measurable macroscopic signals Do this by interactions between particles and matter Which interactions would be useful? Weak?  Too weak at low (nuclear) interaction energies Strong?  Some times useful but often noisy (strong fluctuations, few scattering events per distance) EM?  Underlies most nuclear and particle physics detectors Energies released ≤ E kin (particle) often too small for direct detection  need amplification of signals

4. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 4 12.1 Introduction Particle Ranges a)If smooth energy loss via many steps (i.e. ionisation from light ions)  sharply defined range, useful for rough energy measurement a) b) c) Sometimes several types of processes happen (i.e. high energy electrons)  mixed curves, extrapolated maximum range b)If few or single event stop particle (i.e. photo-effect)  exponential decay of particle beam intensity, decay constant can have useful energy dependence

5. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 5 12.1 Introduction Particles we are interested in photons exponential range (at low E often get absorbed in single events) detect secondary electrons and ions liberated in absorption process. charged particles sharper range (continuously loose energy via ionisation) leave tracks of ionisation in matter  measure momentum in B sometimes radiate photons  can be used to identify particle type neutrons electrically neutral  no first-order em-interaction  devils to detect react only via strong force (at nuclear energies!) long exponential range (lots of nuclear scattering events followed by absorption or decay) need specific nuclear reactions to convert them into photons and/or charged particles when captured by a target nucleus if stopped, measure decay products, electron + proton + anti neutrino

6. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 6 12.2 Charged particles in matter (classification of interactions) If particle or medium emit photons, coherent with incoming particle  radiation process Bremsstrahlung, Synchrotron-radiation: emitted from particle Cherenkov-radiation, Transition-radiation: emittted from medium If not  non-radiating process Ionisation, scattering of nuclei or atoms

7. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 7 12.2 Charged particles in matter (non radiating interactions, what to collide with) What could a charged particle collide with Atomic electrons (“free”)  large energy loss  E≈q 2 /2m e (small m e, q=momentum transfer)  small scattering angle Atomic nuclei  small energy loss (  E=q 2 /2m nucleus )  large scattering angle Unresolved atoms (predominant at low energies)  medium energy loss (  E m e (free))  medium scattering angle  atoms get excited and will later emit photons (scintillation)

8. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 8 12.2 Charged particles in matter (Ionisation and the Bethe-Bloch Formula) Deal with collisions with electrons first since these give biggest energy loss. Task: compute rate of energy loss per pathlength, dE/dx due to scattering of a charged particle from electrons in matter. Remember a similar but inverse problem? Scatter electrons of nuclei = Mott scattering

9. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 9 12.2 Charged particles in matter (Ionisation and the Bethe-Bloch Formula, Rutherford vs. Mott) Rutherford Scattering  (q=Z’e)  nucleus (q=Ze) spin-0  spin-0 point  point  no form-factors non-relativistic M nucl = infinite  no recoil first order perturbation theory (ZZ’  em <<1) Mott Scattering e - (q=1e)  nucleus (q=Ze) spin-½  spin-0 unpolarised electrons (average over all incoming spin orientations) point  point  no form- factors relativistic M nucl = infinite  no recoil first order perturbation theory (Z1  em <<1)

10. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 10 12.2 Charged particles in matter (Ionisation and the Bethe-Bloch Formula, Mott) Differential Mott-scattering crossection Has familiar Rutherford part and new part from spin of relativistic electron It assumes scattering of free electron i.e. V projectile >>V bound-e (deal with bound electrons later) It assumes M nucl >>me  would need recoil corrections to apply results to dE/dx of electrons passing through matter P,V = momentum and relative velocity of electron wrt. nucleus Z = charge of nucleus  = scattering angle of the electron Rutherford term

11. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 11 12.2 Charged particles in matter (Ionisation and the Bethe-Bloch Formula, transforming Mott) Change variables from  to q 2 (q = momentum transfer to electron) to get to frame independent form P P’ q 

12. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 12 12.2 Charged particles in matter (Ionisation and the Bethe-Bloch Formula, transforming Mott)

13. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 13 12.2 Charged particles in matter (Ionisation and the Bethe-Bloch Formula, changing frames for Mott) Change frame to: electron stationary, nucleus moving with V towards electron p in formula is still momentum of electron moving with relativeV  p =m e  V q 2 is frame independent non-relativistic this is obvious (do it at home) relativistic need to define q as 4-momentum transfer Energy transfer to the electron is then defined via:  E= =|q 2 |/2m e  d /dq 2 =1/2m e

14. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 14 Above is crossection for heavy particle of charge z to loose energy between and +d in collision with electron it approaches with velocity V We want kinetic energy lost = -dT per path length dx in material of atomic number density n with Z’ electrons per atom 12.2 Charged particles in matter (Ionisation and the Bethe-Bloch Formula, Mott  Bethe Bloch) number of colissions with electrons in length dx average energy lost per collision p =m e  V |q 2 |= /2m e

15. 21 Jan 2005, Lecture 12 15 12.2 Charged particles in matter (Ionisation and the Bethe-Bloch Formula, simple integral) max via kinematics of “free” electron since E bind << E

16. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 16 12.2 Charged particles in matter (Ionisation and the Bethe-Bloch Formula, min ) But what about min ? can not assume that e is free ≠q 2 /2m e because electron bound to atom can get excited atoms in final state (not just ions)  our integral was wrong for the lower limit! For low need 2-D integral d dq depending on detailed atomic structure Important part of integral but very hard to do result is the Bethe-Bloch Formula

17. 17 12.2 Charged particles in matter (Ionisation and the Bethe-Bloch Formula) Stopping power = mean energy lost by ionisation perpendicularly traversing layer of unit mass per area. Units: Mev g -1 cm 2, Range: 4.1 in H to 1.1 in U I=mean excitation energy; depends on atom type, I≈11 Z [eV]

18. 18 12.2 Charged particles in matter (Ionisation and the Bethe-Bloch Formula, Bethe-Bloch features)  =density correction: dielectric properties of medium shield growing range of Lorenz-compacted E-field that would reach more atoms laterally then normal. Function of  nearly independent of M (except max, relevant at large E)  if know p and measure   get M (particle ID via dE/dx), best in 1/V 2 region Nearly independent of medium (Z/A ≈½) Broad minimum (at  ≈3.0 at Z=100 to 3.5 at Z=7) where stopping power nearly independent of particle and material (values at minimum vary from 1.1 to 1.8 MeV g -1 cm 2 )  particle is called minimum ionising (MIP) Limitations: wrong for very low V (ln goes negative  particle gains Energy = stupid) not useful for very large V (particle starts radiating, see next chapter)

19. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 19 12.2 Charged particles in matter (Ionisation and the Bethe-Bloch Formula, variation with  )  + can capture e - E  c = critical energy defined via: dE/dx ion. =dE/dx Brem.

20. 20 12.2 Charged particles in matter (Ionisation and the Bethe-Bloch Formula, variation with material) M>>me Q=+1e

21. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 21 12.2 Charged particles in matter (Ionisation and the Bethe-Bloch Formula, variation with particle type) P=m  v=m  c usefull for particle ID in drift chamber gas in drift chamber gas

22. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 22 12.2 Charged particles in matter (Radiating Interactions) Emission of scintillation light is secondary process occurring later in time. Has no phase coherence with the incident charge and is isotropic and thus SCINTILLATION NOT A RADIATING INTERACTION in this sense. Other primary radiation processes which are coherent and not isotropic are: Cherenkov radiation is emitted by the medium due to the passing charged particle. Bremsstrahlung and Synchrotron Radiation are emitted by charged particle itself as result of its environment. In both cases radiation is Doppler shifted to high energy by the Lorentz transformation from particle CM frame to the Laboratory frame.

23. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 23 12.2 Charged particles in matter (Cherenkov Radiation) Source of field passing through medium at a v > v phase (light in medium) creates conical shock wave. Like sonic boom or bow wave of a planing speed boat. Not possible in vacuum since v c/n. There is a threshold at  = 1/n use to measure  and thus do particle ID. at x-ray frequencies and above n < 1  Cherenkov radiation is confined to low energy photons, mainly in the visible (blue light in reactors). Simple Huygens secondary wavelet construction gives angle of shockwave as cos  = 1/  n, use to measure direction and . In time that the particle goes from O to P, light goes from O to A. Cherenkov radiation first used in discovery of antiproton (1954). Now often used in large water-filled neutrino detectors and for other particle physics detectors (see Biller). Total energy emitted as Cherenkov Radiation is ~0.1% of other dE/dx.  ct/n  ct O P A particle trajectory

24. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 24 12.2 Charged particles in matter (Cherenkov Radiation) Picture of Cherenkov light emitted by beta electrons in a working water cooled nuclear reactor.

25. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 25 12.2 Charged particles in matter (Transition Radiation) Charged particle radiates when traversing interface between two dielectric media with different n Use this by stacking many such interfaces behind each other (foil stack or foam) to get many photons. Soft x-ray (2-20 keV) part of transition radiation spectrum from particles with  >1000 can be used for particle ID since energy lost into this part of the spectrum ~  Negligible fraction of total energy loss goes to transition radiation

26. 26 12.2 Charged particles in matter (Bremsstrahlung = Brakeing Radiation) Due to acceleration of incident charged particle in nuclear Coulomb field Radiative correction to Rutherford Scattering. Continuum part of x-ray emission spectra. Emission often confined to incident electrons because radiation ~ (acceleration) 2 ~ mass -2. Lorentz transformation of dipole radiation from incident particle centre-of-mass to laboratory gives narrow (not sharp) cone of blue-shifted radiation centred around cone angle of  =1/ . Radiation spectrum very uniform in energy. Photon energy limits: low energy (large impact parameter) limited through shielding of nuclear charge by atomic electrons. high energy limited by maximum incident particle energy. Ze e - 

27. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 27 12.2 Charged particles in matter (Bremsstrahlung  EM-showers, Radiation length) dT/dx| Brem ~T (see Williams p.247)  dominates over dT/dx| ionise ~ln(T) at high T. For electrons Bremsstrahlung dominates in nearly all materials above few 10 MeV. E crit (e - ) ≈ 600 MeV/Z If dT/dx| Brem ~T  dT/dx| Brem =T 0 exp(-x/X0) Radiation Length X0 of a medium is defined as: distance over which electron energy reduced to 1/e. X0 ~ Z 2 approximately. Bremsstrahlung photon can undergo pair production (see later) and start an em-shower (or cascade) Length scale of pair production and multiple scattering are determined by X0 because they also depend on nuclear coulomb scattering.  The development of em-showers, whether started by primary e or  is measured in X0.

28. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 28 12.2 Charged particles in matter (EM-shower model) Simple shower model assumes: E 0 >> E crit only single Brem-  or pair production per X0 The model predicts: after 1 X0, ½ of E 0 lost by primary via Bremsstrahlung after next X0 both primary and photon loose ½ E again until E of generation drops below E crit At this stage remaining Energy lost via ionisation (for e +- ) or compton scattering, photo- effect (for  ) etc. Abrupt end of shower happens at t=t max = ln(E0/Ecrit)/ln2 Indeed observe logarithmic depth dependence

29. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 29 12.2 Charged particles in matter (Synchroton Radiation) Appears mainly in circular accelerators (mainly to electrons) and limits max. energy achievable. Similar to Bremsstrahlung Replace microscopic force from E-field in Bremsstrahlung with macroscopic force from v x B Electrons radiate only to the outside of ring because they are accelerated inward Angle of maximum intensity of synchrotron radiation with tangent of ring  =1/  Synchrotron radiation = very bright source of broad range of photon energies up to few 10 keV used in many areas of science Many astrophysical objects emit synchrotron radiation from relativistic electrons in strong magnetic fields

30. 30 13.1 Photons in matter (Overview) Rayleigh scattering Coherent, elastic scattering of the entire atom (the blue sky)  + atom   + atom dominant at  >size of atoms Compton scattering Incoherent scattering of electron from atom  + e - bound   + e - free possible at all E  > min(E bind ) to properly call it Compton requires E  >>E bind (e - ) to approximate free e - Photoelectric effect absorption of photon and ejection of single atomic electron  + atom   + e - free + ion possible for E  < max(E bind ) +  E(E atomic-recoil, line width) (just above k-edge) Pair production absorption of  in atom and emission of e + e - pair Two varieties:  + nucleus  e + + e - + nucleus (more momentum transfer to nucleus  dominates)  + Z atomic electrons  e + + e - + Z atomic electrons both summarised via: g + g(virtual)  e + + e - Needs E  >2m e c 2 Nucleus has to recoils to conserve momentum  coupling to nucleus needed  strongly Z-dependent crossection

31. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 31 13.1 Photons in matter (Crossections) R  Rayleigh PE  Photoeffect C  Compton PP  Pair Production PPE  Pair Production on atomic electrons PN  Giant Photo-Nuclear dipole resonance Carbon Lead

32. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 32 13.1 Photons in matter (Note on Pair Production) Compare pair production with Bremsstrahlung Very similar Feynman Diagram Just two arms swapped Typical Lenth = Radiation Length X0 Typical Lenth = Pair Production Length L0 L0=9/7 X0 Ze e - e -*  Bremsstrahlung e - Ze e -* e -  Pair production e -

33. 21 Jan 2005, Lecture 12 33 PMT 13.2 Detectors (for photons only, PMT, APD) Photomultiplier: primary electrons liberated by photon from photo-cathode (low work function, high photo-effect crossection, metal, E conversion ≈¼ ) visible photons have sufficiently large photo-effect cross-section acceleration of electron in electric field 100 – 200 eV per stage create secondary electrons upon impact onto dynode surface (low work function metal)  multiplication factor 3 to 5 6 to 14 such stages give total gain of 10 4 to 10 7 fast amplification times (few ns)  good for triggers or veto’s signal on last dynode proportional to #photons impacting APD (Avalanche Photo Diode) solid state alternative to PMT strongly forward biased diode gives “limited” avalanche when hit by photon

34. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 34 13.2 Detectors (for electromagnetically interacting particles, photography) Photography: chemically change silver halide crystals by exchange of a photo-electron from organic sensitiser molecule This was how radioactivity was discovered! Used in bubble chamber images extensively now used in medical imaging or in large emulsion experiments for neutrino induced particle detection Very high spatial resolution Low cost for a single shot Can not be triggered (integrates all the time) but some electronic forms of aiding the readout exist Difficult to read out in computer form and analyse but automatic scanning methods exists

35. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 35 13.2 Detectors (for electromagnetically interacting particles, scintillators) Scintillators Particle excites atom Observe photon from de-excitation of atomic electron (eye, PMT, APD) Takes aprox. 10 * more energy to produce a scintillation photon then one electron-ion pair. Typical 1 photon per 100eV of dE/dx Very old style: Zinc sulphite screens viewed by eye (Rutherford) Scintillators today on the front of every CRT TV-tube. Problem: normally materials re-absorb their own scintillation light Two solutions to this problem exists

36. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 36 13.2 Detectors (for electromagnetically interacting particles, organic scintillators) Solution 1: Organic scintillators Naphtalene, anthracene are organic molecules, low density (  ≈1.3) excitation  non-radiating de-excitation to first excited state  scintillating transition to one of many vibrational sub-states of the ground state (direct transition to ground state is forbidden) molecule can not re-absorb this photon unless it already is in this particular vibration state gives fast scintillation light, de-excitation time O(10 -8 s) often used with wavelength shifters to avoid absorption in light guides

37. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 37 13.2 Detectors (for electromagnetically interacting particles, scintillators) Solution 2: Anorganic scintillators NaI activated (doped) with Thallium, semi-conductor, high density:  (NaI=3.6),  (PbWO 4 )=8.3  high stopping power Dopant atom creates energy level (luminescence centre) in band-gap Excited electron in conduction band can fall into luminescence level (non radiative, phonon emission) From luminescence level falls back into valence band under photon emission this photon can only be re-absorbed by another dopant atom  crystal remains transparent High density of anorganic crystals  good for totally absorbing calorimetry even at very high particle energies (many 100 GeV) de-excitation time O(10 -6 s) slower then organic scintillators

38. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 38 13.2 Detectors (for electromagnetically interacting particles, gas counter classification, see Burcham & Jobes, p.36-39) Gas Counters: 6 MeV  particle stopped in gas gives typically 2*10 5 ion pairs (30eV/ion pair) = 3.2*10 -14 C negative charge Release into C=10 pF  3.2 mV >> V noise (typ. ampl.)  detectable! Charge collected depends on collection voltage low voltage  Ionisation chamber, collect only primary ionisation medium voltage  proportional counter  avalanche (secondary or collision ionisation)  signal still proportional to primary ionisation high voltage  Geiger counter  each particle gives the same response too high voltage  continuous spark (breakdown) E p (Ar)≈10 6 V/m gas filled gap

39. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 39 13.2 Detectors (for electromagnetically interacting particles, ionisation chambers) Ionisation Chambers Used for single particle and flux measurements Can be used to measure particle energy up to few MeV with accuracy of 0.5% (mediocre) Electrons more mobile then ions  medium fast electron collection pulse O(  s) Slow recovery from ion drift replace by solid state detecors

40. 21 Jan 2005, Lecture 12 40 13.2 Detectors (for electromagnetically interacting particles, proportional chambers) Use small wire as positive electrode (anode) E=V/[r*ln(b/a)] high field close to wire  local avalanche near wire  most electrons released close to wire  short average drift distance  fast signal rise time O(ns) Use avalanche amplification to measure small ionisation Problem: UV-photons from recombination spread through volume  catch them on large organic molecules (quencher)  vibrational de-excitation Many such detectors (MWPC) used as large-area position sensitive device Can add drift time measurement to increase position resolution  Drift chamber

41. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 41 13.2 Detectors (for electromagnetically interacting particles, Geiger counters) Geiger counters Construction nearly same as proportional counter Operate with V g <V<V discharge UV photons spread avalance across complete counter volume  same signal for all particles Detection = counting of particles Long recovery time limits counting rage O(100Hz) Not much used for nuclear physics Some use in radiation protection

42. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 42 Semi Conductor Energy Main problems: need very low conductivity (high purity=intrinsic) to see current pulses above dark current avoid trapping of electrons and holes on impurities Two main solutions: junction detectors (for charged particles) Compensated (drifted) detectors 13.2 Detectors (for electromagnetically interacting particles, semi conductor detectors) Semi conductor detectors Move electrons from valence to conduction band via collision with particle  electron-hole pair Band gaps O(eV)  Energy per electron-hole pair = 3-4 eV  1 MeV lost  3*10 5 pairs  only 0.2% statistical fluctuation  excellent energy resolution E fermi band gap

43. 43 13.2 Detectors Silicon as an example semi-conductor Can not get intrinsic silicon easily (impurities) But: Can make “intrinsic” region via p-n-junction diffuse donor (n) or acceptor (p) atoms into crystal (for electromagnetically interacting particles, p-n junction semi conductor detectors)

44. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 44 13.2 Detectors (for electromagnetically interacting particles, p-n junction semi conductor detectors) A p-n junction mobile electrons and holes “anihilate” “depleted” space charge region free of charge carriers  small I leak V bi naturally occurs and stops growth of intrinsic region V bi  0.5 V typical V bi is dropped only in depletion region and produces E Fermi levels equalise extern. V bias grows depletion region: d  V bias ≤ 2mm typical + - V bi d V bias

45. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 45 13.2 Detectors (for electromagnetically interacting particles, p-n junction semi conductor detectors) p-n junction detectors Main application in position sensitive silicon detectors Large area applications in high energy physics up 100’s of m 2 Many ways to pattern the silicon wavers using semi conductor industry processes Very dynamic field of research with large number of new developments today

46. 21 Jan 2005, Lecture 12Nuclear Physics Lectures, Dr. Armin Reichold 46 13.2 Detectors (for electromagnetically interacting particles, compensated semi conductor detectors) If you can not remove charge carriers librated from impurities you can compensate them Best example: Ge is naturally slightly p-type  E gap =0.64 eV  ultra high resolution is possible add n-type impurities (Li) by diffusing them into the crystal at high temperatures freeze the crystal with liquid N 2 to avoid Li diffusing out again (keep frozen forever!!) It is THE  -spectroscopy detector up to 10s of MeV

47. 47 13.3 Radiation Units Activity of a source Becquerel (Bq) is the number of disintegrations per second. 1Bq=2.7*10 11 Curie (Ci) radiation levels sometimes quoted in Bq m -3. Absorbed Dose 1 Gray (Gy) = 1 joule of deposited energy per kg of irradiated mass 1 Gy = 100 rad = 6.24 * 10 12 MeV/kg. Equivalent Dose for biological damage 1 Sievert (Sv) = absorbed dose equivalent in damage to 1 Gy of x-rays,  or . per unit energy deposited: some particles have larger dE/dx then  or  & strong interactions  localised damage  more long term biological risk  higher weight w R then  or  See mext slide for differrent weights 1 Sv = 100 rem (Roentgen equivalent for man) Examples of Sv Lethal whole-body dose 2.5-3.0 Sv  death in 30 days without treatment Limit for radiation workers: 15 mSv yr -1 (UK) or 50 mSv yr -1 (US) Chest x-ray 0.04 mSv CT scan 8 mSv Average UK whole body dose rate 2.6 mSv yr -1 (world from 0.4 – 4 mSv yr -1 )

48. 48 13.3 Radiation Units Average breakdown of 2.6 mSv yr -1 taken from NRPB report (1995). Internally released = ( 40 K, 14 C) Weigth expresses risk from low levels of chronic exposure Main consequences in risk evaluation are cancer and leukemia Cosmic flux at sea level:  cosmic ≈ 1 min -1 cm -2 sr -1

49. 49 13.3 Radiation Units (UK as example) The “other” column contains for example fall-out from Nuclear weapons testing Chernobyl avg. annual dose [  Sv] nuclear testing Chernobyl