The SIMPLE SDD

A. C. Fernandes 1, 2, T. Morlat 2, M. Felizardo 1, 3, J. I. Collar 4, J. Puibasset 5,

G. Waysand 5, 6, H. S. Miley 7, A. R. Ramos 1, 2 , T. A. Girard 2, F. Giuliani 2,

D. Limagne 5, J. G. Marques 1, 2, R. C. Martins 3 and C. Oliveira 1

1 Instituto Tecnológico e Nuclear, Estrada Nacional 10, P-2686-953 Sacavém, Portugal

2 Centro de Física Nuclear, Universidade de Lisboa, Av. Prof. Gama Pinto, 2, P-1649-003

Lisbon, Portugal

3 Department of Electronics, Instituto Superior Técnico, Av. Rovisco Pais, 1, P-1049-001

Lisbon, Portugal

4 Department of Physics, University of Chicago, Chicago, IL 60637, USA

5 Groupe de Physique des Solides (UMR CNRS 75-88), Université Paris 7 & 6, F-75251

Paris, France

6 Pacific Northwest National Laboratory, Richland, WA 99352, USA

7 Laboratoire Souterrain à bas Bruit, F-84400 Rustrel-Pays d’Apt, France

Running title: The SIMPLE SDD

The SIMPLE SDD

A. C. Fernandes, T. Morlat, M. da Costa, J. I. Collar, J. Puibasset, G. Waysand,

H. S. Miley, A. R. Ramos, T. A. Girard, F. Giuliani, D. Limagne, J. G. Marques ,

R. C. Martins and C. Oliveira

ABSTRACT

We describe the fabrication and characterisation of the SIMPLE superheated droplet

detector, a 10 g active mass device of C2ClF5 in 1-3% weight concentrations currently

employed in a direct search for spin-dependent astroparticle dark matter candidates.

INTRODUCTION

SIMPLE (Superheated Instrument for Massive Particle Search) is an experiment (1, 2)

to search for evidence of spin-dependent dark matter using fluorine-loaded superheated

droplet detectors (SDDs). The application of the detector is based on the presence of

fluorine, which possesses the highest figure-of-merit for spin-dependent interactions.

Furthermore, these devices are virtually insensitive to the majority of backgrounds

associated with such searches. For dark matter applications, less than 10 events kg-1 day-1

are expected. The sensitivity to backgrounds is therefore an important issue for detector

use.

Following the thermal spike model of Seitz (3), there are two thresholds for bubble

nucleation: (1) the deposited energy must be larger than the work of formation of a critical

nucleus (Ec), and (2) Ec must be deposited over a distance of the order of a critical radius

(rc). Both thresholds can be tuned by modifying the operating conditions as to render the

SDD insensitive to radiations depositing less than ~ 200 keV µm-1.

Although the SDD can be made insensitive to energetic muons, gamma-rays, X-rays

and electrons with linear energy transfer (LET) below this threshold, response to neutrons

and alpha particles remains problematic. These originate from radioactivity in the rock,

detector shielding and the detector itself. We describe the detector fabrication and response

studies required by the large volume and concentration of the SIMPLE device.

FABRICATION AND INSTRUMENTATION

Standard detectors for dosimetry are loaded with ~ 0.04% w/w active material. In the

SIMPLE project, the SDDs are fabricated inhouse (1) from C2ClF5 (R-115) with a 1-3%

loading. R-115 is selected because of its low solubility (reducing the probability of

unchecked bubble growth by permeation) and large molecular size (for which the

formation of clathrate hydrates is stoichiometrically forbidden (4)). Refrigerant droplets

(diameter 5–100 µm) are fractionated by rapid stirring and dispersed in an elastic, viscous

gel. A homogeneous droplet suspension was produced by adjusting the gel composition in

order to obtain a uniform density, equal to that of liquid freon. The resulting mixture is

outgassed and maintained above its gelation temperature before placing it in the hyperbaric

reactor. The pressure is raised well above the freon vapour pressure to avoid boiling during

the vigorous stirring that follows. After a uniform droplet dispersion has been obtained,

cooling, setting and stepwise adiabatic decompression yield the SDD (Fig. 1). Numerous

practical precautions are necessary for producing stable modules. For example, the

stepwise decompression procedure used is identical to that employed by scuba divers

returning to the surface, in order to minimize the cavitation of dissolved gas bubbles which,

in SDDs, can act as inhomogeneous nucleation centres.

As seen in Fig. 2, the detector is contained in a glass flask of one liter volume, with

an active refrigerant mass of ~ 10 g. Device readout is provided by a piezoelectric

transducer (Murata PKM 13) immersed in a glycerine layer at the top of the flask, which

monitors the acoustic shock wave accompanying a bubble nucleation. The transducer

signal is amplified by a factor of 105, and recorded in a Labview platform, together with the

signals from other detectors, a wide-band hydrophone (Benthos AQ 4) and an acoustic

monitor placed outside the bath/shielding. The Fourier transform of the transducer signal

comprises a well-defined frequency response, with a primary harmonic at 5.5 kHz and a

time span of a few millisecond (Fig. 3). During runs with refrigerant-free ‘dummy’

modules, similar signals arising from pressure microleaks in plastic SDD caps are observed

at a rate of 1 event per day. Even at atmospheric pressure, a residual rate of 0.3 event per

day of characteristic EM noise events is present. These microleaks have in principle been

eliminated.

The SDDs are stored at -4 ºC and 4 atm pressure during storage and transportation to

make the R-115 stably liquid, preventing their response to environmental radiation.

PURIFICATION

The detector response to alpha particles has been examined by uniformly diluting a

400 Bq liquid 241Am source (an alpha emitter) into the matrix before setting the gel. Prior

to an extensive component purification, the spectrum in unspiked devices (Fig. 4,

histogram) had a close resemblance to that produced by the 241Am spiking. The result is

consistent with the presence of a small (~ 10-6 Bq g-1) 228Th contamination which was

confirmed by low-level alpha spectroscopy. Currently, the gelating agent, polymer

additives and glycerol are purified using a pre-eluted ion-exchanging resin (Dowex 21 K)

specifically suited for actinide removal. Each ingredient is pressure-forced through 0.2 µm

filters (Spiral Cap CQS 92 HSPPK, Lewatit MP 500 WS) to remove motes that might act

as nucleation centres. The freon is single distilled and the water is double distilled.

Fig. 4 demonstrates the result of successive purifications of the detector ingredients.

Three regimes of background dominance - alpha-induced recoiling ions (F, C, Cl), alphas

and gamma-rays - are delimited by vertical lines. The sudden rise at 15 ºC originates from

high LET Auger electron cascades following the interactions of environmental gamma-rays

with Cl atoms in the refrigerant (5). The current overall background level is ~ 50 events kg-1

day-1 at 9 ºC and 2 atm.

NEUTRON RESPONSE

The neutron response of SDDs with low freon concentration has been extensively

studied (6, 7) and found to match theoretical expectations. The response of SIMPLE SDDs

has been investigated by irradiating the devices with low-intensity sources of 252Cf (8, 9) and

Am-Be (10). Comparison with simulations yields a generally good agreement for

temperatures higher than 5 ºC (Fig. 5). A complete MCNP-4A (11) simulation of the

calibration set-up takes into account the small contributions from albedo and thermal

neutrons. For calculating nucleation rates (12, 13), cross sections for elastic, inelastic, (n,α)

and (n,p) reactions in the refrigerant are extracted from ENDFB-VI libraries. Furthermore,

deposited energy distribution tables are built using the SPECTER code (14) and the stopping

power of recoiling species are taken from SRIM98 (15). The fluorine threshold recoil energy

corresponding to each temperature is shown in the top axis of Fig. 5, as computed via the

Seitz model, and identified with the maximum recoil energy ERmax = [4A/(1+A)2]En, where

A is the atomic mass of the target and En is the incident neutron energy. The measurements

yielded fitting detection efficiencies of 74 % and 34% for 1 atm and 2 atm operation,

respectively.

The response of SIMPLE SDDs was investigated further in quasi-monoenergetic

filtered neutron beams of 25, 54 and 144 keV implemented at the Portuguese Research

Reactor (16). The results, such as shown in Fig. 6, yielded acoustic detection efficiencies of

~ 70% and 30% at 1 atm and 2 atm, respectively. The results are in good agreement with

the thermodynamic calculations shown in Fig. 7 for 2 atm operating pressure, and confirm

the energy response as a function of temperature. The sharp edges at ~ 54 keV of recoil

neutron energy for fluorine (27 keV for carbon) result from a partial energy deposition

within the critical radius (rc). For lower energies, essentially all the energy deposition is

contained within a droplet. The carbon edge is significantly smoother than that of fluorine,

allowing to distinguish the carbon thermodynamical kinks in the measurement.

SUMMARY

SDDs composed of R-115 distributed in an elastic, viscous gel matrix with a 1-3%

loading, have been developed to address the constraints of the SIMPLE dark matter search.

Inhouse fabrication procedures result in a low cost (USD 200 kg-1) device with ~ 10 g

active mass per liter, and 40 day stability over continuous exposure. Studies of intrinsic

alpha particle background levels in a well-shielded subterranean environment indicate a

rate of ~ 50 events kg-1 day-1, reduced from levels of ~ 104 events kg-1 dy-1 following

extensive purification procedures of the device chemistry. Neutron response studies of

these devices are in good agreement with the thermodynamic calculations, and indicate an

acoustic detection efficiency of 70% and 30% at operating pressures of 1 atm and 2 atm,

respectively.

ACKNOWLEDGEMENTS

This work was partly supported by grants POCTI FNU/43683/2002, POCTI

FNU/32493/2001 and SFRH/BPD/14464/2003 of the Foundation for Science and

Technology of Portugal, co-financed by FEDER.

REFERENCES

1. Collar, J. I., Puibasset, J., Girard, T. A., Limagne, D., Miley, H. S. and Waysand, G.

First dark matter limits from a large-mass, low-background superheated droplet

detector. Phys. Rev. Lett. 85, 3083-3086 (2000).

2. Collar, J. I., Puibasset, J., Girard, T. A., Limagne, D., Miley, H. S. and Waysand, G.

Prospects for SIMPLE 2000: a large-mass, low-background superheated droplet

detector for WIMP searches, New Journ. Phys. 2, 14.1-14.14 (2000).

3. Seitz, F. On the theory of the bubble chambers. Phys. Fluids 1, 2-13 (1958).

4. Sloan Jr. E. D. Clathrate hydrates of natural gases (New York, NY: Marcel Dekker

Inc.) (1998) ISBN 0824799372; Mori, Y. H., private communication.

5. Tenner, A. G. Nucleation in bubble chambers, Nucl. Instr. Meth. 22, 1-42 (1963).

6. Apfel, R. E. The superheated drop detector, Nucl. Instr. Meth. 162, 603-608 (1979).

7. d’Errico, F. Radiation dosimetry and spectrometry with superheated emulsions,

Nucl. Instr. Meth. B164, 229-254 (2001).

8. Ing, H., Noulty, R. A and McLean, T. D. Bubble detectors – a maturing technology,

Radiat. Meas. 27, 1-11 (1997)

9. Tu, C. Q., Guo, S.L., Wang, Y.L., Hao, X. H., Chen, C. M. and Su, J. L. Study of

bubble damage detectors for neutron detection, Radiat. Meas. 28, 159-162 (1997).

10. Mala Das, Roy, B., Chartjee, B. K. and Roy, S. C. Efficiency of neutron detection of

superheated drops of Freon-22, Radiat. Meas. 30, 35-39 (1997).

11. Briesmeister, J. F. (ed.) MCNP - a general Monte Carlo N-Particle transport code,

version 4A, LA-12625 (Los Alamos, NM: Los Alamos National Laboratory) (1993).

12. Collar, J. I. Superheated microdrops as cold dark matter detectors. Phys. Rev. D54,

1247-1251 (1996).

13. Lo, Y. C. and Apfel, R. E. Prediction and experimental confirmation of the response

functions for neutron detection using superheated drops, Phys. Rev. A38, 5260-5266

(1988).

14. Greenwood, L. R. and Smither, R. K. SPECTER, neutron damage calculations for

materials irradiations, ANL/FPP/TM-197 (Argonne, IL: Argonne National

Laboratory) (1985).

15. Ziegler, J. F., Biersack, J. P. and Littmark, U. Stopping and ranges of ions in matter

(New York, NY: Pergamon Press) (1985) ISBN�008021603X; http://www.srim.org/

16. Giuliani, F., Oliveira, C., Collar, J.I., Girard, T.A., Morlat, T., Limagne, D., Marques,

J. G., Ramos, A. R. and Waysand, G. Response of SIMPLE SDDs to monochromatic

neutron irradiations, Nucl. Instr. Meth. A526, 348-358 (2004).

FIGURE CAPTIONS

Figure 1. Simultaneous creation of two metastable systems.

Figure 2. Current SIMPLE SDD.

Figure 3. Signal and noise in current SIMPLE modules: pulse shape (top-left-hand plot)

and its Fourier transform (top-right-hand); pressure microleaks (bottom-left-hand plot) and

EM noise events (bottom-right-hand plot).

Figure 4. The SDD background at 90 m.w.e. and p=2 atm, following cumulative steps of

cleansing: histogram, double distillation of water and microfiltration; (•) single distillation

of refrigerant and purification of glycerine; (o) purification of gelatine and PVP.

Figure 5. Detector response to 252Cf neutron irradiations, in comparison with MCNP

simulation (---) as discussed in the text.

Figure 6. Detector response to filtered beam irradiations (Si+Ti filter) and comparison with

normalised simulation results ().

Figure 7. Energy calibration of SDDs and comparison with calculations based on the Seitz

thermal spike model (3).

0

10

20

30

40

50

60

0 5 10 15 20

tem

pera

ture

(°C

)

pressure (atm)

SO

LG

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approx. boundary(T

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melt )

GAS

LIQUID

agit. slow-down (45 min)

fast stirring (8 hrs) bulk freon

addition

thermalization and rest (overnight)

stop stirring

stop hot-plate

adiabatic decompression (~10-2 atm / s)

SDD!

fast decompression•

•

••

••

• thermal setting of gel (2 hrs)

Fig. 1

Fig. 2

Fig. 3

Fig. 4

0

0.5

1

1.5

2

2.5

3

3.5

-5 0 5 10 15

operating temperature, T (°C)

F recoil threshold energy, P=2 atm (keV)

P=1 atm

P=2 atm

Fig. 5

0

0.05

0.1

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0.2

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0.3

0.35

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[bub

bles

/min

/g]

temperature (ºC)

7406E0

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(C)

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225

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Fig. 6

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50

100

150

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(ke

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temperature (°C)

P = 2 atm

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Fig. 7