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Revision as of 01:04, 15 March 2011
A Positron Source for JLAB
File:PositronFullProposal 4-2009.pdf
Abstract
We propose to develop a partnership between Jefferson Lab's (JLab) Continuous Electron Beam Accelerator Facility (CEBAF) in Newport News, VA and the Idaho Accelerator Center (IAC) at Idaho State University. The partnership will initially be nurtured through a research and development project designed to construct a positron source for the CEBAF. The first year of this proposal will be used to benchmark the predictions of our current simulation with positron production efficiency measurements at the IAC. The second year will use the benchmarked simulation to design a beamline configuration which optimizes positron production efficiency while minimizing radioactive waste. The second year will also be devoted to the design and construction of a positron converter capable of sustaining the heat load from high luminosity positron production. The final year will be used to quantify the performance of the positron source and measure the source's radiation footprint. A joint research and development project to construct a positron source for use by the CEBAF will bring together the experiences of both electron accelerator facilities and solidify this partnership for future projects. Our intention is use the project as a spring board towards developing a program of accelerator based research and education which will train students to meet the needs of both facilities as well as provide a pool of trained scientists for others.
Project Objectives
We propose to develop a partnership between Jefferson Lab's (JLab) Continuous Electron Beam Accelerator Facility (CEBAF) in Newport News, VA and the Idaho Accelerator Center (IAC) at Idaho State University. The partnership will initially be nurtured through a research and development project designed to construct a positron source for the CEBAF. The first year of this proposal will be used to benchmark the predictions of our current simulation~\cite{HuntPos} with positron production efficiency measurements at the IAC. The second year will use the benchmarked simulation to design a beamline configuration which optimizes positron production efficiency. The second year will also be devoted to designing a positron converter capable of sustaining the heat load from high positron luminosity production. The final year will be used to measure the capabilities of the positron source and the source's radiation footprint. A joint research and development project to construct a positron source for use by the CEBAF will bring together the experiences of both electron accelerator facilities and solidify this partnership for future projects.
One of the more common methods used to create positrons is to bombard a target of high atomic number (Z), typically Tungsten, with electrons of sufficient energy to generate a shower of secondary electrons, photons, and positrons. Electron accelerators have used this technique to produce positron beams with intensities approaching /sec~\cite{Ito_1991} at 100 MeV energies that are at least an order of magnitude larger than traditional radioactive source based beams~\cite{Radbeam}. Positron beams have also been produced at GeV beam energies with intensities of /sec for use by the high energy physics community. The current conceptual design of a positron source for use at the CEBAF would rely on the production of positrons at relatively lower energies (MeV). Positrons would be produced using electrons from the current source which have been accelerated to energies between 10 and 20 MeV. Although these low energies produce less intense positron beams our current simulation predicts that we will be able to produce positron currents beyond a nanoamp which are sufficient for our needs. This energy selection has the feature of being close to the 10 MeV neutron production threshold~\cite{n_threshold} allowing us to reduce radioactive waste.
Year 1
The main objective of this proposal in the first year will be to perform positron
production measurements at the IAC in order to benchmark our simulation~\cite{HuntPos}.
As shown in Figure~\ref{fig:PosYieldBrightness-vs-Wthickness},
the simulation predicts an almost linear increase in positron
production efficiency, , as the tungsten converter target thickness
increases when using 10 MeV electrons.
In order to create a beam of positrons for acceleration in the CEBAF, one must also consider
the trajectories of the positrons from the converter.
The emittance () is defined as
where
$x$ is distance of the positron from the beam center along the horizon as it leaves the Tungsten converter
and
$x^\prime$ is the angle of the positrons momentum with respect to the beam axis
given by $\arctan(\frac{p_x}{p_z})$, an example is shown in Figure~\ref{fig:PosYieldBrightness-vs-Wthickness}.
Defined in this manner, the emittance simultaneously accounts for both the location of
emission and the direction. A small emittance will result in less transport loss of
the positrons into the accelerator.
Brightness, defined as
\begin{equation}\label{eq:brightness}
B={ N(e^+)\over \varepsilon_{x}*\varepsilon_{y} },
\end{equation}
is one of the parameters used to evaluate the optimal target thickness by simultaneously
taking into account positron production and emittance. The variable N(e$^+$) represents
the number of positrons from the converter and $\varepsilon_{y}$ represents the emittance in the
vertical beam direction.
The simulation predicts, see Figure~\ref{fig:PosYieldBrightness-vs-Wthickness},
a plateau in the Brightness when the Tungsten target thickness is 0.5 mm.
The simulation also showed this thickness to be optimal in terms of the
positron production efficiency per Watt of deposited energy when using
10 MeV incident electrons. Our objective in year one will be to evaluate
the veracity of the above predictions.
The transport of positrons from the Tungsten converter to the accelerator is envisioned as a three quadrapole system based on the work of Reference~\cite{Sarma2003}. Figure~\ref{fig:QuadPosTransSystem} shows the current conceptual design and the positron trajectories predicted by our simulation of the system. The quadrapole triplet system will select a positron momentum band as well as divert electrons and positrons which are out of the accelerator acceptance. Figure~\ref{fig:PosMomentum} shows the dependence of the positron momentum with positron efficiency as predicted by the simulation. The simulation predicts that a 0.5 mm thick Tungsten target placed in front of a 10 MeV electron beam will produce the most positrons in a momentum band between 3 and 5 MeV. Based on this prediction, the transport system has been designed to deliver positrons with a momentum distribution shown in Figure~\ref{fig:PosMomentum}.
The goal in year 1 will be to confirm these predictions with measurements at the IAC using different Tungsten target thicknesses. A Faraday cup will need to be purchased and installed early in year 1 for these measurements. The magnet system will be provided by our DOE lab partner, the CEBAF, and the remaining beamline components will be provided by the IAC. The IAC has several HP-Ge detectors to measure the energy spectrum at low intensities as well as neutron sensitive scintillators (BC420), and position sensitive ionization chambers to measure attributes of the positrons exiting the converter at higher intensities and perform a preliminary evaluation of the source's radiation footprint.
\begin{figure}[htbp] \centerline{ \scalebox{0.25} [0.4]{\rotatebox{0}{\includegraphics{Graphs/Brightness-vs-Momentum_W0.5.eps}}} %\scalebox{0.15} [0.32]{\rotatebox{0}{\includegraphics{Graphs/PosMom_AfterTarget.eps}}} \scalebox{0.15} [0.28]{\rotatebox{0}{\includegraphics{Graphs/PosMom_AfterColl.eps}}} } \caption{The above figures are predictions from our current simulation. The left figure illustrates the positron production Brightness as a function of the positron's momentum leaving a 0.5 mm thick target. The right image shows the positron momentum distribution leaving the quad tripled transport system shown in Figure~\ref{fig:QuadPosTransSystem}.
Year 2
The second year will use the benchmarked simulation to design an optimal beamline configuration, perform heat load studies at the IAC, and construct a positron converter target which can sustain a high heat load. We expect at least another iteration of measurements using the IAC in order to benchmark the simulation during the first part of year 2. During those measurements we will also perform heat load studies by spanning the accelerator's instantaneous current range from dark current (nA) to 100 mA and increasing the energy from 5 MeV to beyond 25 MeV. Table~\ref{tabl:HeatLoad} is the simulation's prediction for the heat load on each transport element when using a 0.5mm Tungsten target and a 10 MeV electron beam to generate a 20 nA beam of positrons. The final checks between the simulation and data are expected to be completed midway through year two as well as the heat load measurements. A chamber for the Tungsten converter will be designed and constructed which will also house the brushless motor used to rotate the target. A conceptual design of the system's general features is shown in Figure~\ref{fig:CoolingSystem}. A quote for the typical cost of a brushless motor to rotate the converter was used to determine the budget allocation of \$15,000 for the motor.
\begin{figure}[htbp] \centerline{ \scalebox{0.3} [0.2]{\rotatebox{0}{\includegraphics{Graphs/cooling_system.eps}}} } \caption{Conceptual cooling system. Electrons from the CEBAF's present source would impinge a Tungsten converter disk which is rotating to improve its heat load capacity. Water cooled copper plates are heat sinked to the converter and triplet magnet system. The 3 MeV positrons are then injected into the CEBAF. } \label{fig:CoolingSystem} \end{figure}
\begin{tabular}{|c|l|}\hline \multicolumn{2}{|c|}{ Heat Load per Transport System Element}\\ \hline Element & Power (kW)\\ \hline \hline Tungsten Target & 22 \\ First Quad Magnet & 25 \\ Second Quad Magnet & 15 \\ Third Quad Magnet & 5 \\ Collimator & 28 \\ \hline \label{tabl:HeatLoad} \end{tabular}
- Measure production efficiency for different Tungsten converter target thicknesses (100 microns -> 1mm in 100 micron steps)
- Measure positron emmitance and compare to simulation predictions
- Measure brightness as a function of quad setting (quads change emmitance)
- measure positron output as a function of beam current
- Begin designing tungsten converter capable of handling high heat load
- CEBAF can accept particles into acceleration stage as long as
- Electrons are injected at 500 MeV (1 GeV) for a 6 GeV (12 GeV) CEBAF. Currently
Year 3
The final year will be used to quantify the performance of the positron source, measure the source's radiation footprint, and hold a workshop to review the positron source's performance in order to determine a path to implementation at the CEBAF. The first half of the year will be devoted to measuring the positron production rates and emittance at the IAC with in house detectors. The Faraday cup installed in year one will be used to measure the positron current. We expect to only spend a small amount of the beam time match provided by the IAC on this measurement since it was the focus of measurements made during years 1 and 2. The remaining beam time will be spent measuring the amount of extraneous radiation which will not be injected into the CEBAF in order to determine the environmental impact of using the source at JLab. The positron emittance will be measured using two ionization chambers equipped with Gas Electron Amplifiers~\cite{Sauli} that enable spatial resolutions of least 100 $\mu$m~\cite{GEM}. The radiation footprint of the source will be measured using the above ionization chambers for charged particles, a HP-Ge detector for photons, and a BC420 scintillator for neutrons. A workshop will be organized at the end of this project which will bring together CEBAF personnel, IAC personnel, and experimentalists interested in using this positron source. One of the main goals of this workshop will be to present the positron source design for review and formulate a path forward for its implementation in the CEBAF.
- Conceptual Positron source design level 1 (CD1)
- IAC beam is x 40 less power than what will be needed at JLAB
Bibliography
- {HuntPos} M. A. Gagliadi adn A.W. Hunt, Nucl. Instr. and Meth., {\b B 245} (2006) 355-362. S. Golge; {\it et. al.}, Proceedings of PAC07, THPMS067, Albuquerque, New Mexico, 2007.
- {Ito_1991}Y. Ito, {\it et. al}, Nucl. Inst. and Meth. {\b A 305} (1991) 269.
- {Radbeam} K.G. Lynn, {\it et. al}, Rev. Sci. Instr., {\b 51} (1980) 977.
- {n_threshold} S.M Seltzer and S.M Berger, "Photoneutronproduction in Thick Targets", Phys.Rev C Volume.7, p.859
- {Sarma2003} P. R Sarma, J. Phys. D: Appl. Phys. {\b 36} (2003) 18961902
- {Sauli} F. Sauli,Nucl. Instr. and Meth., {\bf A 386}, 531-534 (1997).
- {GEM}Frank Simon, Ph. D. Thesis, Physik-Department, Technische Universitat Munichen, Nov. 2001
- {twoPhoton} P.A.M. Guidal and M. Vanderhaegan, Phys. Rev. Lett., {\b 91} (2003), 142303.
- {DVCS} P.A.M. Guidal and M. Vanderhaegan, Prog. part. Nucl. Physics, {\b 41} (1998).
- {U-Boson} P. Fayet, Phys. Rep. {\b D 74}, (2006), 054034
- {Fatigue}"Fatigue and Durability of Structural Materials", S.S. Manson and G.R. Halford, ASM International, ISBN-10 \# 0-87170-825-6
- {HuntDefects} A. W. Hunt,{\it et. al}, Nucl. Inst. and Meth., {\b B241} ,(2005), 362.
Parts list
Latex File
The LaTex file for this proposal is under Media:Positrons.txt
review
response
2 critics
1.)
critic: "It is somewhat surprising to this reviewer that JLAB would consider a positron source based on 20 MeV electrons, when the yield of positrons is much higher using a GeV electron beam, as available at JLAB."
response: The incident electron energy for the positron source is an input parameter for the design. The current polarized electron source at Jefferson Lab injects 15 MeV electrons into the main accelerator. The proposed positron source would be located in between the polarized electron source and main accelerator. The above was pointed out on page 1 of the proposal in the sentence "Positrons would be produced using electrons from the current source which have been accelerated to energies between 10 and 16 MeV." Using GeV electrons from the main accelerator appears to be cost prohibitive at this time.
2.) critic:"I have some concern that a discussion of implementation strategies will not take place until the end of the project, however. The proposers do not explain the logic of this timing. Perhaps there is a good reason for it, but it would have been more reassuring to know, up front, what JLAB's vision was for the use of the source."
response: The second paragraph in section 5 of the proposal outlines the benefits to JLab's scientific program. The program described in the proposal was reflected in a recent workshop at Jefferson Lab (http://conferences.jlab.org/JPOS09).
3.) critic:"I am slightly surprised that most of the travel budget is requested during the final year ($20K versus $5K in each of the first two years). I would have expected more personnel exchange to be required earlier in the project."
response: The budget was optimized to enable personnel exchanges during the positron source tests at the IAC. JLab personell will be participate in the beam tests. The collaborative efforts involved in designing the source will undertaken through electronic means such as telephone and video conferencing available at both institutions.
4.) attach original letter from JLab endorsing proposal
comments reports
Comments Report
3/21/2008 Review: EPSCoR-State/National Laboratory Partnerships Proposal Number: 107690 Proposal Title: The Development of a Positron Source for JLab at the IAC Investigator: Forest, Tony (IDAHO STATE UNIVERSITY)
1. Scientific and/or technical merit of the project. Proposal 107690 is for development of a positron source based on 20 MeV electrons incident on a 1 mm thick tungsten target, yielding of order 0.001 positron per electron. Such a system will very likely perform as predicted.
2. Appropriateness of the proposed method or approach. It is somewhat surprising to this reviewer that JLAB would consider a positron source based on 20 MeV electrons, when the yield of positrons is much higher using a GeV electron beam, as available at JLAB. Of course, a GeV electron beam is not available at Idaho State U, so the proposal appears to do more what could be done as ISU that what might be best for a JLAB application.
In this context, it would seem that approval of this proposal should be contingent on a clarification from JLAB that they actually prefer a lower performance source that uses a lower beam energy.
A statement from JLAB endorsing this proposal seems to be missing.
3. Competency of applicant's personnel and adequacy of proposed resources and facilities. The ISU team can likely carry out the proposed task.
4. Reasonableness and appropriateness of the proposed budget. The budget is reasonable for the proposed task.
5. Likelihood of the success of the EPSCoR-State faculty laboratory-scientist collaboration. I have no direct knowledge of relevance here, but optimism is a reasonable attitude on this issue.
6. Other Considerations. I am giving this proposal a mark of 5, pending the critical clarification by JLAB that they actually desire a positron source of the typed proposed here.
Such a source would surely work as expected, so there is no need for proof-of-principle R&D. Rather, this proposal deserves to be funded mainly in the context of a clear need for a source with the proposed parameters.
Should a clear case be made by JLAB that this is the kind of positron source they want, then I would evaluate this proposal as as 7-8.
Comments Report
3/21/2008 Review: EPSCoR-State/National Laboratory Partnerships Proposal Number: 107690 Proposal Title: The Development of a Positron Source for JLab at the IAC Investigator: Forest, Tony (IDAHO STATE UNIVERSITY)
1. Scientific and/or technical merit of the project. Under the proposal a positron source will be designed and tested at the Idaho Accelerator Center (IAC) in collaboration with scientists from JLAB. The goal is to install the source at JLAB at the end of the project. The scientific merit of the proposal will be realized once the source is operational at JLAB and used for research. The technical merit of the proposal consists of designing a positron source of sufficient intensity and brightness that simultaneously has a low radiation footprint and can withstand the necessary heat load. The proposers have already done some work on simulating the positron production from a tungsten target and on a transport system comprising 3 quadrupole magnets. The results of this work will be published a the proceedings of a workshop about one year after the end of the project.
2. Appropriateness of the proposed method or approach. The work envisaged in the proposal consists of the following parts: using IAC facilities to benchmark simulations of positron production as a function of target thickness; installing a transport system comprising 3 quadrupoles that will be brought to IAC from JLAB; measurements of source brightness; design of an optimised source and evaluation of heat loads; measurement of radiation produced by the source; and consultation with JLAB partners concerning an strategy for implementing the source at JLAB. All of these steps are reasonable and appropriate. I have some concern that a discussion of implementation strategies will not take place until the end of the project, however. The proposers do not explain the logic of this timing. Perhaps there is a good reason for it, but it would have been more reassuring to know, up front, what JLAB's vision was for the use of the source.
3. Competency of applicant's personnel and adequacy of proposed resources and facilities. The PIs appear to be well qualified to carry out this project. At least one of the PIs (Forest) has worked at CEBAF in the past which should be an advantage for this project. The IAC appears to have the necessary accelerator facilities, workshops and ancillary equipment to carry out the project once three quadrupole magnets are brought from CEBAF and installed. Two relatively inexpensive items (a Faraday cup and a motor) that will be purchased with this grant are necessary for the project. IAC will provide support to this project by defraying the cost of beam time. Release time will be provided by Idaho University for the two PIs to oversee this project.
4. Reasonableness and appropriateness of the proposed budget. The budget appears to be quite reasonable. As mentioned above (#3), there is considerable cost matching provided by IAC and Idaho State University. I am slightly surprised that most of the travel budget is requested during the final year ($20K versus $5K in each of the first two years). I would have expected more personnel exchange to be required earlier in the project.
5. Likelihood of the success of the EPSCoR-State faculty laboratory-scientist collaboration. IAC and CEBAF already have a signed MOU describing the goals of their collaboration. Personal relationships also appear to already exist between the IAC researchers and CEBAF personnel. ISU appears to be supportive of this effort in the sense that they are willing to commit significant resources to it. CEBAF is willing to provide three quadrupole magnets to support this development project and thus appear to be engaged in it. All of these facts augur well for success. While the case is made in the proposal that this positron source will contribute to future scientific discovery at CEBAF, I could find no support for this statement from the CEBAF side. My conclusion is that this faculty/lab-scientist collaboration is likely to be successful during the 3 years covered by the requested grant but that I cannot judge whether it will extend beyond that period because the necessary evidence is not included in the proposal.
6. Other Considerations.
Comments Report
3/21/2008 Review: EPSCoR-State/National Laboratory Partnerships Proposal Number: 107690 Proposal Title: The Development of a Positron Source for JLab at the IAC Investigator: Forest, Tony (IDAHO STATE UNIVERSITY)
1. Scientific and/or technical merit of the project. This proposal seeks to establish a collaboration with Jefferson Labs. The IAC group at Idaho State is proposing to develop a positron source for use by Jefferson Labs. A relativistic electron beam will impinge upon a high-Z target to generate backscattered electrons, photons, and positrons. The IAC approach is to use electrons with energies that are significantly lower than what other groups had used in the past. It is proposed that a nanoamp of positron current can be generated with electron energies between 10 and 20 MeV. The merit of the project is superior and worth supporting
2. Appropriateness of the proposed method or approach. The proposed method is not particularly innovative. High energy physics groups routinely have produced large positron currents using this approach, albeit at much higher energies. Since only about nanoamp is required for the Jefferson Lab scientists, lower energy beams are sufficient. Radioactive waste is decreased, however at these lower energies, a good thing. The appropriateness of the proposed method or approach is acceptable
3. Competency of applicant's personnel and adequacy of proposed resources and facilities. The personnel are perfectly competent to perform these investigations. The proposed resources requested and facilities in place are adequate. The competency and resource adequacy is superior
4. Reasonableness and appropriateness of the proposed budget.
The budget is reasonable and appropriate. Reasonableness is adequate.
5. Likelihood of the success of the EPSCoR-State faculty laboratory-scientist collaboration. This collaboration appears to be quite synergistic and the likelihood of a successful collaboration is high. Likelihood of successful collaboration is superior
6. Other Considerations. This proposal make good use of the infrastructure at the IAC and presents good prospects for a successful long-term collaboration with Jefferson Labs.
Comments Report
3/21/2008 Review: EPSCoR-State/National Laboratory Partnerships Proposal Number: 107690 Proposal Title: The Development of a Positron Source for JLab at the IAC Investigator: Forest, Tony (IDAHO STATE UNIVERSITY)
1. Scientific and/or technical merit of the project. This is an excellent proposal addressing an important technical problem for J-Lab. The development of a high intensity positron source is of definite interest to J-Lab and will increase the breadth of the scientific questions they can address. In addition, the focus on minimizing the creation of activated material will be important to the laboratory in the future.
The proposed plan of developing the source in stages is quite reasonable and achievable. It has a vary good chance of leading to a successful target for J-Lab.
2. Appropriateness of the proposed method or approach.
The combination of theoretical modeling accompanied by experimental studies of test systems is quite appropriate.
3. Competency of applicant's personnel and adequacy of proposed resources and facilities.
The personnel involved in the project have the core competencies necessary to successfully complete this project. The resources available at IAC are ideal for these studies and represent an excellent use of a significant capital investment.
4. Reasonableness and appropriateness of the proposed budget.
The budget is reasonable and should provide sufficient funds for completion of the project.
5. Likelihood of the success of the EPSCoR-State faculty laboratory-scientist collaboration.
This looks like an excellent collaboration with a very good overlap of interests on both sides. This is a good choice of a first project that should lead to a strong collaboration in the future.
6. Other Considerations.