Abstract-Charge-coupled devices (CCD's) have been fabricated on high-resistivity, n-type silicon. The resistivity, on the order of 10,000 Ω-cm, allows for depletion depths of several hundred microns. Fully-depleted, back-illuminated operation is achieved by the application of a bias voltage to a ohmic contact on the wafer back side consisting of a thin in-situ doped polycrystalline silicon layer capped by indium tin oxide and silicon dioxide. This thin contact allows for good short wavelength response, while the relatively large depleted thickness results in good near-infrared response.
We have developed ultralow-noise electronics in combination with repetitive, nondestructive readout of a thick, fully depleted charge-coupled device (CCD) to achieve an unprecedented noise level of 0.068 e − rms=pixel. This is the first time that discrete subelectron readout noise has been achieved reproducible over millions of pixels on a stable, large-area detector. This enables the contemporaneous, discrete, and quantized measurement of charge in pixels, irrespective of whether they contain zero electrons or thousands of electrons. Thus, the resulting CCD detector is an ultra-sensitive calorimeter. It is also capable of counting single photons in the optical and near-infrared regime. Implementing this innovative non-destructive readout system has a negligible impact on CCD design and fabrication, and there are nearly immediate scientific applications. As a particle detector, this CCD will have unprecedented sensitivity to low-mass dark matter particles and coherent neutrino-nucleus scattering, while future astronomical applications may include direct imaging and spectroscopy of exoplanets.
Using WFPC2 on the Hubble Space T elescope, we have isolated a sample of 258 white dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radial distances from the cluster center were observed, and sizable WD populations were found in all three. The location of these WDs in the colormagnitude diagram, their mean mass of 0.51(^0.03) and their luminosity function conÐrm basic M _ , tenets of stellar evolution theory and support the results from current WD cooling theory. The WDs are used to extend the cluster main-sequence mass function upward to stars that have already completed their nuclear evolution. The WD/red dwarf binary frequency in M4 is investigated and is found to be at most a few percent of all the main-sequence stars. The most ancient WDs found are D9 Gyr old, a level that is set solely by the photometric limits of our data. Even though this is less than the age of M4, we discuss how these cooling WDs can eventually be used to check the turno † ages of globular clusters and hence constrain the age of the universe.
We establish a framework for determing absolute ages of Galactic globular
clusters and then use these ages to investigate the age-metallicity and
age-Galactocentric distance relations for the 36 clusters with the most
reliable age data. The clusters span Galactocentric distances from 4 through
100 kpc and cover a metallicity range from $[Fe/H] = -0.6$ to $-2.3$. Adopting
currently plausible choices for the relation between cluster metallicity and
horizontal-branch luminosity, and alpha-enhancement ratios, we find that the
majority of the globular clusters form an age distribution with a dispersion
$\sigma(t)$ about $10^9$ years, and a total age spread smaller than 4 Gyr.
Clusters in the lowest metallicity group ($[Fe/H] < -1.8$) appear to be the
same age to well within 1 Gyr at all locations in the Milky Way halo,
suggesting that star formation began throughout the halo nearly simultaneously
in its earliest stages. We find no statistically significant correlation
between mean cluster age and Galactocentric distance (no age gradient) from 4
to 100 kpc. The correlation between cluster age and horizontal-branch type
suggests that causes in addition to metallicity and age are required to
understand the distribution of stars along the horizontal branches in globular
cluster color-magnitude diagrams.Comment: 26 pages, AASTeX v3.0 with 3 Postscript figure
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