Topic 7. Relevance to the course

Transcription

1 Topic 7 Cosmic Rays Relevance to the course Need to go back to the elemental abundance curve Isotopes of certain low A elements such as Li, Be and B have larger abundances on Earth than you would expect Cosmic rays are the source of spallation We will see that spallation products include Li, Be and B

2 Contents We will look at What Cosmic Rays are How they interact in the atmosphere How we detect them here on Earth The AUGER project Their energy distribution here on Earth How we can explain that distribution using Fermi acceleration Cosmic ray composition First a distinction Primary Cosmic Ray Before a cosmic ray interacts in the atmosphere it is known as primary cosmic ray The interaction products are known as secondary cosmic rays Secondary Cosmic Rays

3 What happens when CRs interact? A cosmic ray is a nucleus of Hydrogen (i.e. a proton) or a heavier element - right up to Iron When a CR interacts in the atmosphere it triggers a wide range of elementary particle interactions that result in a shower of particles at Earth Interactions involving mesons Interactions involving baryons The electromagnetic shower EAS processes EAS = Extended Air Shower Mix of different processes, e.g. Note, at ground level the main signal for primary cosmic rays is a flux of muons

4 Cerenkov effect and angle! Consider such a particle travelling through a medium of refractive index n! Distance travelled by the particle is d = x t = v p t = βct since β = v p /c! Distance travelled by the EM wave is d EM = v EM t = (c/n)t! From simple geometry: cos θ = 1/βn Cerenkov effect! Note that this relationship is independent of time! Considering cos θ = 1/βn we learn a further two things: 1. There is a threshold velocity below which no Cerenkov radiation will be observed. To see Cerenkov radiation we must have β 1/n 2. When a particle is highly relativistic (v ~ c), the Cerenkov angle condition simplifies to cos θ = 1/n

5 Detecting CRs Can be done in 2 basic ways In space/upperatmosphere using satellites, balloons, etc. This studies the primary composition of cosmic rays directly On Earth by detecting the secondary cosmic rays, usually done using either Fluorescence (from excited Nitrogren) Cerenkov radiation The best example of such a detector is AUGER The Design of the Pierre Auger Observatory marries these two well-established techniques The HYBRID technique - product of 6 month design study in 1995 at FNAL Fluorescence AND Array of water Cherenkov detectors 11

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7 Water tanks as seen from Los Leones Fluorescence Site

8 View of Los Leones Fluorescence Site Six Telescopes viewing 30o x 30o each

9 Schmidt Telescope using 11 m 2 mirrors UV optical filter (also: provide protection from outside dust) Camera with 440 PMTs (Photonis XP 3062) Cosmic Ray Composition Experiments such as Auger and other experiments, particularly those deployed in the upper atmosphere/ space have brought a wealth of information on the composition of primary cosmic rays Points to note: 1. Relative to the general level of abundance of elements with Z > 1, protons are less abundant in cosmic rays. 2. Lithium, beryllium and boron are about 10 5 times more abundant. 3. The ratio of 3 He/ 4 He is about 300 times higher.

10 Cosmic Ray Composition (cont.) 4. There is a higher abundance in the region 21 < Z < 25 by a factor of (scandium, titanium, vanadium, chromium, manganese). 5. Transuranic elements are more abundant. 6. There are no antihadrons - essentially no antimatter in primary cosmic rays - presumably no galaxies of antimatter? 7. Electrons are about 1% as abundant as nuclei, and of these about 10% are positrons. i.e.: e + : e : A = 1 : 9 : 1000 Cosmic Ray Composition (contd.) (Relatively) small number of protons may be explained in one of two ways: 1. Cosmic rays originate in or near to Type Ia supernovae which are relatively deficient in H and He 2. High Ionization Potential must be overcome before acceleration can take place

11 Composition and Sources The link between relative cosmic ray abundance and ionization potential suggests that cosmic ray seed material has cooled before acceleration. Why? Since ionization potentials correspond to temperatures like 10 4 K to 10 5 K Spallation The relative over-abundance of certain elements is thought to be the result of spallation whereby medium Z nuclei are broken up into lighter nuclei via collisions with the hydrogen of the interstellar medium In this way, Carbon and Oxygen can be broken up into Lithium, Beryllium and Boron Similarly, Iron can be broken up into Sc, Ti, V, Cr, Mn

12 Spallation For example, the spallation of 12 C: p + 12 C " 11 B + 2p " 10 B + 3 He " 7 Li + 4p + 2n " 6 Li + 4 He + 3 He " 9 Be + 3p + n Similarly, the spallation of 4 He: 4 He + p " 3 He + p + n " 3 H + p + p " 3 He + e - + 2p Cosmic Ray Energy Distribution Cosmic ray flux is well understood over many orders of magnitude Flux has an E -α energy dependence

13 Sources of Cosmic Rays Solar Cosmic Rays Mainly protons, very low energies, flux changes with e.g. solar flares Anomalous Cosmic Rays Created at the outer reaches of the solar system Galactic Cosmic Rays Higher energies, galactic origin, may be accelerated in supernova remnants Energy Dependence How are the cosmic rays accelerated? The Fermi mechanism is where the macroscopic kinetic energy of a moving magnetised plasma is transferred to individual charged particles. 2 basic assumptions: 1. Each time the particle passes through the shock front it receives a fractional energy increase (call this κ) 2. After each collision there is a probability (call it Ρ) for the particle to escape the acceleration region

14 Energy Dependence (cont.) After n collisions the particle will have energy E = E 0 (1 + κ) n Rearranging: n = ln(e/e 0 ) / ln(1 + κ) The number of particles escaping (and hence remaining at E) is: N(E) = P(1 - P) n Rearranging: n = [ ln(n) - ln(p) ] / ln(1 - P) Equating n : ln(n) = ln(p) - α ln(e/e 0 ) or N = P(E/E 0 ) -α A power law spectrum as observed Highest Energy Cosmic Rays Energy distribution GZK effect Far less is known about the highest energy cosmic rays Up to tens of Joules per cosmic ray primary Where do they come from? What produces them? Do they cut-off or not? astro-ph/

15 The GZK effect Cosmic ray protons above a certain energy shouldn t be observed due to the so-called GZK effect Protons with E > ev can interact with CMB photons to produce Δ + baryon which decays to nπ + or pπ 0 effect is to lower energy of proton mean energy (ev) p Δ + γ CMB π + protons travelling >~100 Mpc have effective E max ~ ev n distance (Mpc) Results from AUGER on GZK

16 Results from AUGER on sources Auger 2010: correlation of observed CRs >55 EeV with nearby AGN is present but weak (29/69 within 3.1 o ; expect 15 for isotropic distribution) Note: only possible with UHE CRs otherwise magnetic fields affect pointing Solved problems 1. The nuclear cross-section for a nucleus of mass number A is approximately where R 0 = 1.2 fm and σ is in m 2. Using this information estimate the probability that an incident cosmic ray proton will fail to interact in the atmosphere. 2. A high energy cosmic-ray induced electron is travelling through the atmosphere at a height of about 20 km where the refractive index is Estimate the energy the electron must have in order to emit Cerenkov radiation. If its energy is far above this value, estimate the radius of the Cerenkov light pool on Earth.

17 Rest of course: Week 10: Tues May 5 th No Lecture (Science Council visit) Fri May 8 th Start of Topic 8, Topic 7 class test Week 11: Tues May 12 th end of Topic 8 (possible T8 class test) Fri May 15 th Revision plus T8 class test if not done on 12th