Superheated Droplet Detectors (SDDs) are becoming commonplace in neutron
personnel dosimetry. Their total insensitivity to minimum ionizing radiation
(while responsive to nuclear recoils of energies ~ few keV), together with
their low cost, ease of production, and operation at room temperature and 1 atm
makes them ideal for Cold Dark Matter (CDM) searches. SDD's are optimal for the
exploration of the spin-dependent neutralino coupling due to their high
fluorine content. The status of SIMPLE (Superheated Instrument for Massive
ParticLe Experiments) is presented. Under realistic background considerations,
we expect an improvement in the present Cold Dark Matter sensitivity of 2-3
orders of magnitude after ~1 kg-y of data acquisition.Comment: 6 pages, including 4 figures. To appear in the Proceedings of the
  Intl. Workshop on the Identification of Dark Matter (Sheffield, Sept. 96

Collar, J. I.

Girard, T. A.

Limagne, D.

Waysand, G.

English

arXiv

Superheated Droplet Detectors (SDDs) are becoming commonplace in neutron personnel dosimetry. Their total insensitivity to minimum ionizing radiation (while responsive to nuclear recoils of energies ~ few keV), together with their low cost, ease of production, and operation at room temperature and 1 atm makes them ideal for Cold Dark Matter (CDM) searches. SDD's are optimal for the exploration of the spin-dependent neutralino coupling due to their high fluorine content. The status of SIMPLE (Superheated Instrument for Massive ParticLe Experiments) is presented. Under realistic background considerations, we expect an improvement in the present Cold Dark Matter sensitivity of 2-3 orders of magnitude after ~1 kg-y of data acquisition.Superheated Droplet Detectors (SDDs) are becoming commonplace in neutron personnel dosimetry. Their total insensitivity to minimum ionizing radiation (while responsive to nuclear recoils of energies few keV), together with their low cost, ease of production, and operation at room temperature and 1 atm makes them ideal for Cold Dark Matter (CDM) searches. SDD's are optimal for the exploration of the spin-dependent neutralino coupling due to their high fluorine content. The status of SIMPLE (Superheated Instrument for Massive ParticLe Experiments) is presented. Under realistic background considerations, we expect an improvement in the present Cold Dark Matter sensitivity of 2-3 orders of magnitude after 1 kg-y of data acquisition

Collar, J.I.

Girard, T.A.

CERN Document Server

astro-ph/9610266   31 Oct 19961SUPERHEATED DROPLET DETECTORS AS CDM DETECTORS:THE SIMPLE EXPERIMENTJ.I. COLLAR  §   •, T.A. GIRARD  •, D. LIMAGNE  * , G. WAYSAND  *  § PPE Division, CERN, Geneve 23, CH-1211  •Centro de Física Nuclear, Universidade de Lisboa1699 Lisboa, Portugal  * Groupe de Physique des Solides (URA 17 CNRS), Universités Paris VII/VI75251 Paris, FranceSuperheated Droplet Detectors (SDDs) are becoming commonplace in neutronpersonnel dosimetry. Their total insensitivity to minimum ionizing radiation(while responsive to nuclear recoils of energies ~ few keV), together with their lowcost, ease of production, and operation at room temperature and 1 atm makesthem ideal for Cold Dark Matter (CDM) searches. SDD's are optimal for theexploration of the spin-dependent neutralino coupling due to their high fluorinecontent. The status of SIMPLE (Superheated Instrument for Massive ParticLeExperiments) is presented. Under realistic background considerations, we expectan improvement in the present Cold Dark Matter sensitivity of 2-3 orders ofmagnitude after ~1 kg-y of data acquisition.There is an evident need for large-mass, low-background detectors ableto pervade the neutralino region of the WIMP phase space: the expectedcounting rate in most target materials is <1 / kg-day, and in order todetect  the characteristic ~5 % annual modulation in the WIMP signal [1]within a reasonable time span, detector masses of order >50 kg areneeded.In this regard, Superheated Droplet Detectors (SDD) have beenrecently proposed as a convenient approach [2,3]. These devices consist ofa dispersion of droplets of a superheated liquid (diameter ~10-100 m m),fixed in a viscous polymer or aqueous gel. SDDs are sensitive only tohigh-Linear Energy Transfer (LET) radiation and therefore energeticmuons, gamma rays, x-rays and beta particles fall well below theiractivation threshold, typically >200 keV/ m m. Strong irradiations showthat SDDs are totally insensitive to gamma rays of energies <6 MeV atoperating temperatures <30 °  C [4], while responding to neutron recoils ofonly a few keV. The dense energy deposition by a high-LET particle in adroplet breaks the metastability, causing its vaporization. This process isindependent of the size of the droplet, a major advantage overSuperheated Superconducting Granules (SSG). The resulting bubble(diameter ~1 mm) can be optically recorded and/or the characteristic (infrequency and time duration) sound emitted during the violent2vaporization can be picked up [5]. SDD production involves the use of ahigh pressure reactor, but these detectors operate in a passive and safefashion at room temperature and 1 atm, and have found greatacceptation as neutron pocket dosimeters. While based on the samephysical principles, the problems inherent to using a bubble chamber fordark matter detection [6] (pressure control, safety, duty-time andinhomogeneous nucleation) are absent from SDDs.The following is a brief summary of SDDs' advantages for WIMPdetection:• Total insensitivity to low-LET radiation.• Room temperature and atmospheric pressure continuous and safeoperation.     1001011021035 10 15 20 25 30 35 40threshold energy (keV)operating temperature, T (° C)ClFCaFigure 1: Bubble nucleation threshold energy for recoils from each of the atomicconstituents in a freon-12 SDD, as a function of operating temperature. Alphaparticles can induce nucleation only at T > 27 ° C [2,10].• The low energy threshold for nuclear recoils can be tuned bytemperature control (fig. 1). This is, for instance, ~5 keV at 30° C, ~16keV at 20 ° C for freon-12 SDDs [7]. A differential spectrum is possible bytemperature ramping.• Known response to neutron recoils: monochromatic neutroncalibrations have been performed [8,9] and are in good agreement withtheoretical predictions (fig. 2). Similar predictions can be made for CDMcandidates [2].• Low cost of materials and shielding: less than 200 US $ per kg ofactive mass for "homemade" SDDs (~2,500 times more for commercialSDDs). Large target masses are a realistic possibility.• Longevity and stability of response (measured for > 3 yr [11]).• Simple optical and/or sound recording of bubble nucleation.3• SDDs are fluorine-rich (31% w.f. in freon-12), fluorine having thelargest spin-dependent neutralino cross section [12].• The freon-12 and -134a response falls dramatically for neutronenergies <400 keV. This facilitates an efficient ambient neutronbackground reduction with modest water shielding. This water shieldingconveniently  doubles as a detector temperature regulator (water iscirculated by a bath-temperature controller).     10-710-510-310-110-8 10 -6 10-4 10-2 100Bubbles / n per cm2 / cm3 Incident neutron energy (MeV)40° C30° C20° C   20 ° C (f-134a)Figure 2: A comparison of our preliminary neutron response calculations for f-12and f-134a with the experimental points at 20° C of [8]. Freon-134a has noresponse below 100 keV due to the absence of Cl in its composition [8].• No heavy elements are required in the shielding, due to the SDD'sinherent lack of response to photons. As a result, as far as the neutronbackground is concerned, only a shallow (~100 m.w.e.) underground siteis required (fig. 3). Recoils directly induced by muons may require eitheractive shielding or a slightly deeper site [13].• Other applications such as solar and stellar collapse neutrinodetection can be envisioned, taking advantage of the coherent nuclearelastic scattering cross section. We are exploring the possible use of otherrefrigerant liquids for this purpose (fig. 4).Even for such an advantageous type of detector, some backgroundsmust be considered and reduced [2]:§   SDDs are highly stable against mechanical vibrations [16].§ Homogeneous (spontaneous) nucleation is negligible for theenvisioned degrees of superheat. According to the classical theory ofhomogeneous nucleation [17], a temperature of > 91° C (at 1 atm) wouldbe needed to produce a background of concern in a CDM search.4§ Neutrons from natural radioactivity in rock and detector materials:we have performed MCNP-4a Monte Carlo simulations using the GranSasso ambient neutron spectrum as an input, showing that ~40 cm ofhydrogenated shielding are enough to reduce this background to ~0.01bubbles/kg freon-12/day. The required radiopurity in the shielding itselfis <0.01 ppb U/Th, a level found even in some US tap water.     10-610-510-410-310-21 10 100neutrons / cm2 / sdepth (meter water equivalent)( m ,n) in Pb(m ,n) in rock( m ,n) in H2Osecondary •Figure 3: Components of neutron flux as a function of underground depth. Thegreyed band covers the typical contribution from natural radioactivity in rocks,based on large depth measurements and the average U and Th concentration inthe crust. The contribution to the flux from muon interactions is derived from thesystematics and measurements of [14]. In the absence of high-Z shielding, there isno significant reduction in flux below ~100 m.w.e.. Many urban locations areavailable at such shallow depths. The • (x) marks the Pb-shielded (unshielded)flux in the Stanford underground facility. Similar calculations are  found in [15].§ Neutrons from ( m ,n) in shielding and detector are unimportantbelow few tens of m.w.e. (fig. 3).§ While a particles cannot produce nucleation below ~27 ° C (fig. 1),the accompanying a -recoils can. A radiopurity in the superheated liquidof <10-6 ppb is needed to render this background less important thanthose listed above. Fractional distillation of the freon, if needed, is apossibility. For low filling factors, the short range of the a -recoils (~0.1m m) makes the nucleation efficiency for gel-originated a -recoils verysmall; <0.01 ppb U/Th in the gel are sufficient.§ Auger electrons are high-LET particles, yet the maximum energydeposition for freon-12 is ~2.5 keV, corresponding to the binding energyof the K-shell in Cl and below threshold for this search (fig. 1).Taking the moderated neutron background as dominant, it is shown[2] that an improvement of 2-3 orders of magnitude in the present spin-independent WIMP sensitivity is expected from a 1 kg active mass SDD5after only 1 year of data acquisition, widely exploring the expectedneutralino phase-space.It is evident that optimization of commercial SDDs must be made, forinstance, in order to meet the background requirements listed above. Forthis reason, and foreseeing the construction of inexpensive multi-kgSDDs, a large (97 cm length, 30 cm diam.) pressure reactor, dedicated toSDD production, was built in cooperation with COMEX-PRO. Thereactor withstands 60 bar (allowing us to experiment with a large varietyof superheated liquids) and features two large inspection windows and a16 l tray that can be used to collect the SDD. The reactor presentlyhouses two Ismatec MS-A31016 biological pumps, with external flowcontrols, used to propel freon and gel during production. We have used acoaxial-flow method of production [18] where freon is injected through asmall (200 m m) nozzle formed by electro-erosion. This freon jet issurrounded by a gel flow, the laminar flow is broken spontaneously, andsmall freon droplets are formed. We have recently installed anelectromagnetic stirrer inside the chamber, which allows for the simplerand widely used method of making a freon and gel "mayonnaise". So farwe have only used gelatine, glycerol and Aquasonic [19] -based gels, butwe intend to experiment with polyvinyl alcohol (PVA), polybutylene andacrylamide preparations that should yield a more stable product.A shallow underground location (26 m calcite overburden), withconvenient horizontal access has been found 40 km south of Paris.Measurements of the radiopurity of the rock and tarmac have beenperformed and we will soon start measurement of the (paraffin shielded)ambient neutron flux using commercial SDDs. In a first prototype-stageexperiment, to start immediately, we plan to install a ~100g active massSDD in a 30 lt water bath, held stable within 0.1 ° C by a BioblockPolystat temperature-controller. The bath will be surrounded by ~35 cmparaffin, already available. The sound emission produced by the bubblevaporization will be picked-up by electret microphones, amplified, andthe pulse shape registered in a LeCroy 9350 AM digital storageoscilloscope. Our first objectives are to determine the intrinsicradiopurity of the freon liquid, to measure the dominant backgroundsand to characterize the detector using a weak Cf-252 neutron source (wealso intend to instrument a few commercial SDDs in the same shieldingfor comparison purposes). It is however interesting to note that in theabsence of unforeseen backgrounds, limits comparable to those fromultra-low background Ge detectors could be obtained in ~2 mo. even withsuch a small-mass prototype. The next stage will involve the installationof a multi-kg SDD in the same or a deeper location, depending on theprototype results.6     10-210-11001011021001011021035 15 25 35Ec (keV)dE/dx (keV/m m)operating temperature (° C)13B1218122211513B11222115218Figure 4: Minimum energy deposition and stopping power required for bubblenucleation in some refrigerant liquids (freon-x, labelled by x). Some other suitablemoderately superheated refrigerants are R-134a, -143a, -32, -125. Possible targetliquids contain heavy elements such as I and Br and may be envisioned as solarneutrino detectors. As an indicative value, a B-8 solar neutrino can impart amaximum kinetic energy of ~14 keV to a Cl nucleus.References:1.  A.K. Drukier et al., Phys. Rev. D 33, 3495 (1986) .2.  J.I. Collar, Phys. Rev. D 54, R1247 (1996).3.  L.A. Hamel et al., Proc. of 2nd Workshop on The Dark Side of theUniverse, Rome, Italy, November 1995, prep. hep-ex/9602004.4.  W. Lim et al., Nucl. Instr. Meth. A 370, 568 (1996);S.G. Vaijapurkar et al., Radiat. Meas. 24, 309 (1995) and Radiat.Meas. 23, 753 (1994); M.J. Harper et al., Health Physics 68, 670(1995); C.A. Perks et al., Radiat. Prot. Dosim. 23, 131 (1988); H. Inget al., Nucl. Tracks Radiat. Meas. 8, 285 (1984).5.  R.E. Apfel et al., Rev. Sci. Instrum. 54, 1397 (1983).6.  V. Zacek, Nuovo Cimento A 107, 291 (1994).7.  Freon-12 is a registered trademark of the Du Pont Co.8.  Y.-C. Lo et al., Phys. Rev. A 38, 5260 (1988).9.  M.J. Harper, Ph. D. dissertation, University of Maryland, 199110.  C.K. Wang et al., Nucl. Instr. & Meth. A 353, 524 (1994).11.  J.A. Sawicki, Nucl. Instr. & Meth. A 336, 215 (1993).12.  J. Ellis, Nuovo Cimento A 107, 1091 (1994).13.  P.B. Price, private communication.14.  G.V. Gorshkov et al., Sov. J. Nucl. Phys. 13, 450 (1971).15.  G. Heusser, Nucl. Instr. Meth. A 369, 539 (1996).16.  T. Cousins et al., IEEE Trans. Nucl. Sci. 37, 1769 (1990).17.  M. Blander et al., Am. Inst. Chem. Eng. J.  21, 833 (1975).18.  Y.-C. Lo, Ph. D. dissertation, Yale University, 1987.19.  Aquasonic is a registered trademark of Parker laboratories, NJ.

Superheated Droplet Detectors as CDM Detectors: The SIMPLE Experiment

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Superheated Droplet Detectors as CDM Detectors: The SIMPLE Experiment

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