Searching for a Suitable Salt Structure for SalSa by Allen Odian SLAC

1. Searching for a Suitable Salt Structure for SalSabyAllen OdianSLAC

2. Requirements • Large Volume (Many tens of cubic kilometers) • Long Microwave Attenuation Length (Hundreds of meters for frequencies between 100 MHz and a few GHz)

3. 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

4. Cerenkov Radiation, Coherent and Incoherent • Spectrum, dN/dl ~ (Ze)2/l2 • 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/dl ~ E2 for l > bunch size

5. Cerenkov Spectrum for 1016 ev Shower dN/dlambda (relative units) Wavelength (cm)

6. Salt Properties for Microwaves • Density r = 2.2 gm/cm3 ,X0 = 24 gm/cm2 • Dielectric Constant e = 6, tan d < 10-4 • Ratio Im e/Re e = tan d • Att length Latt : Latt /l = 1/ptand • Latt = f(l) , l = l0/sqrt e • For 1 GHz l0 = 0.3 m, l = 0.12 m • For tan d = 10-4, Latt = 400 m

7. 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

8. View of WIPP WIPP Machine Shop

9. 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

10. Latt vs Frequency at WIPP

11. WIPP as a Salsa Site • Wipp is not suitable for a SalSa site!

12. 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

13. 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 Hockley dome/mine Houston New Orleans

14. 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

15. Layout of Hockley Mine

16. Measurements • Compare Friis formula with experimental results • Ratio of 150 and 300 MHz results • 750 MHz measurement • Polarization and Noise • Ground penetrating radar data

17. Measurement of Signal vs Distance • Friis Formula (no attenuation) • VR/VT = A/(R x l), A = effective area of transmitting and receiving antennae • At 150 MHz (half wave), A = 0.13l2 • At 300 MHz (full wave), A = 0.048l2

18. Latt vs Distance at Two Frequencies

19. Frequency Dependence • For constant tan d • Latt(300MHz)/Latt(150MHz) = 0.5 • Results for Latt from frequency dependence

20. Latt from Frequency Dependence

21. 750 MHz Measurement • Four commercial UHF phased bowtie array • Narrow beam made measurements difficult • Results

22. Latt from Frequency of 750 MHz

23. 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)

24. Ground Penetrating Radar • Masters thesis at Texas A&M • GPR at 440 MHz, n = 2.45 +-0.05 • Results

25. 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.

26. Combined Measurements Dielectric resonator M.Chiba

27. 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

28. 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

29. Backup Slides

30. Properties of materials forUHE Neutrino Detector Rock salt: high density, large refractive index and short radiation length From M. Chiba

31. Salt Measurements at WIPP & Hockley

32. Measurements of complex permittivity of rock salts and lime stones Cavity perturbation method Absorption depends on the surface condition of the samples, e.g. smoothness, stain etc. • 9.4GHz TE107 • Q=4000 • Size: 23x10x155mm3 • 1GHz TM010 • Q=10000 • Size: 230mmf x 30mm

34. Latt from 440 MHz Radar