Looking for the Origin of the Stars A Low Noise Detector

							                                                                          Autumn 1999




                 Looking for the
                  Origin of the
                    Stars                                                A Low Noise Detector
                                           for Millimetre Wave Radio Astronomy

                   In December 1998 Dr Stéphane Claude and Dr Charles Cunningham of the National Research
                   Council, Canada, completed an important upgrade of one of the Superconducting-
                   Insulator-Superconducting (SIS) heterodyne receivers at the James Clark Maxwell radio
                   telescope on the Mauna Kea mountain in Hawaii. The receiver is cooled by an Oxford
                   Instruments hybrid helium cryostat.



          New stars are born           calibration load, and various            junctions mounted in series [1].
             within the large clouds   instrument controls. It is stable and    The mixer operates in a double side
              of interstellar dust     exhibits low noise across its            band mode with no side band
              and gas in our galaxy.   frequency range of 211-276 GHz           rejection. For optimum mixer
             We can learn about        and is fix-tuned across this entire      stability, a magnetic field is applied
            their formation by         band. These features greatly             to the SIS junctions to suppress
         studying the sites of         improve observing efficiency by          unwanted Josephson oscillations.
young stars, such as the Orion         enhancing signal levels and              Stable operation is paramount for
nebulae. These clouds often            permitting quick frequency               completely automatic operation.
contain a wide variety of chemical     changes to detect a number of            The receiver is very sensitive,
species such as carbon monoxide,       spectral lines in each source. The       exhibiting an equivalent noise
hydrogen cyanide and methyl            receiver has a particularly wide,        temperature of 60-80 K across the
alcohol. Such molecules have many      1.8 GHz, IF bandwidth which is           frequency band.
rotational transitions at              important for observation of extra-
millimetre and submillimetre           galactic objects that have a large       The development of low noise
wavelengths and the strength and       velocity range.                          computer controlled receivers is
shape of their spectral lines are                                               particularly important for future
good probes of the chemistry           The Oxford Instruments cryostat          interferometers such as the ALMA
and kinematics of the star             ensures the very stable bath             project (Atacama Large
formation region.                      temperature of 3.7 K (at an              Millimetre Array).
                                       altitude of 4000 m) required for
Detecting these faint, high            low-noise mixing operation of the        Reference
                                                                                [1] A R Kerr, S -K Pan, A W Lichtenberger
frequency signals has required         Niobium SIS junctions (Tc = 9.26 K).     and H H Huang,
the development of dedicated           Importantly, at such a remote site,      “A Tunerless SIS mixer for 200-280 GHz with
                                                                                low output capacitance and
instruments using superconducting      it only needs a weekly refill despite    inductance”, Proceedings of the Ninth
technology. Detection uses hetero-     being tilted during observations,        International Symposium on Space Terahertz
dyne mixing, which combines the        an action which reduces the helium       Technology, pp.195-203,
                                                                                17-19 March 1998.
observed signal with that              hold time. The cryostat also
generated by a local oscillator. The   provides the rigid internal supports
difference in frequency is the         required so that the focus of the
output intermediate frequency (IF).    beam in the mixer is not altered by
The new receiver consists of a         the movements of the telescope.                       Dr S Claude and Dr C Cunningham,
                                                                                             National Research Council,
superconducting SIS mixer, cooled      The new mixer uses a full height        Contact       Herzberg Institute of Astrophysics,
by an Oxford Instruments hybrid        waveguide and a coplanar SIS chip                     Canada
helium cryostat, a liquid nitrogen     which includes four Nb/AlOx/Nb                        E-mail: stephane.claude@nrc.ca