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A liquid xenon ionization chamber in an all-uoropolymer vessel
Nuclear Instruments and Methods in Physics Research A 578 (2007) 409420
A liquid xenon ionization chamber in an all-uoropolymer vessel
F. LePort
a
, A. Pocar
a,
Ã
, L. Bartoszek
a
, R. DeVoe
a
, P. Fierlinger
a
, B. Flatt
a
, G. Gratta
a
,
M. Green
a
, T. Koffas
a,1
, M. Montero D´ez
a
, R. Neilson
a
, K. OSullivan
a
, S. Waldman
a,2
,
J. Wodin
a
, D. Woisard
b
, E. Baussan
c
, M. Breidenbach
d
, R. Conley
d
, W. Fairbank Jr.
e
,
J. Farine
f
, C. Hall
d,3
, K. Hall
e
, D. Hallman
f
, C. Hargrove
g
, J. Hodgson
d
, S. Jeng
e
,
D.S. Leonard
h
, D. Mackay
d
, Y. Martin
c
, A. Odian
d
, L. Ounalli
c
, A. Piepke
h
, C.Y. Prescott
d
,
P.C. Rowson
d
, K. Skarpaas
d
, D. Schenker
c
, D. Sinclair
g
, V. Stekhanov
i
, V. Strickland
g
,
C. Virtue
f
, J.-L. Vuilleumier
c
, J.-M. Vuilleumier
c
, K. Wamba
d
, P. Weber
c
a
Physics Department, Stanford University, Stanford, CA, USA
b
Applied Plastics Technology, Inc., Bristol, RI, USA
c
Institut de Physique, Universite´ de Neuchatel, Neuchatel, Switzerland
d
Stanford Linear Accelerator Center, Menlo Park, CA, USA
e
Physics Department, Colorado State University, Fort Collins, CO, USA
f
Physics Department, Laurentian University, Sudbury, Ont., Canada
g
Physics Department, Carleton Univerisity, Ottawa, Ont., Canada
h
Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL, USA
i
Institute for Theoretical and Experimental Physics, Moscow, Russia
Received 11 December 2006; received in revised form 16 April 2007; accepted 14 May 2007
EXO Collaboration
Available online 25 May 2007
Abstract
A novel technique has been developed to build vessels for liquid xenon ionization detectors entirely out of an ultra-clean
uoropolymer. One such detector was operated inside a welded, He leak tight, all-uoropolymer chamber. The measured energy
resolution for 570 keV gamma rays is s=E ¼ 5:1% at a drift eld of 1.5 kV/cm, in line with the best values obtained for ionization only
detectors run in LXe using conventional, metal vessels.
r
2007 Elsevier B.V. All rights reserved.
PACS: 23.40. s; 26.65.+j; 29.40.Mc; 81.05.Lg
Keywords: PTFE; Fluoropolymer; Double beta decay; Xenon; EXO; Low background
1. Introduction
For over a decade, plastics have been studied and used as
structural materials for particle detectors requiring ultra
low levels of radioactive contamination by experiments
such as Chooz
[1]
, Palo Verde
[2]
, SNO
[3]
, Borexino/CTF
[4]
, KamLAND
[5]
, and MUNU
[6]
. In these detectors,
acrylic, nylon, and EVOH
4
were used for the containers of
the innermost, active liquid volumes. Because of their light
weight, low atomic mass, and production processes that
involve efcient chemical separation stages, selected
plastics have generally shown very low levels of long-lived
radioactivity and, particularly, of the naturally occurring
heavy metals
232
Th, and
238
U and, in certain cases, of
40
K.
Contrarily to what occurs in metals, long-lived radioactive
ARTICLE IN PRESS
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doi:
10.1016/j.nima.2007.05.259
ÃCorresponding author. Tel.: +1 650 725 2342; fax: +1 650 725 6544.
E-mail address:
pocar@stanford.edu (A. Pocar).
1
Now at CERN, Geneva, Switzerland.
2
Now at Caltech, Pasadena, CA, USA.
3
Now at University of Maryland, College Park, MD, USA.
4
Ethyl vinyl alcohol derivative lm. isotopes are not produced by nuclear interactions of cosmic
rays with most plastics.
EXO (Enriched Xenon Observatory) is a program
[9]
aimed at building a ton-class double beta decay detector
with
136
Xe. The plan is to use enriched xenon (80%
136
Xe)
as source and detection medium. While the EXO colla-
boration is planning to build a ton-scale detector with the
ability to retrieve and identify the
136
Ba atom produced in
the bb decay of
136
Xe, an intermediate scale (200 kg of
enriched xenon, 80%
136
Xe) detector without the Ba
tagging feature, called EXO-200, is currently under
advanced construction
[10]
. The Xe in EXO-200 (and,
possibly, in the full size EXO) is kept in liquid phase (LXe)
at a temperature around 170 K and a pressure of 1 atm.
The two electrons produced in the bb decay are detected in
an electric eld of 14 kV/cm by the simultaneous readout
of ionization and scintillation. This technique has been
shown to provide superior energy resolution
[11]
, as
required to suppress backgrounds without sharp energy
features, such as the 2nbb decay of
136
Xe and g-ray
Compton tails. In EXO-200 the vacuum ultraviolet (VUV)
scintillation light (175 nm) is detected by bare (i.e.
without their standard encapsulation) large area avalanche
photodiodes (LAAPDs),
5
while the ionization signal is
collected by crossed wire grids, hence measuring the total
energy of the decay and its three-dimensional location. The
third dimension is provided by the drift time using the
scintillation signal as the start time. The active LXe has
the shape of a cylinder about 40 cm long and 40 cm in
diameter. The vessel with the LXe is submerged in HFE-
7000
6
contained in a low activity, copper cryostat. The
HFE-7000 is an ultra-clean uid that is in liquid phase in a
broad range of temperatures, encompassing 300 and 170 K.
This uid is used as the innermost g and neutron shield and
as the thermal bath to maintain a uniform temperature
around the LXe vessel. The presence of a large thermal
mass of HFE-7000 uid makes it possible to use xenon
containers with poor thermal conductivity such as plastics
or thin metallic shells.
The end-point energy for the bb decay of
136
Xe
(2457.8 keV
[12]
) is substantially higher than that of most,
but not all, radioactive decays in the
238
U and
232
Th decay
chains. In addition,
40
K and other less common back-
grounds are also relevant for a clean reconstruction of the
broad energy distribution of the 2nbb decays. The high
monetary value of the enriched Xe makes it impractical to
use it for shielding the detection volume from background
events originating from the xenon vessel material. The
intrinsic background requirements for all construction
components and, in particular, the relatively heavy LXe
vessel are therefore very challenging to achieve. Additional
constraints are given by the cryogenic temperatures and the
LXe purity requirement with respect to electronegative
contaminations, which would reduce the electron lifetime
in the detector. Polycarbonate has been used for cryogenic,
hermetic vessels, most notably in small bubble chambers
[7]
, because of its mechanical stability, strength, and
compatibility with low temperatures. Unfortunately all
polycarbonate samples (both in the form of raw pellets and
molded plates) measured by our group
[8]
have shown
undesirable levels of radioactive contaminations (especially
40
K), disqualifying them for use in EXO-200.
In this paper we discuss the use of a particular
uoropolymer for the construction of a large cryogenic
vessel containing the LXe. We also present data from an
ionization chamber built entirely out of such uoropoly-
mer. Structural performance and details of the construction
technology used for the chamber will be the subject of a
future publication
[13]
. Although copper was ultimately
chosen for the rst generation EXO-200 chamber for
scheduling reasons, an evolution of the uoropolymer
chamber described here may be used at a future stage of
EXO and by other groups.
2. Fluoropolymer as a vessel material
Fluoropolymers such as polytetrauoroethylene (PTFE)
have been used in the past in LXe detectors as VUV light
reectors and volume displacers
[14]
. It is therefore known
that they are compatible with the clean environment
required to drift electrons over large distances of LXe.
In addition, uoropolymer parts are generally known to
retain their structural integrity at cryogenic temperatures.
DuPont Teon
7
TE-6472
[15]
is a variety of modied
PTFE developed for use in the semiconductor industry and
hence produced with high purity standards. Blanks are
sintered by rst pressing ne pellets in a mold and then
baking the material at a specic oven cycle. Levels of
radioactive contaminants in the raw pellets of a large batch
of TE-6472 set aside for EXO-200 have been measured
with neutron activation analysis (NAA) and were found to
be ð1:65
0:17Þ
10
9
g=g for K ð1sÞ,
o0:26 10
12
g=g
for Th and
o0:35 10
12
g=g for U, at 95% CL. NAA
results for several other elements conrm the extremely low
level of impurities in the material. It was found that other