Large-Mass Ultra-Low Noise Germanium Detectors:
Applications in Neutrino
and Astroparticle Physics
A new type of radiation detector, a p-type modified electrode germanium diode, is presented. The prototype displays, for the first time, a combination of features (mass, energy threshold and background expectation) required for a measurement of coherent neutrino-nucleus scattering in a nuclear reactor experiment. The device hybridizes the mass and energy resolution of a conventional HPGe coaxial gamma spectrometer with the low electronic noise and threshold of a small x-ray semiconductor detector, also displaying an intrinsic ability to distinguish multiple from single-site particle interactions. The present performance of the prototype and possible further improvements are discussed, as well as other applications for this new type of device in neutrino and astroparticle physics (double-beta decay, neutrino magnetic moment and WIMP searches).
pacs:85.30.De, 95.35.+d, 13.15.+g, 25.30.Pt, 95.55.Vj, 23.40.-s
contact author: ]
From an experimental point of view, the detection of the soft nuclear recoils expected from coherent scattering of low-energy neutrinos off nuclei freedman ; drukier remains a daunting challenge. The applications of a capable detector to fundamental and applied physics would be numerous: a large coherent enhancement to the cross section for this process results in expected recoil rates in the hundreds per kg of target per day from reactor antineutrinos, affording a radical detector compaction. Of all man-made neutrino sources, power reactors offer by far the largest flux, with essentially all (anti)neutrino energies being able to interact coherently. The difficulty resides in a majority of these recoils falling well below the energy threshold of conventional detector technologies. Until now, no device has met the minimum target mass ( kg) and energy threshold ( keV) required for such a measurement, even though unrealized proposals abound drukier ; ieee ; blas . Not only the recoil energies themselves are low, but only a fraction of these are generally available in a readily detectable form (ionization, scintillation). A characterization of this fraction (the “quenching factor”) and a precise definition of signal acceptance near the detector threshold are necessary first steps in the development of a coherent neutrino detector. In a companion paper other , an approach to this challenging in itself calibration exercise has been developed.
The advent of a coherent neutrino detection technology would enable true technological applications leocastle , for instance, non-intrusive monitoring of nuclear reactors against illegitimate uses (e.g., fuel rod diversion, production of weapon-grade material) with a truly compact device, improving on proposed methods bernstein1 that rely on standard (charged current) ton-sized neutrino detectors. Beyond these, the interest in observing this process is not merely academic: a neutral-current detector responds essentially the same way to all known neutrino types all . Therefore, the observation of neutrino oscillations in such a device would constitute as close to direct evidence for sterile neutrino(s) as is possible. Separately, the cross section for this process is critically dependent on a neutrino magnetic moment: disagreement with the Standard Model prediction can reveal a finite value presently beyond reach dodd . Recently the strong sensitivity of the process to non-standard neutrino interactions with quarks new and the effective neutrino charge radius bernabeu has been emphasized. Other fundamental physics applications can be listed: a precise enough measurement of the cross section would constitute a sensitive probe of the weak nuclear charge larry . Coherence plays a most important role in neutrino dynamics within supernovae and neutron stars freedman , adding to the attraction of a laboratory measurement. In particular, a measurement of the total (flavor-independent) neutrino flux from a nearby supernova using a massive enough coherent detector would be of capital importance to help clarify the exact oscillation pattern followed by the neutrinos in their way to the Earth beacom .
Ii A First Coherent Neutrino Detector
The Low-Background Detector Development group at the Enrico Fermi Institute has investigated several new technologies, each in principle capable of meeting the three goals (energy threshold, background and minimum detector mass) required for a successful measurement of this mode of neutrino interaction in a power reactor. In this paper we concentrate on what is presently considered the most promising path towards this measurement. The prototype described here exhibits an active mass of 475 g, sufficient for a measurement of the cross section. The technology is however readily scalable to a mass O(10) kg, necessary for most of the applications mentioned above, in the form of a small array of detectors.
While an average of 20-100 eV in deposited ionization energy is required to generate a signal carrier (electron, photon) in proportional chambers or scintillating particle detectors, semiconductors offer the same for a 3 eV modicum. It would then make sense to examine the possibility of using large intrinsic germanium detectors for this application. Unfortunately, the large capacitance of a germanium diode of this mass (1 kg) results in prohibitive electronic noise levels, i.e., ionization energy thresholds of a few keV. When a low-energy quenching factor for Ge recoils of O(20)% ge is taken into consideration, the situation is seen to be hopeless for any man-made low-energy neutrino source. The TEXONO collaboration has recently proposed texono to bypass this impediment by using arrays of commercially-available smaller n-type germanium diodes (5 g) where the low capacitance ( 1 pF) results in 300 eV hardware ionization thresholds. In those conditions, as discussed below, coherent signal rates of O(10) detectable recoils kg day are possible from power reactors. The drawbacks of that approach are evident: a single kg of target mass requires a multitude of amplification channels and a large monetary investment. Near threshold, a much higher rate of unrejectable electronic noise-induced events per unit mass would be sustained than in a single-channel 1 kg crystal of comparable threshold, if such a thing could be built. Finally, the unfavorable peak-to-Compton ratio for detectors this small results in partial energy depositions from environmental radioactive backgrounds being strongly favored, swamping the signal region texono .
Large germanium detectors with 1 pF capacitance and 1 kg of mass can be built, and this is the approach taken here. Modifications of the usual electrode structure in a variety of detector geometries luke1 ; luke2 can lead to these uncommonly low values of the capacitance, often at the expense of degraded energy resolutions, which are evidently not the biggest issue in the present application (the intrinsically-low resolution for small energy depositions being more important). An 800 g modified coaxial detector with a n central electrode (n-type) just 1 mm in radius and 0.5 mm deep achieved a 270 eV FWHM noise level luke2 , a sizeable improvement over a standard coaxial detector of the same mass. The measured capacitance (1 pF) was in excellent agreement with expectations based on the chosen electrode geometry luke2 . In order for the carriers to be efficiently collected along the axis of such a detector a gradient of impurities in the crystal must be present for space-charge effects to establish an effective electric field in this direction. Material with these characteristics is actually commonly present in germanium ingots as a result of segregation processes during crystal growth ingot . The detector in luke2 was inspired by searches for the “Cosmion” cosmion , a low-mass particle dark matter candidate thought at the time to be able to explain the now defunct “solar neutrino problem”. Due to their light mass ( 1 GeV) these particles were expected to produce relatively soft recoils. Cosmions were experimentally ruled out soon after and no further progress nor interest in these detectors appears in the literature.
A noise level of 270 eV (resulting in a threshold two to three times
higher) would still be insufficient for a reactor
experiment looking for coherent neutrino scattering. However, a number of
improvements could in principle reduce this to a threshold 100
eV or even
lower in such a large detector, which would yield a comfortable signal
rate for present purposes. Since
a large fraction
of the noise figure in the seminal work of luke2 was due to the electronic
characteristics of the field effect transistors (JFETs) available 20 years
ago, it seemed timely to reconsider this approach in the light of the most recent technology.
A number of possible improvements were envisioned:
Replacing the resistor-feedback preamplifier used in
luke2 by a lower noise optical- or transistor-reset type is observed to
reduce on its own the electronic noise level by a factor 2 pulsed in
detectors with a capacitance 1 pF using the same JFET as in
luke2 (Texas Instruments 2N4416). This is at the expense of
a reduced energy throughput pulsed ,
which is however not a concern in low-counting rate experiments.
Great progress in reducing JFET capacitance () and noise voltage () has taken place since luke2 . The 2N4416 JFET is now fully outdated outdate (=4.2 pF, = 2 nV/ at 295 K and 10 kHz). A state-of-the-art EuriFET ER105 (=0.9 pF, = 1.6 nV/ at 295 K and 10 kHz), proprietary to CANBERRA Industries, provides a perfect capacitance match to a 1 pF and in the process the theoretically best possible signal-to-noise ratio match ; radeka . The electronic noise level (as described by the energy resolution using a pulser) that can be expected from these technological improvements is described for Germanium by the expression radeka ; pentafet :
with all units as above and s) being the shaping
(integration) time of the (second
spectroscopy amplifier. For s as in luke2 ,
an optimal 45 eV FWHM is to be expected, in contrast to
130 eV for the 2N4416 (this last figure in fair agreement with the observations
in luke2 after introducing the factor 2 noise increase from
the resistor-feedback preamplifier). This noise figure is precisely at
the level of the very best performance presently achievable with
small =1 pF, low-, low- silicon x-ray detectors
outdate ; pentafet :
the conclusion is that, in principle, it could
also be achieved with a modified-electrode
promising expectations may nevertheless be somewhat limited by the effect of
lossy dielectrics lossy and leakage currents leak , which
are seen to start to dominate
noise contributions precisely at the scale of 50 eV FWHM (see
discussion below). It is
nevertheless worth mentioning that JFET technology continues to
improve, and that experimental =0.4 pF proprietary JFETs are available to
CANBERRA Industries. This represents a full order of magnitude improvement
since luke2 .
Motivated by this potential for progress and aiming at obtaining detector specifications sufficient for a demonstration measurement of the coherent neutrino-nucleus scattering cross-section, a modified electrode HPGe detector was built by CANBERRA Industries during 2005. Besides the mentioned improvements to the electronics, another departure from the approach in luke2 was the choice of p-type rather than n-type electrode configuration. The first reason for this was to enjoy a diminished sensitivity to very low energy minimum ionizing backgrounds (surface betas, x-rays, etc.), profiting from the 0.5 mm deep dead layer imposed by the n+ lithium-drifted contact that spans most of the surface of a p-type HPGe. The second was to depart from the observed strong degradation in the energy resolution of the detector in luke2 , which can be ascribed to electron trapping in dislocations as they travel relatively long distances through the crystal to the n+ central electrode in that geometry. It is for this reason that p-type detectors generally exhibit better charge collection than n-type, and this was deemed especially important given the poor drift fields encountered with a modified electrode geometry.
The prototype displays an energy resolution (measured with a pulser) of 140 eV FWHM and a charge collection (Fig. 1) comparable to that from a conventional coaxial-electrode HPGe. To put its performance in perspective, besides a much reduced noise level and threshold, it shows an enhancement in energy resolution from a much degraded value of 15 FWHM (Co, 1.33 MeV) measured in luke2 to a 1.82 KeV FWHM, typical of a modern large coaxial HPGe. Charge collection uniformity along the axis varies by less than 0.15% (Fig. 1), compared to 3% in luke2 . In other words, the device has the mass (475 g) and energy resolution typical of a large HPGe gamma spectrometer while simultaneously displaying the low-noise and low energy threshold (Fig. 2) of an x-ray detector a hundred times lighter. This unique combination makes the device a first of a kind, opening up a number of other interesting applications discussed in the next section. It is the first time that such a combination of mass, energy threshold and resolution has been produced in a radiation detector of any kind.
From the point of view of target mass and sensitivity to very small energy depositions the device is ready for a measurement of coherent neutrino scattering in a reactor experiment: the inset in Fig. 3 displays the fraction of recoils above a choice of three different ionization energy thresholds E (i.e., the fraction of detectable events) as a function of their deposited (ionization) energy. The dotted line corresponds to pulser measurements (black dots) using this prototype (E 330 eV). To make these measurements possible, a careful energy calibration of the pulser was performed with the help of five gamma and x-ray lines identifiable in shielded conditions over the range 10-100 keV. The dashed line corresponds to envisioned minimal upgrades to the JFET (see below), resulting in E 200 eV. The dash-dotted line illustrates the ultimate performance (E 100 eV) that can be expected with present-day JFET technology, matching the state-of-the-art in low-noise x-ray detectors outdate ; pentafet . The combined effect of threshold and energy resolution can then be folded into the expected differential recoil rate in a reactor experiment (main plot, Fig. 3), leading to integrated detectable coherent scattering rates of 2.5, 6 and 28 events / kg day, respectively.
The quenching factor used for these calculations is the standard Lindhard theory prediction for Ge (k=0.2 in the notation of ge ). Measurements of this quenching factor in the interval of recoil energies 0.3-1.4 keV have been recently performed at the dedicated neutron beam described in a companion paper other , using the existing prototype. While the many details of the analysis must be necessarily reserved for another publication new2 , Fig. 4 illustrates the main results from this essential calibration, showing an excellent agreement with Lindhard-based expectations. It must be emphasized that the same range of recoil energies relevant to the upcoming reactor experiment (Fig. 5) has been explored in this calibration (Fig. 4), a crucial requirement before embarking in such a delicate measurement. To our knowledge, it is the first time that recoils this low in energy are directly and individually recorded (the measurements in ge are indirect, i.e., based on the sum of recoil energy and a 69 keV nuclear de-excitation gamma).
Statistical evidence for coherent scattering can be gathered with the present prototype within just a few months of data acquisition near a power reactor, once the background goal depicted in Fig. 5 is reached. The disappearance of the neutrino signal during the refueling of the reactor (spanning 1-3 mo. every 12-18 mo.) would confirm the measurement. Shielded runs at a depth of 6 m.w.e. and extensive MCNP4b simulations mcnp of U,Th, and K gamma and beta emissions in preamplifier parts, crystal holder and endcap indicate that the projected level of background is reachable with adequate active and passive gamma and neutron shielding in combination with a NaI(Tl) anti-Compton shield (Fig. 6). At least two precedents exist prec for achieving O(1) count/keV kg day in the few keV region while benefiting from just a shallow (few m.w.e.) overburden, what is expected at best in a reactor site (e.g., in a “tendon” gallery 20 m from the core adam2 ). An important source of non-radioactive background in the low energy region is microphonic noise: pulse shape discrimination (PSD) techniques (analog morales and digital klapdor ) can be applied against it. With the single precautions of vibration-absorbing pads under the Dewar and attention to mechanically decoupling the cryostat from shielding materials, the prototype is observed to be rather insensitive to these (Fig. 7). This is possibly a result of the very small capacitances involved microph .
It is worth emphasizing that contrary to what is implied in texono , PSD techniques alone cannot be used to reduce an energy threshold imposed by electronic noise, unless an inadmissible penalty in signal acceptance is paid. In other words, any PSD cuts must be justified on grounds of clear physical differences between signal and noise pulses, as in morales . Fig. 7 illustrates the indistinguishability of most true signals and noise below threshold. Concentration should be in reducing electronic noise at the hardware level. Towards this end, the noise components in this prototype have been characterized. As evidenced in the discussion of Fig. 3 the technique has not yet met the limitations imposed by present-day JFET technology: a threshold lower by a factor of should be presently feasible with further effort, leading to a enhancement in the signal rate per kg of detector mass for this neutrino process. An analysis of the electronic noise components in the existing prototype (Fig. 8) indicates that while the expectations described above for the series component have been met (i.e., noise determined by the JFET and detector capacitances, Eq. 1), a non-white component presently dominates. The origin of this is generally traceable to lossy dielectrics. Rearrangement and selection of the JFET package components should be able to lower this component to more manageable levels jf and to generate an electronic noise behavior truly comparable to that of the best pF x-ray detectors presently available. A small decrease in leakage current would also benefit the parallel noise component, leading to the (presently) ultimate performance, i.e., few tens of detectable recoils per kg of Ge per day in the vicinity ( 20 m) of a high-power reactor. We note that up to an additional order of magnitude in signal rate could be obtained from a dedicated effort to reduce noise levels beyond the present state-of-the-art.
While the detection of this intriguing mode of neutrino interaction seems finally within reach, many applications, either technological or in the study of fundamental neutrino properties would need to involve small arrays (10 elements) of these detectors: such HPGe clusters are common nowadays array . Recent advances in HPGe detector encapsulation encaps (leading to simplified handling and enhanced stability of array elements) and microphonic-free cryocooling of HPGe arrays cool can be applied to the construction of truly compact coherent neutrino detectors comprised of a modest number of modified-electrode crystals. In the specific realm of reactor monitoring applications, their sensitivity (hundreds of events per day) would then rival more cumbersome approaches based on underground ton-sized scintillator tanks bernstein1 ; adam2 .
Iii Applications Beyond Coherent Neutrino Scattering
The interest of a large-mass, ultra-low threshold semiconductor detector is not limited to coherent neutrino detection. Experimental searches for Weakly Interacting Massive Particles (WIMPs) revexp , prime dark matter particle candidates, typically assume that these particles are gravitationally bound to our galaxy, resulting in expected speeds relative to Earth of O(300) km/s. This in turn determines the recoil energies they are to deposit in dedicated detectors (few keV to tens of keV, depending on their mass). An alternative WIMP population consists of those that fell into stable closed solar orbits during the formation of the protosolar nebula, following colissionless mechanisms steigman . In that case their maximum speed is limited by the escape velocity from the Sun at Earth (42 km/s), leading to a large concentration of the recoil signal below the threshold of present-day WIMP detectors solar but now within reach of this new device. A second assumption driving most WIMP searches is the false tenet that the only supersymmetric WIMP candidate is the so-called neutralino revexp : as has been recently emphasized kuse , a parallel phenomenology exists for other non-pointlike supersymmetric dark matter candidates (e.g., Q-balls), one that involves very soft recoils, again generally below threshold and hence beyond detection for more conventional detector designs. A third application in this field of dark matter detection is the exploration of the possibility that a light (5-12 GeV/) particle might be responsible for the controversial WIMP DAMA signal dama , a hypothesis not contradicted yet by any other experiments sd ; si . The first experimental limits on these interesting possibilities can be extracted from background characterization runs for the existing prototype new2 , with a sensitivity that should increase as the technique is further developed. Ironically, WIMP interaction rates like those in Fig. 9 would frustrate any attempt to detect reactor neutrinos coherently. Separately from the subject of dark matter searches, it is also worth remembering that the present sensitivity to a finite neutrino magnetic moment should be increased by about an order of magnitude, down to in a detector with these characteristics wmm .
Another important application of a modified electrode HPGe arises from exploiting the characteristic temporal features of the preamplifier traces in this electrode configuration, features already noted in luke2 : Fig. 10 displays two sets of traces, the top panel for a conventional coaxial electrode detector, the bottom panel for the prototype under discussion, both crystals roughly the same mass and dimensions. The top trace in each panel corresponds to the preamplifier output, the bottom one is this same passed through a Timing Filter Amplifier (TFA) set at 10 ns differentiation and 10 ns integration time. Such a TFA trace is a good representation of the time-structure of the arrival of charge to the collecting electrodes klapdor (the same signal processing can be achieved on software using digitized preamplifier traces). For both detectors the event that originated the pulses was a multiple-site gamma interaction. In the coaxial TFA trace this is barely evident, a double “hump” or somewhat broadened trace being the only signature, resulting in a limited ability to differentiate multiple from single-site interactions klapdor . In the present prototype however, the longer distances that charge (holes in this case) must travel to the small p+ electrode result in traces generally stretched by a factor 5 in time, yielding a clear separation of individual site contributions to the overall pulse (a three, possibly four-site interaction in the figure). It is worth noting that the reduced electronic noise is in itself an advantage in this type of pulse shape analysis match . The insets in Fig. 10 graphically illustrate how the radial degeneracy imposed by the coaxial electrode dissolves for the modified electrode: in the first case, the trajectories along the dashed field lines followed by electrons and holes generated at similar radial distances in the crystal are comparable, leading to a trace that would most probably be misidentified as corresponding to a single-site interaction. In the second, for the same initial distribution of charge, the hole trajectories are considerably different, leading to positive identification of the event as a multi-site interaction. In Fig. 11 this effect is illustrated with the help of a 59.5 keV Am collimated gamma source impinging on the crystal perpendicularly to its axis and, given the low gamma energy, interacting via single-site photoelectric effect very close to the surface. Little difference is observed as a function of axial source position in coaxial traces (top), whereas the prototype traces (bottom) are clearly rich in position information. In this respect and as a simple curiosity, keeping in mind that the charge drift speed in depleted Ge is 10 ns/mm, it is possible to extract the rough crystal radius (2.5 cm in actuality) from the rise time of the d=43 mm trace (the closest gamma injection point to the p+ electrode, at the base of the crystal), as well as the approximate crystal length (4.4 cm) from the time dispersion in the onset of the traces.
Besides the evident commercial applications that distinguishing single from multiple-site interactions might have (e.g., an effective higher peak-to-Compton ratio for smallish crystals), next-generation neutrinoless double-beta decay searches using germanium crystals as source and target (MAJORANA maj , GERDA ger ) rely heavily on this distinction: the process of interest, the local emission of two short-ranged 1 MeV electrons indicating a neutrinoless decay, is eminently single-site, while a large fraction of backgrounds is not. The mentioned limited performance in this respect from conventional coaxial crystals has lead these experimental efforts to consider complex multi-electrode segmented germanium detectors, where the additional spatial granularity helps improve the background rejection ability. Evidently, the prohibitive energy resolution degradation observed in n-type modified electrode schemes luke2 would never allow one to consider a neutrinoless double-beta decay application, where the signature sought is a very precisely defined energy deposition corresponding to the Q-value of the nuclear transition (2039 keV for Ge). The optimal charge collection observed in a p-type design leads on the other hand to excellent prospects. Fig. 12 shows the results of a prototype irradiation with a natural Th source. Following th , the Ac gamma ray at 1588 keV typically interacts three times to deposit its full energy, whereas the line at 1592 keV arises from a double escape (i.e., a single interaction) from a 2614 keV emission in Tl. Their neighboring energies provide a convenient benchmark to asses the ability of a detector to distinguish single from multiple-site interactions th . A simple, energy-independent set of cuts based on peak counting of digitized TFA traces such as those in Fig. 10 leads to a background (multiple-site) rejection (BR) of 88% for a signal acceptance (SA) of 93% (Fig. 12). This compares very favorably, even prior to the beneficial effect of close-packing crystals, to an 8-channel segmented four-crystal “clover” HPGe detector (93% BR for 73% SA) steve . In the case of the modified electrode prototype, application of the same cuts to the neutrinoless double beta decay region of interest (2.04 MeV) leads to a reduction in background by 50%, in close agreement with the expectations from a simulation of this particular Th irradiation. In other words, the bulk of multiple-site backgrounds is successfully rejected.
Besides the obvious advantages that use of a single-channel device can bring to next generation germanium double-beta decay searches (increased speed of deployment, reduction in costs, simplified construction and analysis) there are other more subtle ones that can be listed: first, the largest fraction of the backgrounds is expected to arise from front-end electronics and their cabling, evidently much more numerous in a segmented detector approach. Second, the thermal load imposed by the excess cabling involved in segmented schemes is much reduced. Third, long-term instabilities and channel cross-talk are known undesirable features in some segmented configurations. Fourth, segmentation schemes involve use of n-type crystals: leaving aside the slightly lesser energy resolution these usually display (however crucial in double-beta), p-type crystals are more rugged, which is advantageous when arraying. They should also profit from the thick lithium-drifted dead layer to exhibit a much lower sensitivity to surface contaminations, implanted and absorbed alpha-emitting Rn daughters being a major concern in double-beta decay experiments.
A novel type of radiation detector, a large-mass p-type modified electrode HPGe diode has been presented. It features an unprecedented combination of sensitivity to small energy depositions, excellent energy resolution, large mass and built-in information about site multiplicity of the interactions: as is often the case, a promising new addition to the existing arsenal of radiation detection technologies can generate a number of exciting applications. In this case, the device can be considered a first viable coherent neutrino detector, allowing to realistically consider the possible technological applications that smallish neutrino detectors may one day find. In addition to this, a strong role in double-beta decay, neutrino magnetic moment and dark matter searches can be foreseen.
This work is supported by NSF CAREER award PHY-0239812, DOE/NNSA grant DE-FG52-0-5NA25686, and in part by the Kavli Institute for Cosmological Physics through grant NSF PHY-0114422 and a Research Innovation Award No. RI0917 (Research Corporation). We are indebted to J. Lenhert and T. Wilson at NNSA/NA-22 for their resolved support of this project and to L. Darken and D. Gutknecht for their contributions to crystal growing and detector fabrication. Similarly, to C. Aalseth, F.T. Avignone, J. Beacom, R.L. Brodzinski, G. Gelmini, P. Gondolo, A. Kusenko, P. Luke, H.S. Miley, B. Odom, D. Radford, J. Wilkerson and the MAJORANA collaboration for many useful exchanges. This work is dedicated to the memory of R.L. Brodzinski.
- (1) D. Z. Freedman et al., Annu. Rev. Nucl. Sci. 27 (1977) 167.
- (2) A. Drukier and L. Stodolsky, Phys. Rev. D30 (1984) 2295.
- (3) P.S. Barbeau et al., IEEE Trans. Nucl. Sci. 50 (2003) 1285.
- (4) B. Cabrera et al., Phys. Rev. Lett. 55 (1985) 25; A. S. Starostin and A. G. Beda, Phys. Atom. Nucl. 63 (2000) 1297; H.T. Wong, Nucl. Phys. B (Procs. Suppl.) 138 (2005) 333; A.G. Beda, Nucl. Instr. Meth. A531 (2004) 161.; C. Hagmann and A. Bernstein, IEEE Trans. Nucl. Sci. 51 (2004) 2151; I. Giomataris et al., hep-ex/0502033; S.A. Golubkov et al., Instr. Exp. Tech. 47 (2004) 799; K. Scholberg, Phys. Rev. D73 (2006) 033005; C. Braggio et al., Nucl. Instr. Meth. A568 (2006) 412; A. Bueno et al., Phys. Rev. D74 (2006) 033010; A. Bondar et al., physics/0611068.
”Design and Characterization of a Neutron Calibration
Facility for the Study of sub-keV Nuclear Recoils”,
P.S. Barbeau et al., submitted to Nucl. Instr.
See L. Stodolsky’s presentation in
A. Bernstein et al., nucl-ex/0108001;
M.M. Nieto et al., Nucl. Sci. Engin. 149 (2005) 270;
Procs. of the 2006 Applied Antineutrino Physics Workshop, available
- (8) L.M. Sehgal, Phys. Lett. B162 (1985) 370.
- (9) A. C. Dodd et al., Phys. Lett. B266 (1991) 434.
- (10) J. Barranco et al., JHEP 0512 (2005) 021.
- (11) J. Papavassiliou et al., hep-ph/0512029.
- (12) L.M. Krauss, Phys. Lett. B269, 407 (1991).
- (13) J. F. Beacom, W. M. Farr and P. Vogel , Phys. Rev. D66, 033001 (2002).
- (14) K. W. Jones and H. W. Kraner, Phys. Rev. A 11 (1975) 1347; K. W. Jones and H. W. Kraner, Phys. Rev. C 4 (1971) 125.
- (15) H.T. Wong et al. (TEXONO collaboration), Nucl. Phys. B (Proc. Suppl.) 138 (2005) 333; Nucl. Phys. B (Proc. Suppl.) 143 (2005) 501.
- (16) P.N. Luke, N.W. Madden and F.S. Goulding, IEEE Trans. Nucl. Sci. 32 (1985) 457; P.N. Luke, Nucl. Instr. Meth. A271 (1988) 567.
- (17) P.N. Luke, F.S. Goulding, N.W. Madden and R.H. Pehl, IEEE Trans. Nucl. Sci. 36 (1989) 926.
- (18) E. E. Haller, W. L. Hansen and F. S. Goulding, Adv. Phys. 30 (1981) 93.
- (19) G.B. Gelmini et al., Nucl. Phys. B281 (1987) 726.
- (20) D.A. Landis, F.S. Goulding, R.H. Pehl and J.T. Walton, IEEE Trans. Nucl. Sci. 18 (1971) 115; D.A. Landis, F.S. Goulding and J.M. Jaklevic, Nucl. Instr. Meth. 87 (1970) 211; F.S. Goulding, J.T. Walton and R.H. Pehl, IEEE Trans. Nucl. Sci. 17 (1970) 218.
- (21) J. Goulon et al., J. Synchrotron Rad. 12 (2005) 57.
- (22) J. Blair et al., Nucl. Instr. Meth. A422 (1999) 331.
- (23) V. Radeka, Ann. Rev. Nucl. Part. Sci. 38 (1988) 217.
- (24) T. Nashashibi and G. White, IEEE Trans. Nucl. Sci. 37 (1990) 452; Nucl. Instr. Meth. A322 (1992) 551.
- (25) V. Radeka, IEEE Trans. Nucl. Sci. 20 (1973) 182; H.E. Kern, J.M. McKenzie and J.H. McQuaid, IEEE Trans. Nucl. Sci. 19 (1972) 345.
- (26) J.P. Hurley, J.M. Mathiesen and V.L. Dagragnano, Nucl. Instr. Meth. 57 (1967) 109.
- (27) S.A. Pozzi et al., Nucl. Instr. Meth. A513, 550 (2003).
- (28) Y. Messous et al., Astropart. Phys. 3 (1995) 361.
- (29) P.S. Barbeau and J.I. Collar, in preparation.
- (30) MCNP, A General Monte Carlo N-Particle Transport Code, J.F. Briesmeister ed., Los Alamos National Laboratory, LA-12625-M, 1993.
- (31) H.B. Li et al., Phys. Rev. Lett. 90 (2003) 131802; A.G. Beda et al., Phys. At. Nucl. 61 (1998) 66.
A. Bernstein in Neutrino Geophysics, University of
Hawaii, December 2005,
- (33) J. Morales et al., Nucl. Instr. Meth. A321 (1992) 410.
- (34) L. Baudis et al., Nucl. Instr. Meth. A418 (1998) 348; J. Hellmig et al., Nucl. Instr. Meth. 455 (2000) 638; F. Petry et al., Nucl. Instr. Meth. 332 (1993) 107; D. Gonzalez et al., Nucl. Instr. Meth. A515 (2003) 634.
- (35) MD. Rashiduzzaman et al., Nucl. Instr. Meth. 160 (1979) 127; A.H.F. Muggleton, Nucl. Instr. Meth. 101 (1972) 113.
- (36) A. Pullia et al., Nucl. Instr. Meth. A380 (1996) 1; J. Llacer and D.F. Meier, IEEE Trans. Nucl. Sci. 24 (1977) 317.
- (37) D.M. Smith et al., Solar Physics 210 (2002) 33; P. Sangsingkeow et al., Nucl. Instr. Meth. A505 (2003) 183; 1997, P. Mandrou et al., Proc. 2nd INTEGRAL Workshop, ESA SP-382, p. 591.
- (38) J.A. Becker et al., Nucl. Instr. Meth. A505 (2003) 167; J. Eberth et al., Nucl. Instr. Meth. A369 (1996) 135.
- (39) G. Fritz in Nuclear Spectrometry user’s Forum, NPL Teddington, May 2006, available from http://www.npl.co.uk/nsuf/; R.G. Ross et al., Advances in Cryogenic Engineering: Trans. of the Cryogenic Engineering Conference, 49B, ed. J. Waynert et al., pp. 1197 - 1203 (2004); R. Boyle, Cryogenics 44 (2004) 389; R.S. Bathia et al., Cryogenics 41 (2002) 851; R.S. Bathia et al., Cryogenics 39 (1999) 701; B.E. Evans et al., Cryogenics 37 (1997) 695.
- (40) R.J. Gaitskell, An. Rev. Nucl. Part. Sci. 54, 315 (2004).
- (41) G. Steigman et al., Astrophys. J. 83 (1978) 1050.
- (42) J.I. Collar, Phys. Rev. D59 (1999) 063514-1, and references therein; T. Damour and L. M. Krauss, Phys. Rev. Lett. 81 (1998) 5726.
- (43) A. Kusenko and P.L. Steinhardt, Phys. Rev. Lett. 87 (2001) 141301; G. Gelmini, A. Kusenko and S. Nussinov, Phys. Rev. Lett. 89 (2002) 101302.
- (44) P. Belli et al., Phys. Rev. D61 (2000) 023512; Phys. Rev. D66 (2002) 043503.
- (45) C. Savage, P. Gondolo and K. Freese, Phys. Rev. D70 (2004) 123513.
- (46) P. Gondolo and G. Gelmini, Phys. Rev. D71 (2005) 123520.
- (47) H.T. Wong and H. Li, Mod. Phys. Lett. A20 (2005) 1103.
- (48) http://majorana.pnl.gov/
- (49) http://www.mpi-hd.mpg.de/ge76/
- (50) D. Gonzales et al., Nucl. Instr. and Meth. A515 (2003) 634; B. Majorovits and H.V. Klapdor-Kleingrothaus, Eur. Phys. J. A6 (1999) 463.
- (51) S.R. Elliot et al., Nucl. Instr. Meth. A558 (2006) 504.