exploring the universe with particles and rays: α, β, γ, x, cosmic, … toby burnett prof, uw

Exploring the Universe with

Particles and Rays:α, β, γ, X, Cosmic, …

Toby BurnettProf, UW

WALTA workshop 30 July-3 Aug 2001 2

So what is a ray ?

ray :A thin line or narrow beam of light or other radiant energy.

ra·di·a·tion :1. Emission and propagation and emission of energy in the

form of rays or waves. 2. Energy radiated or transmitted as rays, waves, in the

form of particles. 3. A stream of particles or electromagnetic waves emitted

by the atoms and molecules of a radioactive substance as a result of nuclear decay.

Common theme: A source emits a particle, which propagates in space, until it interacts with another particle, . producing secondary particles that are detected


Dictionary

prop·a·ga·tion The act or process of propagating, especially

the process by which a disturbance, such as the motion of electromagnetic or sound waves, is transmitted through a medium such as air or water.


Waves or particles?

Particle: point-like objectWave: spread-out

Quantum mechanics in a nutshell: Light, and all radiation, is discrete. (quantized) The quanta of light are particles called photons, that have

zero mass. (Same for all EM waves.) Emission and absorption: behave like particles. Propagation is wave-like. (Famous example: double-slit) Energy and frequency are the same thing! (E=hf) Same for momentum and inverse wavelength (p=mv=h/)

Special relativity: No particle can travel faster than the speed of light c = 3x108

m/s = 30 cm/ns. E2 = m2 c4 + p2 c2


Experiments and observations

Recall the theme: sourcepropagationinteractionsecondariesdetector

Each stage can provide info about the world around us.Consider a microscope.

detection system

(light) source

interaction

photon detector

Wavelength: limits resolving power. Solution: use higher energy (like electrons)

Same paradigm as particle accelerators, one reason for high energy


The electon Volt

SI unit of energy: joule. We use the electron Volt (eV) energy gained by an electron passing through a electrical potential difference V of 1 Volt. (Energy is force x distance)1 eV = 1.6 x 10-19 J. This is the scale of chemical interactions.

Photons detected from space:

3 degree microwave background

3 x 10-4 eV

Visible light 3 eV

Highest energy ever seen

109 eV = 1 TeV

Prefixes

MeV 106

GeV 109

TeV 1012


A rather big microscope

The Fermilab Tevatron particle accelerator.

accelerates protons to 1 TeV

uses some to make anti-protons (in collisions)

accelerates anti-protons to 1 TeV.

collides them!Why?

Study nature of matter by analyzing produced particles


The CR spectrum revisited

Flu

x

part

icle

s/(

m2

sr

s G

eV

)

Kinetic Energy (GeV)


Solid angle (Greek omega, )

What’s a steradian? Ordinary (plane) angles are a measure of

the opening between 2 lines How can we express a field of view on the

sky in terms of “solid angle”? Recall: 1 radian = 57 degrees

(angle subtended by an arc of length = R on a circle)

[ 1 arcsec = 4.8x10-6 radian]

By analogy: solid angle subtended by an area of 1 m2 on a sphere of radius 1 m = 1 steradian (sr)

Whole sphere = 4 sr Recall: surface area of sphere= 4R2

Hemisphere of sky visible = 2 sr

RR


A little exercise

I am interested in the low-energy part of this spectrum, which is a background, for me!

The total, integrated over energy, flux of cosmic rays near the Earth is ~100 particles/(m2 sr s)

Suppose we have a horizontal square detector, 1 m on a side, exposed to this flux. What is the rate of particles passing through it from above? Assume it is isotropic.


Air showers


What is an air shower?

•Cosmic ray enters the upper atmosphere and interacts..•Secondary particles interact also..•The cascade grows…•The shower front sweeps through the atmosphere•Shower develops in space and time•The shower is axially symmetric


Particles every cosmic rayer should know and love…

Protons smash nuclei and produce pionsPions decay after 10 nanosec (in their own rest frame), into a

muon and a neutrinoMuons also decay radioactively, after 2.2 microsec (in their own

rest frame) into an electron and 2 neutrinosNeutrinos are almost massless, and stable

(but because they are not precisely massless, they can turn into other kinds of neutrinos by a quantum process called neutrino oscillation)

Particle Mass Lifetime

proton (p) 1 GeV Stable

electron (e) 0.5 MeV stable

pion () 140 MeV 10 nsec

muon () 105 MeV 2.2 microsec

neutrino() ~0.1 eV (?) Stable

Rule of thumb from relativity: divide energy of particle by its rest mass to get the Lorentz Factor = E/m: then its lifetime in our

rest frame = tt = lifetime in

particle’s rest frameExample: a 1 GeV

muon has =10,

so its lifetime would be about 22

microsec in our reference frame


The Greek alphabet

alpha -particle (helium nucleus)

beta -particle (electron)

gamma -ray (photon=“particle” of light)

delta (small difference)

epsilon (a very small number)

theta plane angle

sigma error, cross-section (prob.of interaction)

Omega solid angle

Selected letters from the Greek Alphabet (alpha-beta) -- and things they are commonly used for in physics…


Cosmic ray timeline

1911-12 Discovery by Victor Hess: from space

1930s East-west and latitude effects discovered by Compton, Millikan and others: CRs are charged particlesDiscovery of air showers by Pierre Auger

1940s Discovery of short-lived elementary particles produced by cosmic rays: origin of particle physicsFirst large-scale extensive air shower detector built at Echo Lake, CO

1950s EAS detectors around the world find UHE cosmic rays come from all directions

1960s Supernovae found to be likely source of cosmic rays (below 1016 eV)

1970s Fly’s Eye built in Utah(J. Wilkes does thesis research at Echo Lake)

1980s AGASA detector built in JapanCASA detector built in USA

1990s Fly’s Eye, AGASA observe cosmic rays with energy > 1020 eV


Victor Hess after his 1912 balloon flight, during which he discovered cosmic rays come from

space.


Jeff Wilkes and Heather Zorn re-enact Hess’s flight

(Sunday, July 8)

Greg Snow holds a replica of Hess’s electroscope carried on the balloon flight


Pierre Auger, discoverer of cosmic ray air showers.


Neutrinos

An aside: can those neutrinos from interations in the atmosphere be detected?

A big local angle!


The Super-Kamiokande Detector

Control Room

Inner

DetectorOuter Detector

Mt. Ikeno

Entrance 2 km

Water System

Tank

Linac caveElectronics Huts

50,000 ton water Cherenkov detector (40m tall!)ID: 11,146 50cm (20”)PMTs, non-reflective liner OD: 1,885 20cm (8”) PMTs + wavelength shifters, reflective liner (Tyvek)Taking data continuously since 5/31/96Detects solar neutrinos (5-20 MeV), atmospheric neutrinos (0.2—2 GeV), search for proton decay, search for supernovae (10,000 events!)


• Produced by cosmic rays in upper atmosphere (altitude Z=15~20 km)

p+nucleus other particles

• Flight path L to SK detector depends on zenith angle Z:

Atmospheric Neutrinos

cosZ=+0.8

L=25 km

cosZ=-0.8

L=10000 km

cosZ=0

L=500 km

Z

ZENITH

NADIR

SK

• Energy 100 MeV ~100 GeV

Can’t be solar: ESOLAR~20 MeV max

Can’t be astrophysical: flux << atmospheric below ~10 TeV


20” Photomultiplier Tube

Hamamatsu R3600 PMT• Single-photon efficiency 22%• Rise time 1.25~2.5 nanosec


The SK Yacht Club at sea (1/96)


Water Cherenkov Detector

Arrival times shown in nanosec(t=0 when interacts)

Cherenkov light wavefronts

light rays (v=0.75c)

0 7.3 12.7

21.9

20.4

19.0

17.5

17.0

61

66

71

76

80

Relative light intensities shown in arbitrary units

v=c

water (n=1.33)

PMTs


Cherenkov ring

Electron Neutrino Event

Inner Detector

Outer Detector


Solar Neutrinograph

= 1° on the skyExposure: 500 days

exploring the universe with particles and rays: α, β, γ, x, cosmic, … toby burnett prof, uw

Documents