Title: Novel Photosensor Concepts for Future (Anti)Neutrino
University of California, Davis
This meeting will celebrate the first measurement of geo-antineutrinos,
and at the same time it will initiate discussions on how to build a future
super-sensitive detector that may be perhaps named "The Earth Tomograph."
Antineutrinos may reveal unique and precious information about the
structure and processes deep under the ground. This is possible because
antineutrinos easily penetrate through the Earth. But at the same time the
small cross section makes the detection of neutrinos notoriously
difficult. To maximize the detection probability, the size of the
next-generation detector(s) should greatly exceed the dimensions of the
largest current experiments. In the construction of detectors on such a
large scale, no other option remains than to use natural media: deep packs
of ice, or, more likely, water. Charged particles that originate in
impacts of primary neutrinos radiate Cherenkov or sometimes fluorescence
light in transparent media. The created light traverses large distances
before it finally reaches large arrays of photon detectors: the single
most important detector element in this field.
Photosensors play an equally important role in other similar application
areas that also search for rarely occurring phenomena, like in particle
astrophysics, medical imaging and nuclear proliferation control. Unlike
physics, the other application areas may present real, i.e. large and
steady potential markets for new, inexpensive, high-quality, industrially
mass-produced photosensors. Therefore they deserve special attention.
For various reasons, neither the current vacuum photosensor technology,
nor the various modern semiconductor photosensor technologies may be
suitable for such large-area mass applications. In contrast to the
semiconductor photosensor technologies that have rapidly evolved during
the last few decades (but towards small-pixel devices that are unsuitable
for large-area and large-pixel applications), the vacuum photomultiplier
tube (PMT) technology did not make any significant progress since the late
1960s. The complex and bulky construction, and the labor-intensive
manufacture are inherent to the PMT concept; mass production on the
required scale is therefore virtually inconceivable, irrespective of cost.
Also, intrinsic to the PMT concept are some important drawbacks in the PMT
performance: low photoelectron collection efficiency (at most ~70% in
large area PMTs); low quantum efficiency (20-25%), limited in addition
only to a narrow spectral region; complicated and expensive installation
methods; fragility (as dramatically experienced in the Super Kamiokande
massive implosion disaster); high sensitivity to magnetic fields, and
almost complete lack of single-photon resolution (i.e. of the ability to
resolve the number of photons in a photosensor pixel).
In response to this well known problem, some new photosensor concepts have
been proposed recently (few of them by the author of this paper). I will
discuss novel photon detector technologies that may lead to inexpensive
mass production for large markets, and may at the same time revolutionize
neutrino and antineutrino detection, as well as proliferation control.