Solving the Solar Neutrino Problem
Our understanding of stellar processes is
based upon our understanding of our Sun. We have studied
the processes in the Sun and with astronomical
observations developed models of how stars are born, live
and eventually die. If our understanding of the sun is
incorrect then so too would be out models of how stars
develope.
In the early 1970s measurements of neutrino
flux from the Sun seemed to suggest that our
understanding of the processes in the Sun were incorrect.
This was of great concern to astronomers and sparked what
was called the Solar Neutrino Problem. The problem was
that some of the predicted neutrinos were
missing.
For more that 30 years physicists and
astronomers worked to solve this problem. Results were
checked and rechecked. Various experiments were conducted
using several techniques. However, initially they all
found that solar neutrinos were missing. It was not until
2001 that the missing neutrinos were found and were not
missing at all; we were just looking in the wrong
place.
This essay explores the experiment that
first indicated that neutrinos were missing, though to
the resolution of the problem and a little beyond. First
we should briefly describe what a neutrino is and then
explore how they are formed in the Sun
What is a Neutrino?
Neutrinos are sub-atomic particles that have
no charge and due to there nature they interact weakly
with other matter (NOBweb.) As we will soon see neutrinos
are released when certain nuclear reactions take place in
the core of the Sun. The Sun produces neutrinos at such a
prodigious rate that 100 billion of them pass through an
adult’s thumbnail every second (NOBweb.) However, they
interact with matter so weakly that a single neutrino has
a one in 100 billion chance to interact with matter as it
passes through the Earth. The low probability of a
neutrino interacting with matter makes measuring them
difficult. At the commencement of the first solar
neutrino experiment the standard particle model
stipulated that neutrinos had no mass.
Standard Solar Model
During the nineteen and the twentieth
centuries the Standard Solar Model was developed. This
was based on observational evidence and what we can
deduce from studies of related phenomena. One of the
bases of this model is that the Sun is in thermal and
hydrostatic equilibrium (Freedman and
Kaufmann p 403 - 413.)
Observations have shown that the Sun is not
becoming significantly hotter and cooler. This leads us
to the conclusion that all heat generated in the Sun is
radiated away at the same rate as it is produced. Also,
the temperature increases with depth due to the known
fact that compressed gasses become hotter. The
temperature increases with depth but is constant at each
depth. This is the equilibrium state.
Hydrostatic equilibrium is indicated by the
fast that the Sun is neither collapsing nor expanding. At
any particular layer the pressure from the overlaying
material, including the layer its self, is the same as
the upward pressure caused by escaping energy from the
nuclear fusion reactions below.
Within the Sun pressure and density
increases with depth, as indicated by the equilibrium
state. To maintain this situation the energy generated in
the sun there must be transportation mechanism. In the
sun there are two mechanisms: convection and radiative
diffusion. Conduction is also a possibility for the
transfer of heat but it is not considered an important
mechanism. It is believed that radiative diffusion takes
place within the 71% of the Sun’s radius
(Freedman and
Kaufmann p 409 - 411.) In the upper 29% convection
takes place.
Solar models must take into consideration
thermal equilibrium, hydrostatic equilibrium, energy
production and the transportation mechanisms present in
the Sun. The models must also be supported by observation
of the Sun’s surface. One surface observation that has
been useful is helioseismology. Helioseismology has been
used to confirm certain physical propeties of the
Sun.
The standard solar models has given us
important physical properties of the sun’s core. It shows
that the density is 160 000kgm-3, has a temperature of
1.5 x 107K and a pressure of 3.4x
1011 atmospheres. With
the use of the model it has been demonstrated that 94% of
the mass of the Sun is within 0.5 radii of the centre and
that energy production is limited to within the lowest
quarter of the radius.
The production of energy in the sun is via
the conversion of hydrogen nuclei to helium. By far the
greatest process this is achieved through is the
proton-proton (p-p) chain (98.5%.) Approximately 1.5% of
the energy generated by the Sun is via the CNO cycle
(Bahcall, Gonzalez-Garcia and Pena-Garay, 2003.) The
overall p-p reaction is:
4p
+
®4He + 2e+ +
2ν
e +
£25MeV
Every second 600 million tons of hydrogen is
converted to 596 million tons of helium (Miramonti,
2009.) The remaining 4 million ton is converted to energy
which given that the Sun is in thermal equilibrium means
that the sun’s current luminosity is 4 x
1026W.
The dominant reactions in the p-p chain
convert four hydrogen nuclei to helium releasing 4.3 x
10-12 J of energy
(shown as ppI in fig 1.) The process does not use any
intermediate elements. In this process only the first
stage results in a neutrino with a maximum energy of
0.42MeV (fig 2.) The measurement of the neutrino flux
from this reaction constrains the overall rate of
conversion of hydrogen to helium in the p-p chain
(Haxton, 2007.)
There are a number of side branches in the
p-p chain (fig 1.) The two most common are called ppII
and ppIII. The ppII and ppIII branches separate after the
second step in the main branch (ie ppI.) The
3He
collides with a 4He particle to form
7Be. The
7Be then
either undergoes electron capture (ppII) or proton
capture (ppIII.)
In the ppII branch the electron capture
results in the production of a neutrino. The resultant
neutrinos are at two energy levels of 0.38 and 0.86MeV
with the later being in 90% of the cases (Haxton,
1995.)
In the ppIII branch the β decay of
8B
results in the release of a neutrino. This is a high
energy neutrino with a maximum energy of about 15MeV. Due
to their high energies the neutrons formed in this
reaction are the most accessible (Haxton,
1995.)

Figure 1: The
p-p chain reactions and their probability based on the
standard solar model. The reactions producing neutrinos
are highlighted in blue (adapted from Haxton (2007) and
Miramonti (2009.))
Due to the energy barriers that must be
overcome before nuclear fusion can occur (ie Coulomb
barriers) the rate of each of the branches are sensitive
to temperature. The rate of the ppII and ppIII branches
can be determined by measuring the neutrino flux from
7Be and
8B.
There is also rarer branch that results in
the highest energy neutrino (18.77MeV). This results when
the 3He
produced in the second stage of the p-p chain captures a
proton. This is called the hep reaction. The flux from
this reaction is low due to the low probability of it
occurring (Bahcall and Pena-Garay, 2004.)

Figure 2: Solar
neutrino flux from p-p chain reactions based on the
standard solar model (Bahcall and Pena-Garay, 2004) Flux
units are cm-2 sec-1 MeV-1 for continuum sources and
cm-2 sec-1 for line
sources.
Source
|
Max energy (MeV)
|
Predicted flux
(cm-2 s-1)
|
pp
|
0.42
|
5.99 x
1010
|
pep*
|
1.44
|
1.42 x
108
|
hep
|
18.77
|
8.04 x
103
|
8B
|
~15
|
5.28 x
106
|
7Be*
|
0.38 (10%), 0.96
(90%)
|
4.65 x
107
|
Table 1: The energy and predicted flux of
each reaction in the p-p chain producing neutrinos
(adapted from Haxton (1995) and Bahcall and Pena-Garay
(2004))
* line
sources
Neutrinos are also produced in a process
knows as pep. In this process two protons and an electron
collide to produce 2H and a neutrino. This
results in a neutrino with an energy of approximately
1.44MeV.
Unlike the photons produced in the reactions
neutrinos travel quickly out of the core and into space.
This is due to them interacting weakly with other
matter.
The Missing Neutrinos
Experiments
Homestake
The first experiment to detect solar
neutrinos was undertaken by Raymond Davis. The project
got underway after Bahcall showed that Davis’ proposed
experiment would be sensitive to the high energy
8B neutrinos. The detection of neutrinos using
chorine was first proposed by Pontecorvo and Alvarex
(Haxton, 1995.)
The Homestake experiment used a
radiochemical technique using perchloroethylene
(C2Cl4,) a common cleaning fluid.
The chemical was used as it was rich in chorine. When a
neutrino interacted with a chlorine atom a radioactive
isotope of argon is produced in this equation:
37Cl + νe
®37Ar +
e-
The detector used a tank of 390 000L of
C2Cl4 constructed in the
Homestake Gold Mine in South Dakota. To negate the
effects of any other solar radiation the detector was
constructed at a depth of 1480m. The energy threshold for
the generation of 37Ar is 814keV. At that
threshold the detector was sensitive to 8B and
higher energy 7Be neutrinos (as indicated in
fig 2.) It also had sensitivity to neutrinos produced by
the pep reaction and the CNO cycle.
The physical properties of 37Ar
make it a useful medium. It is an noble gas that is
easily removed from C2Cl4. Its
half-life of 35 days gave a reasonable measurement time.
To remove the gas at the end of each recording period
helium was circulated through the liquid. The gas was
then processed with the gas ending up in a charcoal trap.
This was then heated, and passed through a heated
titanium filter to remove reactive gases. After further
concentration through chromatography the gas was then
placed in a counter and counting would continue for one
year. When all factors were taken into considerations the
detector detected 25 neutrinos every year (Haxton,
1995.)
Due to the nature of this experiment the
direction from which the neutrino arrived from could not
be determined.
Bahcall’s calculations predicted a rate of
approximately 7.6 SNU (1 SNU = I
capture/second/1036 atoms) based on the
standard solar model (Haxton, 1995.) The rate measured by
Davis was 2.56 SNU (Miramonti, 2009.) This rate was a
third of the predicted value. This result gave birth to
the Solar Neutrino Problem.
Three major areas were proposed to explain
the discrepancy in the theoretical and observed neutrino
captures in Davis’ detector. Bahcall checked and refined
his model for neutrino production and capture and found
no significant errors. Likewise, Davis tested his
detector in a number of ways and increased its
sensitivity and he found no significant errors. The third
was not taken seriously at the time when Bruno Pontecorvo
and Vladimir Gribov proposed that neutrinos were not
fully understood.
Kamiokande
The Kamiokande detector was originally built
in the Kamioka Mine in Japan to study the stability of
protons and neutrons (Haxton, 1995.). It was later
upgraded with the aim of studying solar neutrinos. The
detector was later upgraded to increase its sensitivity
from 7.5MeV to 7.0MeV. The detector was very sensitive at
high energies (NOBweb.)
The detector utilised 4500 tones of highly
purified water and photomultiplier tubes (PMTs) to
measure Cherenkov radiation (Haxton, 1995.) In this
experiment the Cherenkov radiation was the light emitted
as an electron recoils after a neutrino – electron
collision due to it having a velocity greater than the
speed of light in water. The inner 4140 tons of water was
monitored by 948 PMTs. The outer 1.5m of water served as
an anti-counter and was monitored by 123 PMTs. Only the
events in the inner-most 630 tons of water were observed
to eliminate any gamma-ray events. The imaged volume is
known as the fiducial volume.
Due to the high energy threshold of the
detector it was sensitive to the 8B neutrinos
at the higher end of their spectrum. It detected both
electron and muon-neutrinos with a ratio of 7:1
respectively (Haxton, 1995.)
The Kamiokande experiment had a few
advantages over the Homestake experiment. The most import
of these was that the direction from which the observed
neutrino had originated from could be determined. Over
the course of the experiment it was clearly demonstrated
that the neutrinos originated from the Sun. Also the
energy of the arriving particles could determined. The
spectrum of the neutrinos agreed with the predicted
spectrum of the 8B neutrino spectrum. The
experiment also gave real-time results.
Like the previous Homestake experiment
Kamiokande found that the neutrino flux was less than
expected. After 1040 observing days it was found that the
flux was 46% of that predicted from standard solar model
(Hirata et al, 1990.) The data was checked for error but
no significant errors where found. The data in the result
covers two reported periods. The first from 1978 to 1988
(450 observing days) gave a result of 45%. The second
period between 1988 and 1990 (590 observing days)
resulted in a flux of 45% of the predicted
flux.
The observed number of events was higher
than the Homestake experiment due to the nature of the
detector. This is because the Homestake detector only had
sensitivity to electron-neutrinos. The Kamiokande
detector had some sensitivity to the other types of
neutrinos hence the higher measured flux.
GALLEX
As stated in the section on the standard
solar section the initial stage of the p-p chain gives
the rate of the overall p-p chain reaction rate. For that
reason it is important to measure the flux from this
reaction. The two radiochemical experiments (GALLEX and
SAGE) where designed for this purpose.
The Gallium Experiment (GALLEX) was
undertaken in the Gran Sasso Laboratory in Italy at a
depth of 3300m. The experiment was operational from 1991
to 1997. After maintenance of the chemical plants and
electronics the detector recommenced operations under the
name of GNO (Gallium Neutrino Observatory) which is still
operational. (GNOweb.)
The detector used 101 tons of
GaCl3 solution in water and hydrochloric
acid. The solution contained 20.3 tons of natural gallium
(Haxton, 1995.) When a neutrino interacts with a gallium
atom a radioactive germanium atom is produced via the
reaction below. The half life of the Ge is 16.5 days
(Bellerive, 2003.)
71Ga + νe
®71Ge +
e-
With a threshold of 233keV this detector was
sensitive to the higher energy pp neutrinos. This was
important as Bahcall thought he could more accurately
determine the number of low energy events as the flux of
neutrinos is constrained by the luminosity of the Sun.
Calculations showed that the measured events in the
detector were 53% from pp neutrinos, 27% from
7Be neutrinos, 12% from 8B
neutrinos and 8% from CNO neutrinos (GNOweb.)
Like the Homestake experiment the two
gallium detectors had a run of a set period and therefore
were not real-time detectors. At the end of a run (about
3 weeks) nitrogen gas was pumped through the solution to
extract the 71Ge. It was then converted to
GeH4 and
placed in the counters with xenon gas. The sample was
then observed for six months. The results showed that two
peaks at 10.4keV (K peak) and 1.2kev (L peak.) These
where used to compare the results to natural
radiation.
An important feature of these detectors is
that they could be calibrated by terrestrial sources
(NOBweb.) The GALLEX detector was calibrated by using a
51Cr source.
The original GALLEX experiment measured a
flux of 77.5 SNU over 65 runs. Under GNO a flux of 65.2
SNU was measured. Combining the two the result is 70.8
SNU over 100 runs taken over 2834 observing days. The
standard solar model predicts a flux of 129 SNU
(Bellerive, 2003.) The measured flux was 55% of what was
expected.
SAGE
The Russian-American Gallium Experiment
(SAGE) was built in the Baksan Neutrino Observatory in
the northern Caucasus Mountains in Russia. The detector
was at a depth of 4700m and used 50 tons of liquid
metallic gallium (Haxton, 1995.)
The detector was sensitive to the same
energy level and to the same neutrinos as the GALLEX
experiment. The 71Ge was extracted by
vigorously mixing the target with a mixture of hydrogen
peroxide and dilute hydrochloric acid. This produced an
emulsion where the germanium is first oxidized before
being dissolved by the hydrochloric acid. The germanium
is then extracted as GeCl4 which
is then purified, concentrated and converted to
GeH4.
The extraction of the germanium is about 80% efficient.
The radioactive decay of the germanium is then conducted
in the same manner as in the GALLEX
experiment.
As with the GALLEX experiment SAGE found a
neutrino flux less than that predicted by the standard
solar model. The measured flux was about 64.5 SNU or
approximately 50% of the predicted value.
Super-Kamiokande
Super-Kamiokande was the follow up to the
Kamiokande experiment. Its main aim was to study
atmospheric and solar neutrino oscillations. As with
Kamiokande the detector was an imaging water Cherenkov
detector constructed in the Kamioka mine in Japan
(Bellerive, 2003.) The project underwent two phases
separated by remedial measures following an accident
involving the explosion of PMTs (Cravens et al,
2008.)
The detector contained 50 000 tons of
ultra-purified water (Bellerive, 2003) that was
continually purified. The outer detector provided a
shield for cosmic ray muons and external low energy
background. The outer detector was monitored with 1885
PMTs. The inner detector contained 32 000 tons of water
with a fiducial volume of 25 000 tons. The inner detector
was monitored by 11 146 PMTs. Measures were taken to
minimise background events caused by radon emitted from
the surrounding rock.
With an energy threshold of 5MeV (for the
early part of the experiment the threshold was 6.5MeV)
the detector was sensitive to neutrinos resulting from
the β decay of 8B. One part of the experiment
was to test if there was variation in events between
night and day to test a prediction that neutrinos
underwent oscillation as they passed through Earth. Due
to its large volume the detector provided highly accurate
measurements of neutrino flux.
After 1496 observing days the measured flux
(2.35 x 106 cm-2 sec-1)
from phase one was 46.5% of that expected from the
standard solar model (Hosata et al, 2005.) The experiment
also found that variations in the flux varied due to the
eccentricity of the Earth’s orbit.
The second phase produced similar results.
The measured flux was 2.38 x
106 cm-2 sec-1 (Cravens
et al, 2008.) It found that the flux appeared to be
higher during night-time but uncertainties still allowed
for the flux to be the same for both night and
day.
SudburyNeutrino Observatory
The Sudbury Neutrino Observatory (SNO) is a
1 000 ton heavy-water Cherenkov detector. It was
constructed in the Creighton Mine in Canada at a depth of
2 000m (Bellerive, 2003.)
It differs from the Kamiokande detectors by
the use of heavy-water (D2O.) The vessel
holding the heavy water is surrounded by an array of 9
456 PMTs. The cavity around the detector is filled by 7
000 tons of ultra pure water providing support and
shielding.
The detector is mainly sensitive to
8B neutrinos. and mainly to electron-neutrinos
but the use of heavy water allowed some sensitivity to
muon- and tau-neutrinos. The electron-neutrinos are
detected by a charged-current (CC) interaction while the
other two neutrino types are detected through
neutral-current (NC) and elastic scattering (ES)
interactions (Bellerive, 2003):
CC:
d +
νe
®p + p + e- specific to
electron-neutrinos
NC:
d +
νx
®n + p + νx
where
x = e, μ or τ
ES:
ν
x +
e- ®νx +
e- predominately
electron-neutrinos
For the first time the NC interactions were
observed in this experiment. This was important as this
interaction measures the total flux of
neutrinos.
Detection of the rates at which each
reaction takes place can determine if neutrinos oscillate
on their journey from the Sun’s core to Earth. The
determination of which neutrinos react is given from the
following relationships:
Φ
CC = øe
Φ
ES = øe +
0.15øμτ
Φ
NC = øe +
øμτ
These relationships show that CC
interactions only take place with electron-neutrinos, ES
with predominately electron-neutrinos but also with the
other two flavours and NC with all neutrino
types.
In the first of three phases of the SNO
experiment the vessel contained pure heavy water.
Cherenkov light is produced when neutrons are captured.
The energy threshold was about 5MeV. The results of this
stage where as follows (Bellerive, 2003):
Φ
CC = ~1.76 x
106 cm-2 sec-1
Φ
ES = ~2.39 x
106 cm-2 sec-1
Φ
NC = ~5.09 x
106 cm-2 sec-1
The higher value of ΦNC over
the other two interactions indicates neutrino
oscillation. Furthermore the measured
ΦNC was close to the predicted
8B neutrino flux (~5.05 x
106 cm-2 sec-1)
by the standard solar model (Bellerive, 2003.) This was
clear statistical evidence for neutrino oscillation. This
phase did not find any clear evidence for a variation
between day and night variations in the neutrino
flux.
The second phase of the SNO project used
heavy water with about 2 tons of NaCl added to enhance
neutron detection. The addition of the salt also provided
a more accurate measure of the NC interactions by
eliminating some assumptions about the CC and ES energy
spectra. The results were comparable with the first stage
of the experiment and provided further evidence that
neutrino oscillation does occur. The results from this
stage were as follows (Bellerive, 2003):
Φ
CC = ~1.59 x
106 cm-2 sec-1
Φ
ES = ~2.21 x
106 cm-2 sec-1
Φ
NC = ~5.21 x
106 cm-2 sec-1
The third and final phase of the project
used 3He proportional counters immersed in the
heavy water (Aharmim, 2008.) Thirty six active ‘strings’
of detectors where used. This arrangement allowed for
more accurate measurement of the NC interactions and
hence the total solar neutrino flux. The result was in
agreement with the other two phases of the
experiment:
Φ
NC = ~5.54 x
106 cm-2 sec-1
The SNO experiment provided the answer to
the solar neutrino problem.
Neutrino Oscillations
What SNO proved was that neutrinos change,
or oscillate, as they travel between the core of the Sun
and Earth. The electron-neutrino emitted in the core can
change to muon- and tau-neutrinos. An important
implication of this is that neutrinos are not mass-less
as dictated by the standard particle model
(NOBweb)
In 1978 Wolfenstein (1978) proposed that
neutrino oscillations take place within the sun. This
occurs due the forward scattering of neutrinos. This
could occur even if the neutrinos where mass-less. The
model has been subsequently improved through the work of
Mikheev and Smirnov such that the oscillations can
exhibit resonance behaviour due to the propagation
through matter with different densities (IPNweb.) The
effect is known as the Mikheyev – Smirnov – Wolfenstein
(MSW) effect. The effect is particularly strong on
electron-neutrinos as they can propagate
‘while having
charged-current [CC] interactions with electrons in
addition to the neutral-current [NC] interactions.’
However, this process is only significant at higher
neutrino energies (NOBweb) so it didn’t explain why low energy neutrinos also
appear to oscillate.
At lower energies neutrinos undergo vacuum
oscillation. For this to occur the different states must
have finite masses (INDweb.) Neutrinos can be described
in terms of their mass or by the particles that they are
associated with (ie electron, muon or tau,
NOBweb.) The relationship
between these two descriptions are constrained in what
are called mixing angles. For oscillation to occur each
favour must have different masses (INDweb.) This allows
for changes as a neutrino passes through a vacuum and the
probability that a neutrino will oscillate is based on
its energy and the distance travelled. The measurement of
the mixing angle started in the SNO project and is still
being refined.
As we will see shortly there is a transition
between the low energy mass-related oscillations and the
low energy vacuum oscillations.
Following
Experiments
Following the SNO experiment the nature of
neutrino oscillation has been continued to be
studied.
KamLAND
The detector used in the KamLAND experiment
is housed in the cavern excavated for the original
Kamiokande experiment. It consists of 1 000 tons of
liquid scintillator within a container immersed in
non-scintillating oil and surrounded by 1879 PMTs (Araki
et al, 2004.) The instrument is contained within a 3 200
ton water-Chenenkov detector to shield it from gamma rays
and neutrons and indentifying cosmic ray
muons.
The detector measured the flux of
anti-neutrinos emitted from nuclear reactors that
surrounded it. The flux from the reactors could be
determined and hence the number of expected events in the
detector could be deduced. When an anti-neutrino
interacts with the scincillator an inverse β decay
reaction occurs with an energy threshold of
1.8MeV:
Anti-neutrino
+ p ®e+ + n
Over a period of 766 ton-years the
experiment observed 258 anti-neutrino events as opposed
to the predicted 365 events if there was no oscillation
present (Araki et al, 2004.) Arake et al (2004) concluded
that neutrino oscillation was due to MSW effect and that
it corresponds directly to neutrino oscillation in a
vacuum. Evidence for this comes due to the distortion of
the energy spectrum of the observed anti-neutrinos.
Furthermore, it eliminated all but the large mixing angle
solution of MSW (LMA-MSW).
Borexino
The significance of this experiment is that
it measured 8B neutrinos at lower energy
levels than in previous experiments (Bellini, 2008.) The
LMA-MSW theory dictates that at energies below 2 MeV
vacuum oscillations should dominate and at energies above
5MeV matter-driven oscillations should be dominated.
Between the two there should be a smooth transition. A
feature of this experiment is that this could be
tested.
The Borexino detector is similar to the
KamLAND detector. It consists of a nylon vessel
containing 278 tons of liquid scintallator. The vessel is
surrounded by 2212 PMTs which is contained in a 2 100 ton
water-Cherenkov detector with 208 PMTs. The detector was
carefully shielded to isolate it from background
radiation to allow detection of lower energy events. It
has an energy threshold of 2.8MeV.
Over 246 days of observations it measured a
flux of approximately 2.65 x
106 cm-2 sec-1 which
was in agreement with the LMA-MSW. In effect the
experiment demonstrated that there is a transition
between matter-driven neutrino oscillations and
oscillations in a vacuum as described in the LMA-MSW
theory (Bellini, 2008.)
Recent and Ongoing
Experiments
The nature of neutrino oscillation is still
an active area of study. The K2K project used a beam of
muon-neutrinos fired at the Super-Kamiokande detector
250km away (Ahn et al, 2006.) They studied the parameters
of oscillation (predominantly between muon- and
tau-neutrinos.) The results were consistent with
oscillations observed in atmospheric neutrinos generated
from cosmic rays.
The MINOS experiment utilised a
muon-neutrino source 720km from Fermilab (NIMweb.) After
an exposure period of two years they found that there was
a disappearance of a given neutrino type due to
oscillation (Adamson et al, 2008.)
Work is still ongoing for the OPERA
experiment which is housed in the same laboratory as the
GALLEX experiment (Lutter, 2009.) Using a neutrino source
originating at CERN 730 km away they are studying
tau-neutrino interactions. To do this they are using a
nuclear emulsion and looking at muon-neutrino to
tau-neutrino oscillations. They also hope to study
muon-neutrino to electron-neutrino
oscillations.
Conclusion
From our understanding of the processes that
occur in the Sum we can determine the number of solar
neutrinos that pass through a given area. This idea was
the foundations of testing if our understanding of solar
processes was correct. If we could measure the correct
number neutrinos we would have direct physical proof that
our models are correct. This was important as prior to
the solar neutrino experiments all our modelling and
calculations were mainly from observations rather than
measurements.
The early radiochemical experiments
indicated a deficit in neutrino triggered events in the
detectors. In the case of Homestake the results were
about a third of what was expected due to only being
sensitive to electron-neutrinos. The following
water-based experiment (ie Kamiokande) resulted in about
45% due to being partly sensitive to other neutrino
types.
The radiochemical gallium experiments of
GALLEX and SAGE where sensitive to lower energy neutrinos
from the pp reaction. The results were still less than
expected at 55% and 50% respectively. These experiments
demonstrated that both high and low energy neutrinos were
missing.
The larger water-based experiment of
Super-Kamiokande still found only 46% of the neutrinos
expected.
The missing neutrinos were a great concern
to astronomers and physicists alike. The experiments were
designed to test the standard solar model. However, the
number of neutrinos was not as expected. The idea that
the model was wrong did not fit with other observations.
It appeared that our understanding of the way neutrinos
behaved was not quite correct.
The heavy-water SNO experiment conclusively
found that indeed our understanding of neutrino physics
was incomplete. The experiment found the missing
neutrinos and gained insight into the processes that
changed the neutrinos as they travelled from the core of
the Sun to the Earth. This discovering had the
implication that neutrinos were not mass-less as
previously stipulated in the standard particle mode. The
standard particle model had to change.
Since the completion of the SNO experiment
other projects have further constrained our understanding
of neutrinos and how they oscillate as they travel
through both matter and a vacuum. It turns out that
neutrinos with energies above 5MeV are predominately
changed to other types of neutrinos in matter (i.e.
within the sun.) For particles with energies less than
2MeV they oscillate predominately in the vacuum between
the Sun and Earth. Between the two there is a smooth
transition. This behaviour is described in the LMA-MSW
theory.
==========================================
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