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Detection of Ultra High-Energy Cosmic Particles
with the Use of Radio and Radar Methods
December 7, 2005
Oscar Stål
Physics in Space Programme
Swedish Institute of Space Physics, Uppsala
Dept. of Astronomy and Space Physics, Uppsala University
Supervisor: Bo Thidé
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Outline
●
Presentation of objectives
●
Introductory high-energy cosmic particle physics
●
Radar studies of EAS ionisation columns
●
Lunar satellite detection of Askar’yan radio pulses
●
Discussion and outlook
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Background and framework
●
●
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●
LOIS radio sensor network and deep space radar project
Initiated by Bo Thidé, collaboration with LOFAR (Netherlands)
Provides new methods to study fundamental physics in space
Can this facility or these methods be of use in astroparticle physics?
If not usable, at least it is necessary to quantify what radio/radar
background to expect from UHE cosmic particles
Ultimate goal is low-frequency array on the Moon – LIFE (LURBO)
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Objectives
Our objectives for this diploma work have been two-fold:
●
●
To determine, by approximate analytical methods,
the radar cross section of the ionisation columns
created by Extensive Air Showers
To investigate the feasibility of using a lunar satellite
for in situ detection of Askar’yan radio pulses from
cosmic particle interactions with the lunar regolith
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Outline
●
Presentation of objectives
●
Introductory high-energy cosmic particle physics
●
Radar studies of EAS ionisation columns
●
Lunar satellite detection of Askar’yan radio pulses
●
Discussion and outlook
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Cosmic rays
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Charged particles: p, 
Steep decrease of flux with
energy:
 ~ E-2.7
 ~ E-3.1
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E < 1015 eV
E > 1015 eV
Flux extends to the highest
energy ever observed (UHE)
Isotropic flux, B obfuscation
Sources of UHE particles not
yet determined
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Extensive Air Showers (EAS)
●
Cosmic particle interacts with constituents
of the atmosphere
●
Shower of secondary particles generated
●
Hadronic, muonic and EM components
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Transverse scale given by Molière radius,
90% of the energy contained within rM
For air, rM = 70 m at sea level and
increasing with altitude to several
hundred metres
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Detection of UHE cosmic rays
●
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Air shower arrays
Haverah Park, AGASA, KASCADE, Pierre Auger
Detects shower particles at ground level
Fluorescence telescopes
Fly's eye, HiRes Fly's Eye
Excellent energy resolution, sensitivity
Low duty cycle
Radio methods
Pioneered in the 60's, now LOPES
Coherent geosynchrotron emission
Highly polarised short radio pulses
Falcke et al., Nature 435 (2005)
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
The Greisen-Zatsepin-Kuzmin (GZK) cutoff
●
For E > 1019.5 eV, cosmic rays interact with CMB photons
●
Pion photoproduction: p +   p0
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+ p p +   p+ +
n etc.
Intergalactic medium no longer
transparent over Mpc scales
Still, cosmic rays have been
observed beyond this “cutoff”
A most interesting question
in astroparticle physics is why?
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Ultra high-energy neutrinos
●
UHE neutrinos should be produced in the GZK process:
p+  + +   e+ + e +  + 
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More sources of UHE neutrinos
suggested, but none confirmed
No magnetic field influence
Detection using optical, radio or
acoustic methods
Best limits on UHE neutrino flux
obtained using radio methods
Gorham et al., Phys. Rev. Lett. 93 (2004)
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Outline
●
Presentation of objectives
●
Introductory high-energy cosmic particle physics
●
Radar studies of EAS ionisation columns
●
Lunar satellite detection of Askar’yan radio pulses
●
Discussion and outlook
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Nishimura-Kamata-Greisen (NKG) model for EAS
Longitudinal development parametrised by shower age:
“slant depth”
Total number of particles in the shower:
Transverse distribution of shower particles:
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Ionisation columns
●
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Atmospheric ionisation yield calculated from transverse particle
density and ionisation parameters for air
Ionisation over long
distances, > 10 km
Evaporation time
scale uncertain:
20 s – 20 ms
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Ionisation columns
●
Ionisation volume modelled as collisionless, cold, non-magnetised
plasma with inhomogeneous density
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Radar cross section and scattering width
●
●
The radar cross section [m2] is the projected area of a perfectly
reflecting sphere giving a reflected power equivalent to that of the real
target
In a two-dimensional problem, the cross section is replaced by the
scattering width [m]
Cross section (RCS):
Scattering width (SW):
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Inhomogeneous wave equations
●
●
To determine the radar cross section of the EAS ionisation columns,
the scattered E and B fields are required
Wave equations for the spatially inhomogeneous medium derived
from first principles (Maxwell):
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Separation of the wave equations
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For radially inhomogeneous n(x) it is possible to separate the wave
equations in cylindrical coordinates
The Ez and Bz component equations decouple for infinite cylinder
Shower aging is effectively neglected, we treat only maximum
TM and TE mode scattering for normally incident wave
m is the azimuthal “quantum number”
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Scattering theory in cylindrical geometry
The Sommerfeld radiation condition for n = 2 allows the total field
to be written as incoming plane wave + scattered cylindrical wave:
Asymptotic dependence of scattered wave is chosen consistently
with plane wave expansion when there is no scatterer:
The phase shifts m contain all information about the scattering:
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Phase-integral approximations
●
How to obtain the phase-shifts? Usually by analytic matching if the
inner solution is known. We use the phase-integral (WKB) method
The general second order linear ODE of a complex z
has approximate phase-integral solutions in terms of
where q(z) is generated asymptotically from an arbitrary basefunction, for order 2N+1 of approximation:
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
First order approximations to the phase-shifts
●
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Phase-shifts in phase-integral method obtained from the asymptotics
of the solutions
Connection formula used to
cross the turning point from
q2 < 0 to q2 > 0
In first order approximation,
it is possible to use a path
along the real line
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Choice of base function
●
The base function needs to be consistent with the physics, and this
choice is in general a difficult problem. No generic method exists
From direct transformation of the radial equations, we obtain
for the TM mode equation
By using instead the modified base function
we reproduce the zero phase-shifts in the no-scattering limit,
which is desirable. Corresponds to the Langer modification in a
spherical problem and was previously studied by Berry et al.
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Scattering widths – numerical results
●
●
Numerical integration of the phase-shift formula using the NKG
refractive index for horisontal EAS at maximum development
Calculations of the scattering width for various shower altitudes, radar
frequencies and primary particle energy
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Longitudinal length scale and 3D cross section
●
Shower not infinitely long – what is the longitudinal length scale?
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Scattering theory is formulated in the extreme far-field?
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Consider the first Fresnel zone, the longitudinal dimension is given by
inverting the far-field condition:
Affects the range of the radar system, and thereby the sensitivity to
UHE particle showers. No simulations performed on this
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Outline
●
Presentation of objectives
●
Introductory high-energy cosmic particle physics
●
Radar studies of EAS ionisation columns
●
Lunar satellite detection of Askar’yan radio pulses
●
Discussion and outlook
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Background
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Chandrayaan-1 Moon mission
●
India's first mission to the Moon
●
Scheduled for launch in 2007-08
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100 km polar orbit
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ELVIS instrument proposal
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HF/VHF radio receiver of LOIS type
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Can it be used for detection of UHE
cosmic particles?
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Detection principles
●
3. Coherent V-C
radio emission
2. First interaction,
shower initiated
4. Rays refracted
at interface
5. Detection by satellite
or surface-based aerials
1. Neutrino enters
the Regolith
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Radio emissions from particles in dense media
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Showers very localised in dense media, rM ~ 10 cm
Wavelengths longer than shower dimensions means
emission becomes coherent
Ouput power scales quadratically with E,
dominates optical output at UHE
Radio transparent material required
Ice, very dry rock, permafrost, giant
underground salt domes suggested
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Coherent Vavilov-Cerenkov emission
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Particle track is not infinite, so the output is smeared around the
Cerenkov angle
Coherent radiation for higher frequencies closer to the Cerenkov
angle qC
Long wavelengths means the shower radiates as a single particle,
hence only the net charge contributes
v > c/n
cos(qC)=1/(nb)
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
The Askar`yan effect
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Neutral shower gives no emission
The particles in an initially neutral shower will undergo scattering
processes as they traverse the material...
Compton scattering:
 +e-atom   + e-
Bhaba scattering:
e+ + e-atom  e++ e-
Annihilation:
e+ + e-atom   + 
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... and acquire a negative charge excess of 20-30%
●
This process is called the Askar’yan effect
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Radiation properties
●
Zas-Halzen-Stanev (ZHS) Monte Carlo based on simulations of
showers in dense media confirms the 20-30% charge excess
Radio emission at Cerenkov angle well parametrised by
0 specific decoherence frequency, 2.5 GHz for regolith
The angular spread is given by a Gaussian:
where the width is frequency dependent, decreasing with 
E. Zas, F. Halzen and T. Stanev, Phys. Rev. D 45 (1992)
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Radiation properties
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Experimental confirmation
●
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Experiment at SLAC using 3.6 tons Si target, GeV photons, total
shower energy up to 1019 eV
The ZHS simulation results confirmed to a factor of two, radiation
coherent and linearly polarised
D. Saltzberg et al., Phys. Rev. Lett. 86 (2001)
P. W. Gorham et al., Phys. Rev. D 72 (2005)
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
The Moon as an UHE particle detector
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Moon opaque to neutrinos with
E > 1016 eV
UHE cosmic rays of minor interest
Target (upper 10 m) is regolith:
Si grains and tiny rocks
Dielectric properties depends on
TiO, FeO contaminants
Radio transparency for  < 1 GHz
if 5% contaminants assumed
Olhoeft and Strangway, Plan. Sci. Lett. 24 (1975)
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Threshold energy
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●
Threshold determined from the ZHS parametrisation and
the minimum detectable signal
Using Chandrayaan-1 and ELVIS parameters:
Altitude
Centre freq.
Bandwidth
Sensitivity
h


Pmin
= 100 km
= 100 MHz
= 50 MHz
= -135 dBm/Hz
=> Threshold neutrino energy becomes 5*1019 eV
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Detection aperture
●
Detection rate is given by the effective aperture (“cross section”),
determined through simulations
Sensitivity dependent on:
- Primary neutrino mixing and branching ratios
- Primary energy and neutrino-nucleon cross section
- Dielectric properties, attenuation in the regolith
- Surface effects (refraction, reflection etc.)
- Distance from surface to observation point (geometry)
- Measurement frequency, bandwidth and minimum signal
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Monte Carlo sensitivity simulation implemented in Matlab
●
Simulations performed for different primary neutrino energy
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Simulation results
2pAmoonNdetected/Ntotal
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Model dependent event rate
●
Using model for minimum GZK neutrino flux (Engel et al., 2001)
●
The event rate can then be determined from
=> 2.2 detectable
events per year =)
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Outline
●
Presentation of objectives
●
Introductory high-energy cosmic particle physics
●
Radar studies of EAS ionisation columns
●
Lunar satellite detection of Askar’yan radio pulses
●
Discussion and outlook
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Discussion, part 1
●
●
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●
We have presented how an EM scattering problem in cylindrical
geometry can be conveniently treated by the phase-integral
approximation
The results have been applied to scattering of radio waves from EAS
ionisation columns for determination of radar cross sections
The physics of the EAS has been modelled in a simplistic manner,
which might put restrictions on the applicability of our results from this
aspect
Applications of radio wave scattering in cylindrical geometry also exist
for meteor trails, ionospheric striations and for lightning ionisation
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Discussion, part 2
●
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We have considered observation of coherent radio pulses from
showers induced in the lunar regolith by UHE cosmic neutrinos
A simple simulation program has been constructed for estimating the
efficiency of a satellite experiment with this purpose
This program offers generous possibilities for further variation of
different experimental parameters
Porting to a faster code (e.g. FORTRAN) is desirable before more
extensive simulations can be performed
We believe that optimisation might further increase the 2.2 events to
something really useful, although 2.2 yr-1 is still competitive (!)
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
THE END
Thank you for listening!
Special thanks to everyone who has supported me during this work:
Bo Thidé, IRF-U
Jan Bergman, IRF-U
Gunnar Ingelman, THEP, UU
John A. Adam, ODU (Norfolk, VA)
Fellow diploma students at IRF-U and elsewhere
All members of the friendly staff at IRF-U
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
Surface effects
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Radio waves refracted at surface
Beam becomes wider  Better opening angle, but lower
field strength
Total internal reflection,
qTIR = p/2 – qc
q'
TIR more important at high frequencies
CR detection supressed since all rays
are down-going
Vacuum
n=1
q
Regolith
n ≃ 1.7
Smooth surface or detailed topographic map
Oscar Stål
[email protected]
IRFU seminar
Uppsala, 7/12/2005
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