Searching for a Suitable Salt Structure for SalSa

by

Allen OdianSLAC

Requirements

• Large Volume (Many tens of cubic kilometers)

• Long Microwave Attenuation Length (Hundreds of meters for frequencies between 100 MHz and a few GHz)

Types of Large Volume Salt Deposits

• Evaporites (Ancient sea beds) DOE Site WIPP in New Mexico 600 meter thick Low Level Radioactive Waste Disposal

• Salt Domes (Geologically Processed Evaporite) United Salt Co’s Hockley Mine in Texas

• Both are ~ 300 meters below ground surface

Cerenkov Radiation, Coherent and Incoherent

• Spectrum, dN/d ~ (Ze)2

• Incoherent, e-s and e+s, Ntotal ~ Energy (E)

• Ne-/Ne+ ~ 1.2, Charge excess, Nexcess ~ 0.1 Nt

• Coherence length ~ bunch size (few cms) dN/d ~ E2 for > bunch size

Cerenkov Spectrum for 1016 ev Shower

Cerenkov Spectrum

0.1

1

10

100

1000

10000

100000

1000000

0 5 10 15 20 25 30 35 40 45 50

Cerenkov Spectrum

Wavelength (cm)

dN

/dla

mbda

(rela

tive

unit

s)

Salt Properties for Microwaves

• Density = 2.2 gm/cm3 , X0 = 24 gm/cm2

• Dielectric Constant = 6, tan < 10-4

• Ratio Im /Re = tan • Att length Latt : Latt / = 1/tan

• Latt = f() , = 0/sqrt

• For 1 GHz 0 = 0.3 m, = 0.12 m

• For tan = 10-4, Latt = 400 m

Maximizing the Signal

• Power P is radiated for about a nanosecond

• P ~ E2 ~ E2 ~ 1/R2: E ~ 1/R ~ Voltage on Antenna

• V ~ E x Antenna Design (frequency)

Cerenkov Radiation lobes are linearly polarized (Antennas need to preserve this)

Energy Threshold set by R, Noise, and Antenna

View of WIPP

WIPP Machine Shop

Test of WIPP Site

• Ceiling of salt tunnel had two sets of three 10 cm diameter holes, depth 6.7 meters, 22.9 meters apart

• Transmitter at end hole, ratio of signal at two separations gives attenuation length

• Study Latt ~ f(Frequency and Depth)

• Results show presence of water n = 2.8 not 2.45

Latt vs Frequency at WIPP

WIPP as a Salsa Site

• Wipp is not suitable for a SalSa site!

How to Make a Salt Dome

• Start with Evaporite

• Bury it deep by Geologic Movement

• When it becomes plastic, expel impurities

• Rise toward surface from density difference

• Shape like a high altitude Helium balloon

U.S Gulf coast salt domes

Salt dome demographics:

• Several hundred known—some are good source of oil

• Typical ~3-5 km diameters, 5-15 km deep

• ~200 km3 water equiv. in top 3-5 km for many domes

HoustonNew Orleans

Hockley dome/mine

Hockley Salt Dome

• Active Salt Mine, United Salt Company• About 3 by 4 Km in horizontal, many Km

deep• No Boreholes, but Tunnels and Chambers• Complications from Matching Antennas to

Salt• Change in beam pattern• Measure many distances

Layout of Hockley Mine

Measurements

• Compare Friis formula with experimental results

• Ratio of 150 and 300 MHz results

• 750 MHz measurement

• Polarization and Noise

• Ground penetrating radar data

Measurement of Signal vs Distance

• Friis Formula (no attenuation)

• VR/VT = A/(R x , A = effective area of transmitting and receiving antennae

• At 150 MHz (half wave), A = 0.132

• At 300 MHz (full wave), A = 0.0482

Latt vs Distance at Two Frequencies

Frequency Dependence

• For constant tan

• Latt(300MHz)/Latt(150MHz) = 0.5

• Results for Latt from frequency dependence

Latt from Frequency Dependence

750 MHz Measurement

• Four commercial UHF phased bowtie array

• Narrow beam made measurements difficult

• Results

Latt from Frequency of 750 MHz

Polarization and Noise

• At 150 MHz, test salt for birefringence

• No cross polarization observed within the rejection of antenna

• Observed noise characterized by a <600 K blackbody (not inconsistant with 310K blackbody)

Ground Penetrating Radar• Masters thesis at Texas A&M

• GPR at 440 MHz, n = 2.45 +-0.05

• Results

Samples measured around 10GHz

•Rock salt is fragile, so that it is not easy to make small stick samples ( 1mm x 1mm x 10.2mm ). Lime stone (especially Jura lime stone ) is rigid. The small stick samples are obtained using a milling machine.

in situ measurementsP.Gorham et al

NaCl, Dielectric Materials and Applications (A. R. von Hippel ed.), 1954

tanδ=2×10-4

Rock salt Hockley mine, USA

Rock salt, Halstadt mine, Austria

NaCl synthesizedLime stone, Kamaishi, Japan

Lime stone, Mt. Jura, France Rock salt, Asse mine, Germany

Hippel 25GHz

1GHz cavity 5.5GHz

8.2GHz

11GHz

Nacl powder

Teflon

Dielectric resonator

tan

1

L

Combined Measurements

M.Chiba

Summary of WIPP/Hockley results• Rock salt is perhaps the clearest medium known for EM

propagation– Usable frequency range from few MHz to ~10 GHz– Evaporite beds (WIPP) have problems with impurities, but salt domes appear

to be often purified through geologic processes

• No measureable bi-refringence or depolarization– Allows for possibility of polarization tracking– May be better than ice in this respect

• Several other salt domes known to be as good or better than Hockley– Avery Island (LA), Weeks Island (LA)– Cote Blanche (LA), Grand Saline (TX)– Many others expected to be excellent but as yet unmeasured– Typical salt volume 50-100 cubic km per salt dome– Several hundred known known salt domes in Gulf coast area, probably

thousands throughout the world

Conclusions

• At least one salt dome may be suitable for SalSa

• A search for a candidate SalSa dome should start

• Preliminary tests should be made to test properties of dome

Backup Slides

Properties of materials for UHE Neutrino Detector

MaterialProperties

Air(STP)

Ice(H2O)

Rock salt

(NaCl)

Lime stone

(CaCO3)

Density ρ (g/cm3) 0.0012 0.924 2.22 2.7

Radiation length X0 (cm) 30420 39 10.1 9.0

Refractive index n = 1.000293 1.78 2.43 2.9

Cherenkov angle (deg) 1.387 55.8 65.7 69.8

Cherenkov threshold energy(keV) …. 107 50 33

Rock salt: high density, large refractive index and short radiation length

From M. Chiba

Salt Measurements at WIPP & Hockley

Measurements of complex permittivity of rock salts and lime stones

Cavity perturbation methodAbsorption depends on the surface condition of the samples, e.g.

smoothness, stain etc.

9.4GHz TE107 Q=4000 Size: 23x10x155mm3

1GHz TM010Q=10000Size: 230mm x 30mm

Latt from 440 MHz Radar