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The Sun provides all of the energy that our planet needs for life and has been doing so for five billion years. Understanding our Sun and its interior is one of the major goals of the NASA Science program. Still this is a very difficult task because very little makes it directly out of the Sun’s interior. The energy we see today, that warms the Earth, was made 50,000 to 80,000 years ago and is only now coming to the surface to make light. However, neutrinos penetrate matter almost without interaction and make it to Earth in only eight minutes from creation. Since neutrinos interact only weakly they are hard to detect; never-the-less within the last ten years neutrino detectors on Earth have started to reliably detect neutrinos from the fusion reactions in the interior of the Sun and scientists have started to use this information to investigate the Sun’s nuclear furnace.
Changes in solar neutrino flux make it advantageous to take a neutrino detector into space since the solar neutrino intensity changes dramatically as the inverse square of the distance from the Sun, by five orders of magnitude when going from the Earth to the Sun. Launch of a neutrino detector into space toward the Sun will: a) significantly reduce the required detector size while allowing for improved detector energy resolution and performance, b) attempt to completely eliminate background terrestrial neutrino sources for improved measurement accuracy, and c) conduct unique science experiments near the Sun not achievable with much larger detectors on the Earth. Furthermore, NASA has other interest in developing spacecraft that can go close to the Sun for Science and as a means to accelerate a spacecraft to higher speeds to send missions to nearby stars.
NASA's interest in deep space exploration has been a key factor in its unmanned spacecraft development and launch of exploration science satellites and spacecraft. NASA has done exceptional experiments in space where science benefits from the unique platform of spacecraft that provides unprecedented views. For example the Hubble Space telescope is really a small and very common instrument, but when it is put into an orbit high above the Earth, it becomes one of the most powerful optical observatories man has ever made. Moving neutrino observations to space follows this natural progression and it would only need a small 250 kg detector and support equipment.
The concept of putting a neutrino detector in close orbit of the sun is completely unexplored and innovative. Its scientific objective is to vastly enhance the understanding of the solar interior which is a NASA major goal as stated in the decadal survey. Preliminary calculations show that such a spacecraft if properly shielded, can operate in this environment both taking data of neutrino interactions which can be distinguished from random background rates of solar Electromagnetic emissions, Galactic charged cosmic-ray and gamma-rays by using a double pulsed signature. The idea of using a Heavy Liquid Scintillator where the CH2 has all the Hydrogen atoms changed to Deuterium would give sensitivity to multiple neutrino interactions. If successful this spacecraft concept will enable a whole new type of mission to explore and study our Sun, in details that could not be done with the largest neutrino detectors on Earth nor other type of spacecraft measurements that are not using neutrino detection. This MSFC CAN Dual Use proposal would develop a complete simulation of the science and technical aspects for building a spacecraft to accomplish this mission.
The project achievements anticipated from this project include, identifying a Neutrino Detector Technology capable of operating close to the Sun to study the solar interior for the purpose of better understanding our Sun, its future expected changes such as long term forecasting of solar energy output as well as fundamental physics such as Dark Matter searches; Particle Physics Neutrino Oscillations; and Nuclear Physics Matter effects of Neutrino interaction. On Earth the intensity of solar neutrinos is very low, by going very close to the Sun in a solar orbit the neutrino rate can be 10,000 times higher and a small detector inside the de- coherence neutrino radius would study the interior nuclear reactions of the Sun. Light from the Sun comes from these same Nuclear fusion reactions inside the nuclear furnace core but takes
>50,000 years to reach the surface, while neutrinos will come directly out of the solar core and they will tell us much more about the current solar interior than measuring any other solar emission. Current neutrino technologies are limited and need large detectors to make a small number of measurements, and all current neutrino detection technology are Earth based and have never flown in space. By advancing and developing neutrino detector technology to permit it to fly and operate in space, this will lead to a new ability to study the Sun, one that would enhance our ability to predict long term Solar output, Solar storms as well as fundamental new science studies that are currently unattainable.
This one year joint study between the NASA/MSFC Astrophysics Office, the NASA Advanced Concepts Engineering office, and Wichita State University will allow us to develop a new technology for a neutrino detector design through detailed computer simulations and address the aspect of background signal rejection, shielding, and various options for neutrino signal identification. The technical challenges and details will be addressed in cooperation with Les Johnson of the MSFC through regular monthly video meetings, where he will use NASA contacts to solve challenging problems. In addition, with the help of the theoretical physicists, the project will explore options of how to design a detector that would allow the most Physics to be studied. Such Physics topics are stellar interior measurements that could then be compared to the standard solar model; particle physics for neutrino oscillation; dark matter trapped in the solar interior; and nuclear physics matter effects on oscillation of neutrinos. The result from this physics study and technical detector parameter design will directly lead to new technology best suited to answer the Science questions of Stellar Neutrino imaging, rare nuclear fusion reaction rate studies and particle physics of neutrino oscillations inside the de-coherent radius limit which extends out to 35 solar radii for higher energy solar neutrinos. At the end of this one study we will know what detector technology design can do the most science, advance the TRL of this innovative approach, and that would then be the lead information for a major technology development to overcome the limitations of terrestrial based experiments.
There are two possible ways to put a detector into beyond earth orbit, both include possible Space Launch System (SLS) flight add-ons. Several SLS test flights will go to the Moon or near Earth asteroids and they could take a small add-on payload, but another SLS mission currently under consideration to the far edges of the solar system will first go close to the Sun within 5 solar Radii and that too can take a small test mission payload to be dropped in a solar orbit.
There are many different techniques to detect the neutrino. Since we are limited in scientific payload weight we can put into a satellite, we need to choose the best possible option. Preliminarily studies at Wichita State University and NASA/MSFC show that developing a small 250 kg sensitive volume is best and putting that in solar orbit at 8 solar Radii would make the rates of solar neutrinos from fusion high enough to do some substantial physics studies. We can further enhance this detector by using a process to replace all the Hydrogen in Liquid Scintillator with Deuterium which can be done 100% complete with a catalytic process giving us a “Heavy Liquid Scintillator”, a new novel neutrino detector technology idea that could have direct applications in new ways to search for nuclear material. We would then add dopants to this heavy liquid scintillator to enhance the sensitivity to other neutrino interactions such as Ga, as well as using the neutrino capture on C-12 into N-12 to study the higher energy neutrino reaction region. An important aspect of this design study that can be answered with simulations is the different rates from backgrounds and the proper shielding needed. Measurements from the Helios 1 Probe that the charged protons went up to 200 MeV but had an extremely small rate of 10-2 (cm2 sr s MeV)-1 and Helios 2 electron spectrum went up to only 20 MeV.
Cosmic ray rates seen on Earth would be smaller due to the Solar Modulation Theory, this expectation is supported by observations made by Helios-1 and 2. Between the two factors, of the exceptional quiet Sun in Cycle 23/24 and the assumption of a smooth cosmic-ray intensity gradient as a function of distance, it was seen that the expected ‘upper limit’ of cosmic-ray intensity at a nominal distance of 10 solar radii are well within the error bars of PAMELA’s 1-AU data. Galactic gamma ray rates as seen by Egret and Fermi satellite are reasonably low at 10-4/s cm2 sr. Solar neutrino rates are expected to be one or two per hour, and through the double signal coincidence in time will be identified in subsequent data analysis. Shielding and active veto detectors will keep the background low by reducing these rates.
The main source of count rate in the detector will be that from solar electromagnetic radiation emission. This is expected to be in the region of 0.5 to 10 MeV and at 10 down to 10-1/s cm2 sr, again as seen by the Helios spacecraft. We have been able to do a detailed study of the shielding needed for a 25 kg detector. The final optimized shielding was determined to be 9 cm of Tungsten and less than a meter in diameter. This is very reasonable and the rates from electromagnetic solar radiation would be low.
The planned physics studies that the detector needs to perform will greatly influence the detector technology design. Open questions are the signal resolution, timing responses, communication rates and multi-event triggering. All of these parameters are tied to the science goals to be performed. The plan of work will hence be a detector technology study combined with the science goals that such a future space mission would like to achieve and match their performance for best results. All of this can be done with the planned detailed simulations.
As an example of this complicated coupling, let’s look at the specific case of de-coherent neutrinos and energy resolution needs. Energy resolution is a crucial factor in making sound scientific measurements on the neutrino. The electron neutrino survival probability curve from low energy neutrinos up to 10 MeV has the ability to allow for distinction between many different models of neutrino oscillations, including sterile neutrinos. However, in order to see the difference between these various theories, the converted neutrino energy resolution must be high. In very large experiments, the energy resolution inherit in the various detector techniques is washed out because of two factors: First, the large size of the detector limits the amount of light reaching the photo detector, stemming from light acceptance issues reducing the photon statistics, and second, the size of the detector means that the attenuation length of the detection media will make identical signals, whether near to or far from the photo detector, very different because the events far away have much reduced light in the photo sensor due to photon absorption in the media. In a detector designed within a spacecraft, the detector volume is very small so both effects are mitigated which should leave the experimental measurements with good energy resolution, in addition to a high rate of solar neutrino interactions. From this benchmark study we will learn more precisely the science potential achievable on a spaceflight mission.
Hence, it is possible in a one year study through a simulation of detector technology and physics processes to reach a conclusion of what detector technology is needed to achieve which physics parameters for performing the best science study. There are also applications to solar weather prediction and nuclear material surveillance from this project that could have further applications outside of the basic sciences and may be of interest to industry.
The specific tasks to be performed during this effort are listed below by institution:
Wichita State Univ. with Prof. Nick Solomey and Graduate student Caleb Gimar, aided by undergraduate physics majors, Prof. Meyer and Aerospace Prof. Atri Dutta will be studying the following three things:
a pulse height identical to within 20% of the expected X-ray energy of our neutrino interaction events.
South Dakota State Univ. with Prof Robert McTaggert
NASA/MSFC Mark Christl, Les Johnson
The deliverable for this study will be a detailed report documenting the best approach to achieve the science objectives identified above and an evaluation of its performance. This will include a conceptual design for the payload supported by the results from the simulations completed during the study; identifying the detection strategy and background signal rejection; and key spacecraft capabilities to realize a mission in near-solar orbits. These results will be further disseminated to the scientific community through conferences and submission of an abridged version for publication in a journal.
The overarching objective of this study breaks new ground by extending current ground based measurements to a space-based platform. The principal risk for this project is not achieving satisfactory results. Preliminary calculations indict that spacecraft resources should be adequate and background signal can be suppressed sufficiently to permit meaningful scientific measurements and only the level of performance and degree of optimization is yet to be determined.
The requested NASA contributions to the project are identified in budget details. The project support by NASA scientists is small and falls within their normal duties and therefore do not have a specific value assigned.
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