Overview
Scientists at the University of Pennsylvania are on the frontline of neutrino research, contributing to experiments and discoveries at some of the world’s leading solar neutrino detectors.
See the articles below for more information on the Homestake Gold mine and Sudbury Neutrino Observatory.
Dr. Kenneth Lande’s research is directed at understanding the energy-generating mechanism in the core of the Sun and experimentally investigating the details of the nuclear fusion reactions that are presumed to be responsible for that energy generation. Since the neutrinos that arise in these fusion reactions provide the only direct measure of the rate of each of the involved reactions, the experimental problem is to measure the energy spectrum of the emitted neutrinos.
The first detection of these neutrinos was by a 615-ton chlorine-based solar neutrino detector, which Dr. Raymond Davis built in the Homestake gold mine in South Dakota. This detection represented the beginning of neutrino astrophysics, a field that has had enormous growth and great scientific excitement. Lande has been a researcher there since 1972, and assumed the directorship of the experiment in 1990.
Initial observations at Homestake, which began in the 1960s, found that the detected electron neutrino flux was only about 1/3 of that expected from the thermal emission of the Sun and has generated considerable interest in the physical process responsible for this apparent reduction of neutrino flux.
The Homestake chlorine detector observes electron neutrinos from two of the three distinctive energy groups emitted by the sun, the high energy range, 8B, and the 1 MeV energy range, 7Be, PeP and CNO. The total flux of these electron neutrinos is 2.56 ± 0.23 SNU (solar neutrino units). This is about 1/3 of that predicted by the standard model of the sun.
Recently, the SNO detector has measured the flux of electron neutrinos from 8B. In chlorine detector SNUs, the SNO measurement is 2.01± 0.14. By subtracting the SNO measurement of 8B electron neutrino flux from the chlorine measurement, we find that the 1 MeV region electron neutrino flux is 0.55 ± 0.27 SNU compared to the solar model prediction of 1.8 SNU. Thus, the detected electron neutrino flux in the 1 MeV region is also about 1/3 of that predicted.
The presumption is that 2/3 of the electron
neutrinos initially emitted by the fusion reactions in the
core of the sun convert into other neutrino species, muon
and tau, by the time they reach the earth.
The goal of future investigations is to
(1) understand the physics responsible for the neutrino flux suppression by
more specific investigations of solar neutrino emission and by controlled
neutrino flavor transition terrestrial experiments using long-range neutrino
beams from high energy particle accelerators,
(2) apply that physics to convert the observed flux into the neutrino flux
generated in the solar core, and then
(3) set up and operate a long-term neutrino monitor of energy generation in
the solar interior.
Dr. Alfred Mann
Dr. Alfred Mann is the Bernard and Ida Grossman Professor of Physics, Emeritus. His theoretical and experimental work in neutrino physics has used neutrinos as a probe to study the structure of elementary particles and of stars. Over three decades he carried out key research on neutrino interactions with matter, ranging from observation of the scattering of neutrinos from nuclei, protons, and electrons at Fermilab and Brookhaven to observations of the neutrinos from Supernova SN1987a, from the sun, and from cosmic rays at the Kamiokande-II detector.
Dr. Mann along with collaborators from Harvard, Wisconsin, and Fermilab designed and built a large-scale neutrino detector, which enabled the scientists to make one of the earliest measurements of the weak neutral current and of a new hadronic “bare charm.” Later with collaborators from Brown, KEK (a Japanese acronym that translates as High Energy Accelerator Research Organization), and Brookhaven, he went on to develop another sophisticated detector to study neutrino-proton and neutrino-electron interactions, which yielded, among other quantities, an upper limit on the neutrino magnetic moment.
It was on the basis of his expertise in neutrino physics and neutrino detectors,
that Dr. Mann was invited by 2002 Nobel laureate Masatoshi Koshiba to form
a Japan-America collaboration at Kamiokande-II to attempt the directional
detection, in real time, of the solar neutrinos from 8B-decay. Kamiokande-II
successfully detected an electron neutrino flux coming from the sun, which
was one-half of that predicted by the Standard Solar Model, thereby also
confirming that the neutrinos detected by Ray Davis at the Homestake mine
experiment were in fact solar neutrinos. Dr. Mann has also been intimately
involved in the Kamiokande-II study of extraterrestrial neutrinos, the so-called
cosmic ray or atmospheric neutrinos. That research produced strong evidence,
now confirmed by the Sudbury Neutrino Observatory, for neutrino oscillations
(from the observed ratio of the electron-neutrino and muon-neutrino fluxes).
He tells the story of the Penn-Tokyo collaboration and the group’s successful
hunt for the elusive neutrinos from an exploding star in his book Shadow of
a Star: The Neutrino Story of Supernova 1987A (W.H. Freeman, 1997).
The Sudbury Neutrino Observatory project (SNO)
has solved one of the great puzzles of20th-century physics
and astrophysics – the anomalously low flux of neutrinos
coming from the sun. Since the late 1960s when Ray Davis
first announced that he detected about one-third the number
of neutrinos predicted by models of stellar evolution, scientists
were in a quandary regarding the source of the discrepancy.
Was his experiment wrong? Was our understanding of stars
wrong? Or was there something else, perhaps an inadequate
understanding of the properties of neutrinos?
SNO has shown that Davis's experiment was correct,
and that the model of the sun is also correct. The puzzle
was solved when SNO showed that some of the Boron-8 electron-neutrinos
that are produced in nuclear fusion reactions that power
the sun transform to another type of neutrino which does
not produce a signal in Davis's detector.
Unlike previous solar neutrino experiments,
the SNO detector is sensitive to three different neutrino
reactions. One of the reactions is, like Davis's experiment,
only sensitive to the electron-neutrinos that the sun produces.
The other two reactions are sensitive to electron-neutrinos,
and, in different proportions, to mu-neutrinos and tau-neutrinos – types
that are not produced in the solar fusion reactions. The
three reaction types can be separated using the position,
angle, and energy information of the events observed.
By comparing measurements of the flux of solar
Boron-8 electron neutrinos (nu_e) to the total flux of all
neutrino types (nu_x) coming from the sun, SNO has shown
that Davis's original measurement was correct – the
nu_e flux is suppressed – but that the flux of all types
of neutrinos is in agreement with the predictions of the
model. The conclusion is that some of the electron-neutrinos
that were produced in the sun transform into the other types
of neutrinos before they are detected on earth. The most
likely mechanism for producing this transformation requires
that neutrinos have small, but non-zero mass. This is an
indication of exciting new physics beyond the Standard Model
of elementary particle physics. Although the mass of the
neutrinos is tiny, the total mass of all the neutrinos in
the universe is comparable to that of all the visible stars.
The unique feature of SNO is the use of a kiloton
of heavy water, D2O, as a neutrino target. The valuable D2O
is securely contained in a spherical acrylic vessel which
is twelve meters in diameter. The vessel is surrounded by
light water, H2O, and is viewed by 9500 photomultiplier tubes.
To limit backgrounds introduced by cosmic radiation at the
earth's surface, the entire laboratory and detector are located
two kilometers underground in a cavity in one of the world's
most productive nickel mines. The SNO cavity, which is the
size of a ten-story apartment building, is maintained as
a “clean room” to exclude trace contamination from
mine dust.
The SNO experiment began taking calibration
and neutrino data in May 1999. The program of calibrations
determines the optical parameters; the spatial, angular,
and energy responses of the detector; the response to signals
from neutrinos and processes that produce background; and
systematic effects which might bias interpretations. The
calibrations are taken routinely to track the time dependence
of the detector response.
Neutrino data will be acquired in at least
three configurations of the detector. The initial configuration
was the simplest, with only heavy water inside the acrylic
vessel. In June 2001, the detector configuration was altered
to the first of two configurations that will enhance the
detection capability for nu_x. Measurements in these two
configurations will produce independent measures of the flux
of nu_x and serve as checks on each other and on the result
from the initial phase. The additional data will also permit
accurate measurements of possible distortions in the electron
energy spectrum and day-night spectral differences for nu_e
induced events. These measurements will lead to precise evaluation
of the physics parameters responsible for neutrino flavor
transformation in the solar sector. Additional topics of
study include atmospheric neutrinos and a search for anti-neutrino
interactions. SNO's neutron detection capability is a unique
asset for this work. A program of data acquisition and analysis
lasting at least through 2005 is envisioned, in order to
obtain the highest precision results possible.
Penn researchers were deeply involved in commissioning the detector and are now active in operations and data analysis at all levels. The University of Pennsylvania group constructed and is responsible for maintaining all the front-end signal processing electronics for the detector. This includes PMT signal detection and digitization, triggering, and GPS timing electronics. The effort has required three custom-designed integrated circuits and fourteen custom-designed printed circuit boards. Graduate students contributed to or were solely responsible for nine of the circuit boards. This represents one of the many substantial and crucial contributions to the SNO experiment by students.
Additional links:
- Homestake Gold Mine home page.
- Penn at the Sudbury Neutrino Observator