COSMIC RAYS

What are cosmic rays?

Early 1900’s : Electroscopes were mysteriously discharging without the presence of radioactive materials – “background radiation”.1911: Victor Hess took an electroscope up in a series of balloon experiments and found, surprisingly, that this radiation increases with altitude (a factor of 8 between 0 and 17,500 ft.)He concluded that the radiation is entering the atmosphere from space and gave it the name “cosmic radiation” or “cosmic rays” Hess awarded the Nobel Prize in 1936

What are cosmic rays? (continued)

Cosmic rays were thought to be part of the electromagnetic spectrum until the 1930’s when it was shown that they were affected by the Earth’s magnetic field and thus consisted of charged particles.

“Cosmic Rays” (charged particles arriving at earth) consist of..Galactic Cosmic RaysAnomalous Cosmic RaysSolar Energetic ParticlesSolar Wind Other: Energetic Storm Particles, Co-rotating interact. particles

STEREO -

Solar TerrestrialRelationsObservatory

Two identical, sun-pointed observatories in heliocentric orbit drifting away from the earth, one leading and one lagging

Scheduled for launch in February 2006

LET – Low Energy TelescopeTop Cover

M.I.S.C. Electronics Board

Upper Shields

L2 Detectors

L3 DetectorsInner Detector Housing

Detector Collimators

L1 Detectors

L1 Detectors

Main Housing Lower

ShieldsFront-End Electronics

Bottom Covers

Particle detection on LET is done with silicon semiconductor detectors:

L1: 20 μm x 2 cm2 L2: 50 μm x 10.2 cm2 L3: 1 mm x 15.6 cm2

LET uses the standard dE/dx vs E' technique for particle identification

Detectors are made of ion-implanted silicon

Bethe-Bloch formula for relativistic ionisation loss:

Where -dE is the total energy loss of a particle with atomic number z in length dx, e is the electron charge, ε

0 is the permittivity of free space,

me is the mass of the electron, I is the mean ionisation potential of the target,

Ne is the number density of electrons, v is the particle's velocity and

γ

it's

lorentz factor. If the medium through which the high energy particle passeshas atomic number Z and number density N, then N

e=NZ and the above

equation reduces to:

Bragg curve for 7 MeV alphas in copper

)(')( 22 vfzvNZfzdx

dE

2

222

220

42 2ln

4 c

v

I

vm

vm

Nez

dx

dE e

e

e

Range of high energy particle is found by integrating energy loss ratefrom the particle's initial energy until it stops:

E=(γ-1)Mc2 ; dE=d(γMc2)=Mvγ3dv

So R=R(z,M,E0)

Yielding:

This can be approximated bya power law:

Where k and α are empirical quantities (1)

0

02

0

0

)('/

EE

E vfz

dE

dxdE

dEdxR

0

0

3

2 )('

E

vf

dvv

z

MR

M

E

z

MkR 0

2

The integral is a function only of v0 – a measure of the initial kinetic energy per nucleon of the particle

With two stacked detectors, an incoming particle (Z,M,E,θ) will deposit energy = ΔE in the first detector and E' in the second detector.

(2)

To find z, assume M is a monotonic, continuous function of z:

M = 2z z ≤ 20

40 + 4.772(z – 20) 20 < z ≤ 21 2.132z z >21 {

Solve for z using (1) and (2), then solve for M assuming integer z.

)',,()',,()sec( EMzREEMzRt

Therefore when dE/dx is plotted against E', particles seperate into bands corresponding to their charge (z) and sub-bands corresponding to their mass (M).

From earlier, we have for the energy deposited in the ΔE and E' detectors:

So:

)('2 vfzdx

dE )(*' vgME

)(*)('*'* 2 vgvfzMEdx

dE

Si bandgap = 1.1 eV

N-Type P-TypeJunction

Detection of chargedparticles by solid state devices: p-n diffused junction type

Band Structure of Semiconductor

It is important to determine the minimum reverse-bias voltage necessary to fully deplete the detectors: too little will yield poor charge collection and too much may cause damage over time.

-120 V

-130 V

-140 V

-150 V

LET Detector: L3-11Bias

Electrons swept to n-side, holes to p-side

Net electric signal is fed to preamp→shaping amp→ADC

ADC channel varies linearly with energy, can convert ADC channel to electron-volts

This is the energy deposited in the detector by the incident particle

As described earlier – can then deduce Z, m

Mass Resolution

Using the power-law approximation for the range-energy relation (1), substitute this into (2) yielding:

Taking partial derivatives with respect to each measured quantity will show how the mass resolution couples to measurement uncertainties.

Each partial is proportional to M, so total mass res. is proportional to mass

1

1

2')'(

)sec(),,;',,,(

EEE

ZL

kZkEELM

22

2'

22

22

22

' EELM E

M

E

MM

L

M

Detector Thickness Uncertainty (σL)

Mass resolution is proportional to the accuracy of the detector maps: a variation of 1 um in a 20 um thick detector will introduce a 5% variation in signal. Thickness map resolution required to distinguish 22Ne/20Ne is ~1%, these thickness maps are good to ~0.5%.

Energy Loss Fluctuations (σΔE

)

Multiple Coulomb Scattering (σθ)

Charge State Fluctuations (σΔE

)

Incident angle uncertainty (σθ)

Other contributions to mass resolution include…

Instrument engineering model test at LBNL 88” Cyclotron

March 2004

boron

nitrogen

Galactic Cosmic Rays – ACE/CRIS

CRIS

Studying Galactic Cosmic Rays with ACE/CRIS

Galactic Cosmic Ray factoids:

GCRs are made up of ~90% protons, ~9% helium and 1% heavy nuclei

GCR energy density in the galaxy is 1 eV/cm3 – comparable to the energy density of galactic magnetic fields, thermal energy in the interstellar medium, and starlight.

GCR number density in the vicinity of earth is 10-10 /cm3

The highest energy GCRs (one per square kilometer per century) ever measured have energies >1020 eV (~50 J) - equivalent to the kinetic energy of a 100 mph fastball, enough energy to raise a teaspoon of water by a few degrees C.

Galactic cosmic ray particles are charged and thus interact with magnetic fields resulting in a loss of directional information during their propagation through the galaxy from their source to earth.

Map of the Galaxy in Cosmic Rays

…we can’t discern their origin from sky maps!

Where do the galactic cosmic rays come from?

No directional information from particles, but we can look at the electromagnetic radiation produced by cosmic rays. Supernova remnants such as the Crab Nebula are known to be a source of cosmic rays from the radio synchrotron radiation emitted by cosmic ray electrons spiraling in the magnetic fields of the remnant.

Supernovae provide the necessary energetics to accelerate cosmic rays and occur frequently enough to maintain their population in the galaxy for energies less than ~1015 eV. Above these energies, the acceleration mechanism is still a mystery. Candidates include: quasars, gamma-ray bursts, dark matter decay, topological defects, active galactic nuclei, superstrings,…

Power law spectrum of galactic cosmic rays

N(E)dE xdwith x: 2.2-3

Can be predicted by this simplified Fermi acceleration mechanism in the vicinity of the SNR shock…

Average energy of particle after 1 shock crossing:

P = prob. particle remains in accel. region after 1 crossing

0*EE

After k collisions there are N=N0Pk particles with energies E=E0βk. Eliminate k:

dEEconstdEEN P )ln/(ln1*)(

Yields:

ln/ln

00

P

E

E

N

N

Differentiating…

ln

ln

)/ln(

)/ln(

0

0 P

EE

NN

Which is the power-law!

Model propagation of GCR in galaxy

Use “leaky box model”

Calculate Source Abundances from CRIS isotope data:

A model used to describe the propagation of cosmic rays in the galaxy. Particles are emitted by a number of point source objects distributed throughout a finite volume (the Galaxy). The particles freely diffuse inside the volume and when they encounter the boundary they can either be reflected or with some probability, escape.

Assume sources are distributed homogenously throughout Galaxyand continuously emit a time independent spectrum of high-energy particles (valid for secondary cosmic ray particles). The finite volume is instead modeled by escape due to a “catastrophic” loss term in the diffusion equation. The following equation is for the steady-state leaky box model: injection, deceleration and catastrophic loss terms are balanced.

')]',()',()['(

)(])()([

))()(()()()(

0)(

0

dEvEEnEEnEN

ENvnEnE

ENEbE

EQENEN

t

EN

jijpHijHej

j

iiHipHei

iiii

i

e

ii

Ni(E) = cosmic ray number density of species i at energy/nuc E, τi = decay lifetime for unstable nuclei, γ = lorentz factor, Qi(E) = source term (constant in space and time), bi(E) = ionization energy loss per nuc, σαj and σpj = total destruction cross-sections of nuclei by alphas and protons, nHe and nH = mean number density of He and H in ISM, σαji and σpji = spallation cross-sections for production of nucleus i by bombardment of target j by alphas and protons, E and E’ = kinetic energy per nuc of product (i) and parent (j), vi = velocity of

particle i, τe = lifetime against escape, Λesc = ρv τe = mean free path against escape

Using this leaky box model for cosmic-ray propagation in the galaxy with known values for spallation cross-sections of heavier nuclei into lighter ones, we can determine the source composition of cosmic rays.

A lot can be learned from isotope ratios of these source abundances. Any differences between the cosmic ray source composition and the solar system abundances can give clues about the environment where cosmic ray material originates.

22Ne/20Ne Anomaly

The 22Ne overabundance compared to solar system values can be explained by contributions by Wolf-Rayet stars to the GCR-origin environment

(“superbubbles”)

Sulfur, calcium and argon (Z=16, 18, 20) abundances can be derived with unprecedented precision due to CRIS’s large geometry factor and 7-year data set along with new results for the experimental cross-sections needed to nail down the secondary contribution to the isotopes of these three elements.

Evidence for Galactic Chemical Evolution in the Cosmic Rays

Models of GCE make predictions about the abundances of various isotopes as a function of time in the Galaxy. Lower-right plot shows the increase of the more neutron-rich isotopes of oxygen as a function of metallicity (which itself increases as a function of time, shown in the lower-left plot). With the statistics provided by CRIS along with accurate cross-section estimations, we could possibly see evidence for galactic chemical evolution in the cosmic rays. This has never been seen before!

- Constrains on the masses of stellar progenitor to supernovae

Numerical calculations have been performed to determine the composition of the material returned to the interstellar medium in supernovae over a range of progenitor star masses (circles). The horizontal solid lines are the GCR abundances for these isotopes and dashed lines are (+/- 1σ). Vertical line is boundary between SN Ia and SNII.

- Determine if FIP or volatility is the method for preferential acceleration in GCRs

GCR source elemental abundances are ordered by first-ionization potential (lower-left plot), possibly because the source ejects it’s outer envelope over a long period of time before a SN shock sweeps up and accelerates this material. The outer envelope is preferentially populated by ions because they are more efficiently carried here by the progenitor star’s magnetic fields. But FIP and volatility are correlated (lower-right) – perhaps the non-volatiles are efficiently accelerated as grains while the volatiles are accelerated as nuclei.