Measuring Neutrino-Nucleus Cross Sections
at the Spallation Neutron Source
Submitted to the ORNL Physics Division by:
on behalf of the future
Table of Contents
I. Executive Summary
high beam intensity and short duty-factor of the Spallation
Neutron Source (SNS) at ORNL would allow an exciting program to measure
charged-current neutrino-nucleus cross sections, ZA(ne,e-)X.
These cross sections are of great interest to the fields of nuclear structure
physics and astrophysics; in particular, they would be of immediate use
in the study of core collapse supernova physics. This is especially true
due to the fortuitous similarity between the SNS neutrino spectrum and
the neutrino spectrum produced in a supernova explosion as shown in Figure
the termination of the neutrino programs at Los Alamos and at ISIS, there
are no stopped-pion neutrino facilities, which
are necessary to do such neutrino experiments. We propose to build at the
SNS a shielded, instrumented enclosure which will host an initial set of
two detectors in order to carry out a long term program of cross section
measurements on a range of appropriate nuclear targets. The tentative name
for the proposed facility is Supernova Neutrino Studies
at the Spallation Neutron
present, only neutrino cross sections on 2H,
have been measured well (~4-10% errors). The only other reported
results are for 2H, 56Fe and 127I (~40%
errors). We show below that at SNS2
it is feasible to measure the charged-current neutrino-nucleus cross section
for any selected nuclear target species to a statistical accuracy of 10%
in one year. With clever detector designs that allow reuse of the detector
with different target materials it becomes feasible to develop a true program
to measure the cross sections for many nuclear species. Theoretical
interest in these cross sections spans the entire range of nuclear species
and must be tempered by the realization that neutrino cross sections are
exceedingly small (~10-41 cm2) so that even with
the prodigious neutrino flux at the SNSthe
required target size is of order 10 tons. Therefore viable target materials
must be naturally monoisotopic and affordable
in large quantity. Based on these considerations an initial three-year
program might consist of 10% measurements for the neutrino cross section
on 56Fe, 209Bi, 12C, 16O.
Future directions would be guided by the results of this initial program
and by theoretical interest. For instance, total cross sections on additional
targets; additional data on a smaller number of targets to allow for differential
measurements as a function of energy and/or angle; or neutral-current measurements.
The neutral current measurements, which are of significant theoretical
interest due to the complete lack of experimental data to test theories,
are perhaps possible at SNS2
given appropriate detector development.
II. Scientific Motivation
star more massive than 8-10 solar masses ends its life in a catastrophic
explosion known as a core collapse supernova. These explosions are extremely
important for galactic dynamical and chemical evolution because they disperse
almost the entire mass of the star into interstellar space. Neutrino-nucleus
weak interactions play a central role in both supernova dynamics and supernova nucleosynthesis.
These interactions are also central to existing or proposed terrestrial
facilities to detect neutrinos from the next Galactic or near-extra-Galactic
supernova, which in turn will provide detailed neutrino ?light curves?
from which supernova models and supernova nucleosynthesis
models can be diagnosed and improved. Although many nuclei participate
in these weak interactions, and it is impossible to measure all of the
relevant cross sections, measurement of several key cross sections would
provide invaluable checks on the elaborate theoretical models used in current
computations.A new aspect of the
supernova mechanism, the importance of electron-capture on nuclei heavier
than Fe, has recently been recognized. No information is currently available
for calculations, or even estimates, of these electron-capture rates. The
proposed neutrino facility would enable measurements of the inverse reaction
on e.g. 75As, 93Nb and 127I, in the relevant
nuclear mass region. From these measurements, the electron-capture rates
can be estimated. Fortunately the neutrino energy spectra available at
the SNS are exactly what is needed for these measurements.
Proposed enclosure and location
neutrino detector and shielding enclosure would be located in the SNS target
hall.In discussions with the SNS
Target Systems Division we have identified a mutually acceptable location
on the north side of the beam line, at a mean distance of 21m from the spallation
target, and at an angle of 160 degrees relative to the incoming proton
beam direction. The available floor space is ~ 4.5m X 4.5m with a clear
height of 6.5m. This is shown in Figure 2.
full power (1.4 MW) the SNS will bombard its mercury target with a 1.1 mA,
1.3 GeV proton beam, producing~0.2
neutrinos per proton. The resulting neutrino flux through this volume will
be ~2.5 X 107n/s/cm2,
giving several tens of neutrino interactions per day for a ten
ton detector. This must be compared with the cosmic ray muon
(neutron) flux through this volume of ~2.5 X 108 (1.4 X 106)
events per day. Such events must be suppressed through a combination of
the SNS time structure, an active veto counter and shielding.
SNS time structure (~700 ns proton pulses at 60 Hz) allows us to eliminate
a large amount of both types of cosmic backgrounds by turning off the detector
except for the small fraction of time during which neutrinos can come from
the target. Target neutrinos, which result from the p®m®n
decay chain, will all arrive within several muon
decay lifetimes (tm
= 2.2 ms).
This results in an active time of only 4 X 10-4 seconds for
every second of machine operation, thus reducing the effective cosmic ray muon
(neutron) flux through this volume to 1 X 105 (5.6 X 102)
events per day.
active veto system can only provide rejection for the charged component
of the cosmic background. We assume an achievable efficiency of 99% for
this veto system which would further reduce the cosmic ray muon
flux to 1 X 103 events per day. Most of the remaining muons
can be rejected by their signature in the detector, but those which produce
a neutron in the shielding can be confused with the desired signal. To
a reasonable approximation we are only concerned with those muons
which produce a neutron in the last interaction length of shielding. The
yield is 4X10-5 n/m/(g/cm2),
giving a background rate of ~5.2 events/day coming from neutrons generated
in the shielding enclosure by cosmic ray muons
which failed to fire the active veto system.
shielding the primary cosmic neutron flux is (as discussed above) ~560/day,
two orders of magnitude above the irreducible background from cosmic muons.
This sets the scale for the thickness of shielding: a steel enclosure,
with a 1 m thick roof and 0.5 m thick walls will reduce the primary cosmic
neutron flux by two orders of magnitude. Preliminary calculations, incorporating
the SNS target and shielding assemblies and materials from nearby neutron
scattering instruments show that this shielding is also sufficient to shield
against SNS-generated neutrons.
calculations show that the weight of the resulting shield package, plus
the detectors is within the load limit of the SNS floor at our desired
location. The resulting 3.5m X 3.5m X 5.5m interior space would be sufficient
to house the active veto-counter system and one or two neutrino target/detector
different types of detectors are being investigated; the two that appear
most promising are:
Segmented detector - a fine-grained tracking calorimeter with the target
material distributed in the form of solid cylindrical tubes;
Homogeneous detector - a liquid-filled tank with close-packed phototubes
on the interior that could be operated either as a scintillator
detector or as a Cherenkov detector.
elements of the segmented detector would be composed of a position-sensitive
gas proportional counter surrounded by a thin-walled cylindrical tube made
of the target element. Signals would be read out from both sides of each
individual channel to provide three-dimensional position information. Particle
energy is reconstructed from the range of the particle track or by the
total number of fired cells. Direction information can be extracted from
the reconstruction of the track. In principle, this detector can be constructed
in such a way that the detector elements are reusable; when a measurement
with one target material is complete the target/detector combinations would
detector elements would be removed and loaded into a new set of target
tubes, and the new target/tube combinations re-stacked. As a result, neutrino
interactions with different nuclei can be studied with the same detector.
This has great benefits for systematic error reduction as well as for long-term
ease of operation and cost. From the list of nuclei of interest given above,
cylindrical tubes of 9Be, 27Al, and 56Fe
are easily obtainable; suitable tubes of many other target other elements
can possibly be manufactured as powder contained in a plastic matrix.
with gas tubes as the sensitive elements have a number of advantages compared,
for example, to those with scintillator
rods. In general, gas tubes are less expensive and do not require an expensive
light-readout system. In addition the low detector mass eliminates the
necessity to subtract interactions in the target from the interactions
in the detector itself. The energy resolution obtained by measurement of
the track length is as good as that obtained by measurement of the visible
energy deposition for electrons in the energy range of a few tens of MeV.
Carlo studies show that a reasonable design for the target tubes would
be 1.0 cm diameter tubes with a wall thickness of 0.5 mm. An
Fe target with a 10 ton fiducial mass would
be 1.85 m X 1.85 m and would have 2.5 m long tubes. The total target/detector
volume should include an additional 20 cm beyond this fiducial
volume such that accepted events are totally contained..
This results in a size of 2.9 m X 2.25 m X 2.25 m - or a total of 50,600
2.9 m target/detector tubes. For an Fe target
with 10 ton fiducial mass, at a mean distance
of 21 m from the SNS spallation target,
and with an expected cross section of ~2.6X10-40 cm2,
the expected reaction rate is 61 per day.We
estimate a detector efficiency of 30%, leading to a signal rate of 18 neutrinos
per day or about 3600 per year.
nuclei are difficult or impossible to obtain as solid compounds. It is
more efficient to measure such targets (e.g., 2H, 12C,
in the form of a liquid or an aqueous solution. Therefore we propose to
have a second, homogeneous detector that can be filled with various liquids.
The detector would be built as a steel, spherical or rectangular light
tight vessel, with a size of ~27 m3. Scintillator
or Cherenkov light would be detected by
~300 8? PMTs (or other type of photodetectors)
mounted on the inner wall.
first target for this detector would likely be 12C (liquid scintillator).The
cross section has been previously measured by both KARMEN and LSND, but
better data are desired. For SNS2
the expected rate is 10 events per day assuming a detection efficiency
of 50%, or 2000 events per year. In one year this is four times the total
number ofevents measured by KARMEN.
An additional benefit of an early 12C
measurement is that it will provide a calibration of the SNS neutrino flux.
This is critical to allow a systematic accuracy comparable to our statistical
expect to submit a letter of intent discussing the physics motivation of
the proposed neutrino facility and requesting the identified floor space
to SNS management by August 2003. Preparation of this document is well
along and prior to submission we will be hosting a workshop/collaboration
meeting. By January 2004, we expect to have a proposal submitted to the
Nuclear Physics Division of the DOE.
experimental schedule is guided by the expectation of full-power beam at
the SNS by the end of 2008 - at that point we could efficiently begin commissioning
our detector. In order to achieve this we would need to perform detailed
design studies and some R&D in FY04-06; begin detector construction
in FY06; erect the shielding enclosure in FY07; and install the detector
is our intention that SNS2
will operate as a user facility for the neutrino community and that experimental
priority will be set by an independent Program Advisory Committee.
following institutions are currently a part of the SNS2
Ridge National Laboratory
of South Carolina
discussions are actively under way with additional collaborators. We anticipate
that this collaboration will be open to all users, and we welcome interested
parties to join.
Figure 1. Energy spectrum of neutrinos produced by the
SNS (top) and by a supernova (bottom).
Figure 2. Proposed SNS2
facility location at an angle of 160 degrees
relative to the incoming proton direction and a distance of 21 m from the