Title: Novel Photosensor Concepts for Future (Anti)Neutrino Detectors

Daniel Ferenc
University of California, Davis

Abstract: 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.