arXiv:physics/0406114v1 [physics.ins-det] 24 Jun 2004
Negative Ion Drift and Diffusion in a TPC
near 1 Bar
C. J. Martoff, R. Ayad, M. Katz-Hyman
Department of Physics, Temple University, Philadelphia, PA 19122, USA
G. Bonvicini, A. Schreiner
Department of Physics & Astronomy, Wayne State University Detroit, MI 48202,
Drift velocity and longitudinal diffusion measurements are reported for a Negative
Ion TPC (NITPC) operating with Helium + CS2 gas mixtures at total pressures
from 160 to 700 torr. Longitudinal diffusion at the thermal-limit was observed for
drift fields up to at least 700 V/cm in all gas mixtures tested. The results are
of particular interest in connection with mechanical simplification of Dark Matter
searches such as DRIFT, and for high energy physics experiments in which a low-Z,
low density, gaseous tracking detector with no appreciable Lorentz drift is needed
for operation in very high magnetic fields.
A TPC which drifts negative ions (in this paper, CS−
was invented to reduce diffusion in three dimensions to its thermal (lower) limit
without applying a magnetic field[1,2,3]. This provides the highest 3-D space-
point resolution attainable for long drifts, without the power requirements and
expense of a magnet.
2) rather than electrons,
Such characteristics are particularly important for the development of the
DRIFT series of direction-sensitive gaseous detectors searching for WIMP dark
matter . Three coordinates of good resolution on the recoil track are essential
for DRIFT, in order to measure the length and direction of tracks from low-
energy atom recoils produced by elastic scattering of massive WIMPs. The
standard solution of a TPC with magnetic field along the drift direction would
give good resolution in just two (transverse) coordinates. Furthermore the
Preprint submitted to Nuclear Instruments and Methods2 February 2008
necessary large magnet is impractical for underground experiments due to
cost and electric power requirements.
Unlike the light electrons, negatively charged molecular ions are much more
efficiently thermally coupled to the bulk of the gas than drifting electrons
would be. In appropriate gas mixtures, the negative ion drift mobility is near
constant, and the rms 3-D diffusion follows the “low field” limiting behavior:
often up to reduced drift fields E/P of several tens of V/cm·torr .
Of course there are details to be reckoned with; the ions must form before the
primary ionization electrons drift far from their point of origin, or the resolu-
tion will be spoiled from the beginning. Also the negative ions must relinquish
their extra electron and produce a Townsend avalanche in the endcap gain
region. Both of these requirements have been shown to be amply met by CS2
at 40 torr and by mixtures of CS2 with small amounts of noble gases at 40
torr total pressure.
The DRIFT I experiment (active mass 0.16 kg) operates with a pure CS2
fill at 40 torr , which is near-optimal pressure for a direction-sensitive WIMP
search with the DRIFT I spatial resolution.
To achieve the much higher target masses planned for the DRIFT II (5 kg)
and DRIFT III stages, higher target gas pressure and hence higher resolution
are required. However, the low recoil atom energy places an absolute upper
limit on the target gas density of less than 1 mg/cm3. For pure fills of the
medium-mass target gases that are most interesting as WIMP targets, this
would require running well below atmospheric pressure. The vacuum vessel
and support then become a major element of cost and complexity, as they are
in DRIFT I.
It is therefore of great interest to see whether a 1-bar gas mixture could be
found which would not shorten the recoil tracks below any hope of directional
detection, but would still give all the benefits of negative ion drift. In a pre-
vious paper, we reported on operation of GEM micropattern gain elements in
negative ion drift mixtures near 1 bar. Raising the total pressure using a
Helium buffer gas is a natural solution to consider, since equal pressures of
Helium and CS2 have densities in the ratio of approximately 4/76 = 0.05.
This report is to show that indeed mixtures of this kind do work well as TPC
gases. The 0.9 bar limit in the present work was imposed by the apparatus and
is not a limit of the technique itself. Such mixtures can therefore be considered
for next-generation DRIFT detectors.
Operation at 1 Bar also permits entirely new applications for NITPC. For
example, a NITPC using a helium mixture has been proposed for use as a
main tracking detector in the NLC . The low drift speed of negative ions
(only tens to hundreds of meters per second) allows arbitrary orientation of the
drift direction relative to the momentum-measuring magnetic field, without
producing any significant E × B effects. The slow pulse repetition rate and
low duty factor of NLC-like machines greatly mitigates the negative effects of
the slow ion drift.
When combined with the very small diffusion broadening obtainable, the slow
negative ion drift velocity also brings phenomenal z-resolution (along with
good transverse resolution). With sufficient gas gain and amplifier sensitiv-
ity, single electrons or clusters could be detected individually as in the TEC
scheme, giving a number of statistical advantages for particle measurement.
2 Experimental Methods
The tests were carried out with a small test TPC in a stainless steel bell jar
with a simple gas manifold. A sketch of the test TPC is shown in Figure 2.
The drift volume was rectangular, 50 x 60 mm transversely and 80 mm long
in the drift (z) direction. The field cage was made of bare 500 micron diameter
wires spaced 5 mm apart in z, stretched around nylon supports at the four
corners. The drift-cathode was a solder-coated PCB with an Sn photocathode
attached to it with conducting epoxy.
Charge was liberated from the photocathode by pulsed UV illumination from
an EG&G Flash-Pak . The Flash-Pak’s short-wavelength limit in air is ∼230
nm. The standard internal capacitors of the Flash-Pak were augmented with
additional HV capacitors to give a stored energy of about 0.2 Joule per pulse.
The Flash-Pak was triggered by an external pulser, from which a time-zero
signal was also derived. This system was a very convenient and cost-effective
solution for generating variable-amplitude pulses of charge (photoelectrons)
in the TPC, which were sharply defined in time and space. It was found to be
essential to scrape the photocathode clean each time the detector was exposed
to air between gas fills.
UV light entered the bell jar through collimating apertures and a quartz win-
dow, passed through a hole in the endcap cathode of the test-TPC, and struck
the Sn photocathode. The endcap structure was an 8-wire MWPC. One cath-
ode of this MWPC (the “grid”) terminated the drift-field region. This grid
was a transparent electrode made by epoxying a stainless steel mesh under
tension, to a solder-coated PCB which had a 50 x 50 mm window milled out of
it. The mesh pitch was nearly 40 cm−1and the geometrical transparency
Fig. 1. Schematic cross-sectional view of Mini-TPC.A: Sn photocathode, B: Drift
cathode PCB, C: Field cage voltage divider chain, D: Field cage, E Grid support
PCB, F: Grid mesh, G: Endcap MWPC anode wires, H: MWPC cathode PCB, I:
UV admittance aperture, VD: Drift voltage, VM: MWPC gain voltage
was 81%. The anode plane of 8 Au-plated W wires 15 µm in diameter, at a
pitch of 6 mm and with 50 g tension, was placed 6 mm behind the grid. The
anode wires were attached to a PCB anode frame using cyano-acrylate adhe-
sive and soldered to contacts on the PCB. The MWPC structure ended with
a second cathode (the “MWPC cathode”), which was simply a solder-coated
Negative “drift voltage” up to -10 kV was applied to the drift cathode; the grid
and MWPC cathode were grounded, and positive high voltage was applied to
the MWPC anodes. This setup has the obvious advantage that drift and gain
voltages can easily be adjusted independently.
Anode wires were read out individually through 1 nF high-voltage decoupling
capacitors and amplified (and inverted) by Amptek A225 amplifiers. The
resulting positive signals were re-inverted by home-made common-emitter am-
plifiers, and then sent to the data acquisition system.
The gas system was based on a simple soft-soldered copper-tube manifold with
attachments for introduction of various gases and of organic vapor (CS2
from a small liquid reservoir. Base pressure with the rotary pump used was
about 50 milli-torr. Negative ion drift chambers are very insensitive to con-
tamination with anything less electronegative than CS2 (for example, air). It
has been found that extraordinary precautions with gas purity are unneces-
sary. Gas mixtures were prepared by admitting the minor component into the
chamber first, followed by the major component. Pressures were monitored
with a mechanical gauge which is insensitive to the nature of the gas being
Longitudinal diffusion was measured using the Amptek outputs and a digi-
tal oscilloscope. The NITPC output pulse width and its delay relative to the
FlaskPak trigger signal were measured as a function of drift field, at con-
stant MWPC voltage and constant FlaskPak amplitude. The delay time was
converted into drift speed using the known geometry. The amplifier shaping
time was subtracted in quadrature from the measured width. The result was
then converted from pulse width in time to pulse width in distance by multi-
plying with the drift speed measured at that drift field. Space charge effects
were shown to be absent by checking the results at high and low FlashPak
3Results and Discussion
Results for the four gas mixtures studied are shown in Table 3. To obtain the
tabulated diffusion “temperatures”, the curves of FWHM diffusion width of
the anode signals vs. 1/√EDwere fitted to straight lines. The slopes were set
equal to the expected value for the FWHM of one coordinate in a 3-D diffusion
problem, obtained from Equation 1:
and the corresponding temperature computed.
The deviations of the tabulated temperatures from the actual room temper-
ature of 293 K probably reflect the limitations of the simple diffusion model
used, rather than any real physics. Ohnuki et al using different detectors
and methods, also found diffusion temperatures differing significantly from
room temperature. Typical data is shown in Figures 3 and 4 for the case 200
torr CS2 plus 500 torr He. A line fitted to the diffusion curve has a y inter-
cept of -.01 rather than zero, and slope 1.30 rather than 1.49 expected from
For the lower total-pressure mixtures, the He acts essentially as a buffer gas,
hardly changing the drift properties. The drift mobility in the 700 torr mixture
CS2 (torr) He (torr) ion mobilitydiffusion
400 0.22258 K
40120 0.18 281 K
40 1600.17 281 K
2005000.0071 229 K
Fig. 2. Longitudinal diffusion results summary
200 300 400 500 600 700 800
Fig. 3. Typical Drift Velocity Data. Measured drift velocity as function of drift field
for 500 torr He + 200 torr CS2 .
however is significantly lower than would be expected if it were dependent on
the partial pressure of CS2 alone. The drift velocity itself drops by nearly a
factor of six compared to the lower total-pressure mixtures.
Mixtures of CS2 with a helium buffer gas were found to have thermal-limit
diffusion up to drift fields of over 12 V/cm·torr. These mixtures also allow
stable operation of a Negative Ion TPC near 1 Bar total pressure. This opens
the way to numerous applications of the NITPC technique.
0.03 0.035 0.04 0.045 0.05 0.055
0.06 0.065 0.07 0.075 0.08
Fig. 4. Typical Longitudinal Diffusion Data. Longitudinal diffusion for 80 mm drift,
measured as described in the text, for 500 torr He + 200 torr CS2 .
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 Unique Wire Weaving, Hillside, N.J.
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