Jamie Leech

Abstract:

The design and preliminary results from testing the performance of a 230GHz unilateral finline SIS mixer, fabricated on a silicon substrate are presented. The mixer will be employed in the single baseline heterodyne interferometer-GUBBINS (220-GHz Ultra-BroadBand INterferometer for S-Z), which aims to measure the null frequency in the Sunyaev-Zel'dovich spectrum at ~227GHz. The mixer is operated in the frequency range of 180GHz~280GHz with an IF bandwidth of 1-13 GHz. An important feature of this mixer is its ultra-wide IF bandwidth, so as to achieve very high brightness sensitivity in the observation of the galaxy clusters in the faint cosmic microwave background radiation. This SIS mixer described is deposited on an 80um silicon substrate with the dielectric constant of 11.9. The incoming RF signal from the feed horn is coupled via the waveguide mode to a unilateral finline, which is tapered to a 2.5um slotline. The signal is coupled from the slotline to the microstrip with the aid of two radial stubs. The employment of a silicon substrates gives a slotline impedance of 36Ω, which is ideal for coupling to the microstrip, where the SIS junction is fabricated. Also, the employment of silicon allows the generation of the trenches around the device so that individual devices can be separated from the batch without dicing. A 2-stage notch was fabricated at the front end of the substrate to match the loaded waveguide to the free space. The capacitance of the SIS junction was tuned out using a wide band circuit consisting of two stubs and a 3-stage Chebyshev transformer. This is followed by a 3-stage RF choke to prevent RF signal from leaking into the IF output port. The mixer chip was fabricated at KOSMA, Cologne using Nb-AlOx-Nb tunnel junction defined with E-beam lithography. The device used in this experiment had a normal resistance of 20Ω, a current density of 14KA/cm2, an area of 1μm2 and an intrinsic capacitance of 75fF. Measurement of the mixer performance was done using a local oscillator which gives sufficient power to pump the mixer in the frequency range 200GHz~260GHz using 12μm beam splitter. Measurement of the mixer sensitivity was done using the hot/cold Y-factor method. Uncorrected noise temperature of 75K was obtained at 208GHz, with 30 K contributed by mismatch to the IF system. Future characterization and improvements of the IF system and the measurement with better-tuned devices will also be reported.

Abstract:

An analysis of large-area CO J = 3-2 maps from the James Clerk Maxwell Telescope for 12 nearby spiral galaxies reveals low velocity dispersions in the molecular component of the interstellar medium. The three lowest luminosity galaxies show a relatively flat velocity dispersion as a function of radius while the remaining nine galaxies show a central peak with a radial fall-off within 0.2-0.4r25. Correcting for the average contribution due to the internal velocity dispersions of a population of giant molecular clouds, the average cloud-cloud velocity dispersion across the galactic discs is 6.1 ± 1.0 kms-1 (standard deviation of 2.9 kms-1), in reasonable agreement with previous measurements for the Galaxy and M33. The cloud-cloud velocity dispersion derived from the CO data is on average two times smaller than the HI velocity dispersion measured in the same galaxies. The low cloud-cloud velocity dispersion implies that the molecular gas is the critical component determining the stability of the galactic disc against gravitational collapse, especially in those regions of the disc which are H2 dominated. The cloud-cloud velocity dispersion shows a significant positivecorrelation with both the far-infrared luminosity, which traces the star formation activity, and the K-band absolute magnitude, which traces the total stellar mass. For three galaxies in the Virgo cluster, smoothing the data to a resolution of 4.5 kpc (to match the typical resolution of high-redshift CO observations) increases the measured velocity dispersion by roughly a factor of 2, comparable to the dispersion measured recently in a normal galaxy at z = 1. This comparison suggests that the mass and star formation rate surface densities may be similar in galaxies from z = 0 to 1 and that the high star formation rates seen at z = 1 may be partly due to the presence of physically larger molecular gas discs. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

In previous work, we have described novel smoothwalled multiple flare-angle horns designed using a genetic algorithm. A key feature of these horns is that they can be manufactured very rapidly and cheaply in large numbers, by repeated direct drilling into a single plate of aluminium using a shaped machine tool. The rapid manufacturing technique will enable the construction of very low cost focal-plane arrays, offering an alternative to conventional electroformed corrugated horn arrays. In order to experimentally demonstrate the new technology, we constructed a 230 GHz focal-plane array comprising 37 smooth-walled horns fabricated by direct drilling. We present the measured beam patterns for a large sample of these horns across the array, demonstrating the suitability of our manufacturing techniques for large format arrays. We have measured the cross coupling between adjacent feeds and have shown that it is negligible. We also present high quality beam patterns measured for a much smaller 700 GHz horn, showing the promise of extending this technology to THz frequencies.