Progress in the construction of the Soudan 2 nucleon decay detector which is being built at the Soudan iron mine in Minnesota is discussed. The expected event rate and characteristics of low energy neutrino events, muon events, multiple muon events, and other cosmic ray phenomena are discussed

Ayres, D. S.

Barrett, W. L.

Dawson, J. W.

Fields, T. H.

Goodman, M. C.

Hoftiezer, J.

May, E. N.

Mondal, N. K.

Price, L. E.

Schlereth, J. L.

English

The authors reported here on the progress in construction of the Soudan 2 nucleon decay detector which is being built at the Soudan iron mine in Minnesota. The authors also report the expected event rate and characteristics of low energy neutrino events, muon events, multiple muon events, and other cosmic ray phenomena which the authors might be sensitive to

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Construction of the Soudan 2 detector

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267
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CONSTRUCTION OF THE SOUDAN 2 DETECTOR
D. S. Ayres, W. L. Barrett, J. W. Dawson,
T. H. Fields, M. C. Goodman, J. Hoftiezer, E. N. May_
N. K. Mondal, L. E. Price, J. L. Schlereth, J. L. Thron
Argonne National Laboratory, Argonne, IL. 60439
T. Kafka, W. A. Mann, R. Milburn, A. Napier, W. Oliver, J. Schneps
Tufts University, Medford MA. 02155
H. Courant, K. Heller, S. Heppelmann,
T. Joyce, M. Marshak, E. Peterson, K. Ruddick, M. Shupe
University of Minnesota, Minneapolis MN 55455
W. W. M. Allison, G. Barr, C. B. Brooks,
J. H. Cobb, D. H. Perkins, P. Shield
University of Oxford, Oxford OXI 3RH, England
D. Cockerill, P. J. Litchfield, G. F. Pearce, E. W. G. Wallis
Rutherford Appleton Laboratory, Didcot, OXII, England
paper presented by M. C. Goodman
We report here on the progress in construction of the Soudan 2
nucleon decay detector which is being built at the Soudan iron mine in
Minnesota. We also report the expected event rate and characteristics
of low energy neutrino events, muon events, multiple muon events, and
other cosmic ray phenomena which we might be sensitive to.
i. Description of the Detector.
- The detector is an iron calorimeter with high density (2 g/cm 3)
which emphasizes dE/dx measurement and excellent tracking. It consists
of 256 identical modules with a total mass of I.I kilotons (Fig. i.)
Each module consists of formed steel sheets in a honeycomb pattern, with
I
0.5 meter drift cells read out by 2.5 m by 1.0 m proportional wires and
cathode strips (Fig. 2.). Each drift cell is 14 mm in diameter and
layers are separated by 1.6 mm of iron. A gas mixture of 85% Argon with
15% CO 2 will be used, and a drift field of 200 V/cm, which will give a
drift velocity of 0.92 cm per microsecond. For each wire and cathode
" strip, the pulse height will be recorded in each of 256 time bins which
are 200 ns long. Vertical and horizontal positions of hits will be
determined by matching anode and cathode pulse heights. Positions along
• the electron drift direction will be measured in 2 mm bins using timing
information. Sampling every 0.2 radiation lengths coupled with
ionization information will give good event reconstruction often
including particle identification. The direction from stopping tracks
will be determlned by the rise in pulse height at the end of a track.
This will greatly aid in vertex identification, which is important in
distinguishing proton decay events from neutrino interactions.
https://ntrs.nasa.gov/search.jsp?R=19850027828 2020-03-20T17:10:32+00:00Z
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The modularity of the detector will allow calibration in a neutrino
beam. We believe previous proton decay experiments indicate that thls
will be an important ingredient in separating proton decay candidates
from neutrino induced backgrounds, as well.
To reduce the number of electronics channels required, the cathodes
and anodes will be separately multiplexed by a factor of 8. Associating
a module with a given hit will require demultlplexlng of both anode and
cathode signals, but for the simple topologies of events expected (small
volume clusters of hits and tracks along straight lines) this will not be
difficult.
The experiment will be located at a depth of 2200 m of water
equivalent. This will shield the detector from the hadronlc and
electromagnetic component of cosmic ray showers, and from muons with an
initial momentum of less than 700 Gev. Triggers will be caused by high
energy muons, neutrino events, and radioactivity In the mine. A muon
rate of 0.3 Hz is expected. Some of the muons may interact outside of
the detector and give hadrons and electrons which enter the detector.
Therefore a two-layer shield is being constructed around the entire
detector, made out of 23 ft long aluminum proportional wire planes.
2. Trigger.
The trigger will consist of some multiplicity of hits in a local
region, probably around 4 out of 16 wires. We will have the ability to
finely tune the trigger in each area of the detector, which will minimize
effects of radioactivity from the cavity walls while maxlmlzln_
acceptance to more difficult proton decay modes, such as p + KT_.
3. Current Progress.
The first 5-ton module has been constructed and is being tested with
cosmic rays. By the time of the meeting, several more wlll have been
built and tested, so that considerable experience will have been gained
on the construction and performance of the modules. The testing program
wlll reconstruct large numbers of stralght-through cosmic ray muons. We
will use spatial reconstructions of the tracks to identify the individual
tubes struck by each cosmic ray. We will evaluate the performance of
each tube in a module to obtain statistics on efficiency, attenuation,
pulse-helght resolution, and position resolution. Such an analysis has
already been performed on data from a 1-ton prototype of the detector and
shows excellent performance, with position resolution along the drift
direction limited by the flash ADC clock. A sample of stopplng muon data
demonstrates the directlon_tagging ability of the detector. We expect to
begin installing modules in the mine in November 1985 and complete
installation of all 256 modules in December of 1987.
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4. Cosmic Ray Capabilities.
The excellent tracking capabilities will enable this detector to do
certain studies of cosmic ray physics as a byproduct to searching for
nucleon decay. In particular it is desirable to try to confirm the time
correlations of multiple muon events seen in the Soudan I detector, as
well as single muons which may be coming from Cygnus X-3 and other
astrophysical sources 1,2. For single muon events, the Soudan 2 detector
will have 15 times the acceptance that the Soudan I detector had. The
multimuon acceptance should increase by more than this, the actual rate
depending on the lateral spread of the muons when they reach the level of
the detector. It will certainly be possible to look for Cygnus X-3 and
other possible astrophysical cosmic ray sources as well. It is also
possible that the multlmuon event distributions will give some
information about the elemental composition of cosmic rays in the 1014 eV
energy range.
Neutrino events are expected at a rate of about I00 per year. It is
possible to calculate the _/_e ratlo_ charged current to neutral current
ratio_ and flux as a function of energy and angle that are expected from
cosmic ray neutrinos produced in the earth's atmosphere. Neutrino
oscillations could change the up/_e ratio as a function of angle. Any
new weakly interacting neutral particle might manifest itself as an
excess in the apparent neutral current to charged current ratio. Any
astrophysical sources of neutrinos, such as high energy particles
produced in solar flares, would give neutrinos associated with a
particular direction in the sky.
The detector would be sensitive to a monopole flux of 1.0 x 10"14
cm'2s'Isr "I in one year. The memory time for each event is 50 _sec, so
it will be possible to see monopoles (if they ionize) with a velocity
down t_-2 x 10-4 c.
5. Conc lu sion.
A i.I kiloton tracking calorimeter is being constructed for use in
- the Soudan mine in Minnesota. As well as a tool in the search for
nucleon decay, it will be possible to study cosmic ray muons and
neutrinos and look for other cosmic ray phenomena.
I. J. Bartelt et al., submitted to Phys. Rev. D.
. 2. M. Marshak et al., Phys. Rev. Lett. 54, 2079 (1985).
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