NEUP 2009 V1
Photofission of Actinides with Linearly Polarized Photons
A PDF version of the RFP is given below
- Proposal Due date
- 11:59 am March 16,2009
- 3/4/09
- preproposal + Budget to dave harris
- 3/10/09
- first draft
- 3/13/09
- final version (of course not Dan's working the weekend)
10 page narrative
Guidelines
Go/NoGo requirements
- Commitment to reporting
- If DOE Lab Work and safety Health program
- Export Control
- Standard Research Subcontract
- Quality Assurance if non-academic partner
- commitment to prepare additional contracts
Proposal Outline
Title: \begin{center} {\bf \Large Photofission of Actinides with Linearly Polarized Photons}\\ Technical Workscope : AFS-2 \end{center}
Technical Work Scope and Mission Relevance (20%)
This project involves the investigation of nuclear photofission at a fundamental level, with the ultimate goal of developing a new technique to quantify actinides for safeguards and quality control applications. In this proposal, we outline a plan to (1) measure the asymmetric neutron angular distribution resulting from anisotropic fission fragment emission when actinides are actively interrogated by linearly polarized photons and, (2) evaluate the potential for utilization of this phenomenon in the areas of materials control and accountability. The method for the production of linearly polarized photons from a bremsstrahlung beam described in this proposal is well established and can be readily implemented as an add on to other systems using bremsstrahlung beams for active interrogation. The fundamental measurements of the neutron angular distributions resulting from the angular dependence of fission fragment emission constitute a new, previously unexplored area of investigation.\\
Proposed scope description and Scientific Merit (20%)
It has long been known that the nuclear fragments resulting from photon induced fission of heavy nuclei are emitted an-isotropically when measured with respect to the incident photon direction. The first such measurement~\cite{Win1956}, performed with unpolarized photons in 1956, found an angular distribution given by $a + b sin^2\theta$, where $\theta$ is the polar angle and $a/b$ was found to depend on the energy of the incident photon, the target, and the type of fission fragment observed. The introduction of linear photon polarization breaks the azimuthal symmetry by imposing a preferred direction in space perpendicular to the incident photon beam. This $\phi$ asymmetry was observed in 1982~\cite{Geissen1982} with the measurement of fission fragments from the polarized photofission of thorium. Here, we propose a technique which has the potential to be sensitive to these azimuthal asymmetries, but avoids the technical difficulties associated with directly detecting the short ranged fission fragments.\\
For linearly polarized photons and considering only electric dipole (E1) transitions, the photofission of an even-even nucleus ($J^{\pi} = 0^+$, such as $^{238}U$) gives an angular distribution of the fission fragments\cite{Geissen1982}:
\begin{equation} W(\theta,\phi) = A_o + A_2 (P_2(cos \theta) + P_{\gamma} f_2(1,1) cos 2\phi P^2_2(cos \theta)) \end{equation}
The angular distribution coefficients $A_o$ and $A_2$ depend on the transition state (J,K), where $K$ is the projection of the total spin $J$ on the symmetry axis of the deformed nucleus. For $J = 1, K = 0$, we have $A_o = 1/2, A_2 = -1/2$ and for $J = 1, K = 1$, we have $A_o = 1/2, A_2 = 1/4$. $P_2 = \frac{1}{2}(2 - 3 sin2 \theta)$, $P_{\gamma}$ is the photon polarization, and $f_2(1,1) = 3 sin2 \theta$. $\theta$ is the polar angle with respect to the beam and $\phi$ is the azimuthal angle ($\phi = 0$ parallel to the electric field vector and $\phi = \pi/2$ perpendicular to E). \\
The calculated angular distributions of the fission fragments are shown in figure \ref{fig:w} for fragments emitted parallel and perpendicular to the polarization vector for a photon polarization of 30\% and for both K = 0 and K = 1. It can be seen that the asymmetries are large, even for such a low polarization.
\begin{figure}[ht]
\centering
\psfig{figure=w.eps,height=5.5in,width=5.0in}
\caption{$\theta$ distribution of fission fragments with photon beam polarization of
30\% for $K = 0,1$ and $\phi = 0, 90^{\circ}$. $K$ is the quantum number denoting the
projection of the spin of the transition state on the symmetry axis of the deformed fissioning
nucleus.
Solid lines are for unpolarized photons.
}
\label{fig:w}
\end{figure}
For any target thicker than a few $mg/cm^2$, of course, the fission
fragments are not easily detectable due to their heavy ionization energy loss and
resulting short range. The question we wish to address here
concerns whether or not the angular asymmetry in the fission fragments
is manifest in the angular and energy distributions of the prompt neutrons which
they emit, thus providing a possible unique signature for the presence of
photofission. Such a technique exploits the unique kinematics of the fission
process in conjunction with the relative penetrability of the fission neutrons.\\
To a very good approximation, the recoiling fission fragments emit neutrons isotropically in their center of mass\cite{Ploeg95} with a statistical energy distribution. A fully accelerated fission fragment travels with a speed of about 4\% of the speed of light. In the center of mass of the fragment, a typical 1 MeV neutron travels with a $\beta$ of about $4.6$\% of $c$. Thus, the neutron energy spectrum measured in the laboratory is a convolution of the "primordial" spectrum which the excited fission fragment emits, and the kinematical boost effects due to the fragment recoil. \\
These effects were examined in a Monte Carlo simulation employing the following assumptions:
\begin{itemize}
\item{}The fission fragment mass distribution was sampled uniformly between $85<A<105$ and $130<A<150$.
\item{}A fixed amount of total kinetic energy, 175 MeV, is shared between the two fission fragments in a manner consistent with momentum conservation.
\item{}Neutrons are emitted isotropically in the center of mass of the fully accelerated fission fragments with an energy distribution given by:
\begin{equation} N(E) = \sqrt{E} exp(-E/0.75) \end{equation}
This reproduces the fission laboratory neutron energy distribution as measured with $(n,f)$.
\item{}The fission fragment angular distribution is sampled in both
$\theta$ and $\phi$ as noted previously for either K = 0 or K = 1, and the
neutrons were given the appropriate kinematic boost.
\item{}The resulting rate asymmetry $\frac{N(\theta = \pi/2 ,\phi = 0)}{N(\theta = \pi/2,\phi = \pi/2)}$ is calculated for $P_{\gamma} = 0.3$. This degree of polarization is commensurate with that attainable using off axis bremsstrahlung, as described below.
\end{itemize}
For a pure $K = 0$ transition, the resulting $\phi$ distribution is shown in figure~\ref{fig:k0k1}. The resulting rate ratio, $\frac{N(\theta = \pi/2,\phi = 0)}{N(\theta = \pi/2,\phi = \pi/2)}$ is 1.25, reflecting the angular distribution of the fission fragments as well as the "washing out" effect of the neutrons emitted isotropically in the frame of the fission fragment. This rate ratio is for the case in which there is no cut on the neutron energy. If one imposes the cut $E_n
> 2$ MeV in the simulation, the asymmetry rises to 1.37. For the pure
$K = 1$ case, the resulting $\phi$ distribution is also shown in figure~\ref{fig:k0k1}. The corresponding rate asymmetry in this case is 0.84 without any neutron energy cut.\\
\begin{figure} \vspace*{-0.5in} \begin{center}
\begin{minipage}[t]{0.58\linewidth}
\epsfig{file=k0k1.eps,width=\linewidth} \end{minipage}\hfill
\begin{minipage}[t]{0.38\linewidth} \vspace*{-5cm}\caption{Monte Carlo
$\phi$ distribution of neutrons at $\theta = 90^{\circ}$. Solid: $K =
0$. Dashed: $K= 1$.} \label{fig:k0k1} \end{minipage} \end{center}
\end{figure}
%These asymmetries are merely representative of what one {\em might}
%observe.
The veracity of the above calculations is based on well founded physics,
but will need to be confirmed.
In the measurements proposed here, fission in actinides will be
induced by a bremsstrahlung beam of continuous energy. In practice,
fission may proceed via multiple channels, and one cannot directly
calculate the expected neutron asymmetries. As such, we propose a
complete experimental investigation of the $\theta$ and $\phi$
distributions of the neutron yields for a number of actinides to
ascertain the magnitude of the asymmetries and to determine the
sensitivity to the presence of fission and the selectivity of the technique to the
identity of the target nuclide. \\
\noindent {\bf Proposed production of linearly polarized photons }\\
As described in the {\em Resources and Capabilities} section of this proposal, the experimenters have ready access to a low energy electron linac. The energy of this accelerator is well suited to produce photons in the energy region of the giant dipole resonance of actinides, where the photofission cross section is large. We propose to optimize our existing beam line for the production of linearly polarized bremsstrahlung photons modeled after that constructed by the Geissen group\cite{Geissen1987}. It is well known that when an electron beam strikes a thin radiator, the resulting bremsstrahlung radiation which lies off axis from the incident beam is polarized as shown in figure \ref{fig:offaxisc}\cite{Geissen1987}. If one collimates the photon beam at an angle of $\theta = \frac{m_e}{E_{beam}}$ where $m_e$ is the rest mass of the electron and $E_{beam}$ is the energy of the incident beam, the polarization is maximized. A schematic of the relevant geometry is shown in the figure. %The energy dependence of the polarization of such a beam, as %measured by a polarimeter which is based on the large analyzing power of %the photodisintegration of the deuteron, is shown in figure %\ref{fig:pol} where it can be seen that modest polarizations of the %bremsstrahlung beam can be obtained. For such a set up, linear polarizations of the photon beam of around 30\% are typical. The rate of the photons as compared to the uncollimated case is decreased by approximately a factor of 20 to 30 due to the photon beam collimation as well as the need for a thin radiator to minimize multiple scattering of the incident electrons. The latter requirement is necessary to preserve the correlation between the angle of the incident electron, that of the resulting photon, and its polarization.\\
\begin{figure}[ht] \centering \psfig{figure=offaxisschematica.eps,height=3.25in,width=5.5in} \caption{Production of linearly polarized photons by the off axis bremsstrahlung technique. An electron beam strikes a thin ($\sim 10^{-3}$ radiation length) radiator, and a downstream collimator passes photons whose trajectories lie off axis from the incident electron beam. The optimal angle, $\theta$ in the figure is $m_e/E_{beam}$ or about $2^{\circ}$ for the proposed experiment.} \label{fig:offaxisc} \end{figure}
We propose to optimize our linearly polarized bremsstrahlung
beam line and construct a polarimeter to measure its performance. This facility
will be used to measure the angular dependence of the photofission
neutron yields as a function of the target, the bremsstrahlung endpoint energy,
and the neutron energy to evaluate the sensitivity of this technique to
the presence of photofission events. A schematic of the proposed
experimental setup is shown in figure \ref{fig:hrrl_val}. It consists of a
pulsed electron accelerator delivering approximately 14 MeV electrons
which are bent 90 degrees in the horizontal plane. The electrons are
then bent up or down in the vertical plane approximately two degrees
before they strike a thin ($10^{-3}$ radiation length)
bremsstrahlung converter.
The bremsstrahlung photons propagate down the beam line
in a cone with a characteristic opening angle of $m_e/E_{beam}$ radians.
A fixed collimator
is placed downstream of the bremsstrahlung radiator and is offset in the
horizontal plane. This collimator passes bremsstrahlung photons which are
off axis from the primary electron beam, and therefore are linearly
polarized 45 degrees with respect to the horizontal, with an orientation
depending upon the angle of incidence of the electron beam on the
bremsstrahlung radiator. Between the radiator and the collimator, a
magnet deflects charged particles in the horizontal plane away from the
photon collimator. The photons exit the vacuum via a thin window at the
downstream edge of the collimator on the downstream side of a four foot
thick concrete wall. This window serves as a thin converter
for a pair spectrometer luminosity monitor which is used for relative
normalization of the photon flux in the two polarization states. In addition to the
converter, the pair spectrometer consists of a 0.35 T, 20 cm long dipole
magnet and two plastic scintillator detectors to measure the electrons
and positrons in coincidence. The size of the scintillators will be small
in the direction of the dispersive plane of the magnet, thus providing
momentum analysis of the electrons and positrons. The yields of the
electron-positron
pairs will be directly proportional to the photon flux at a photon energy
given by the position of the plastic scintillator detectors and the
pair spectrometer magnetic field setting, thus providing a relative
flux normalization between the two polarizations states. The pair converter thickness,
electron and positron detector size and position, and magnetic field
setting are chosen to provide electron-positron coincidences for about
one out of every ten beam pulses in order to minimize accidental
coincidences. Electron-positron timing distributions will be recorded
in a time to digital converter (TDC). A schematic of the
proposed pair spectrometer is shown in figure~\ref{fig:ps-val}.\\
\begin{figure}[ht] \centering
\psfig{figure=Hrrl_valeriia_1.eps,height=4.5in,width=5.5in} \caption{A
schematic of the proposed experimental setup. 14 MeV electrons are produced
in a linear accelerator (left, back) and bent by a magnet (left, front) 90 degrees in the horizontal plane. They
are then bent up or down 2 degrees ($=m_e/E_{beam}$) before impinging
on a thin bremsstrahlung radiator. The bremsstrahlung photons then pass
through a hole in a
wall, consisting of a collimator placed off axis from the electron beam.
The photon flux is measured by a pair spectrometer
(red magnet to the right of the wall). The resulting polarized photon beam
impinges on a target (white), and neutrons are detected at 90 degrees to the beam in plastic
scintillator detectors using the time of flight technique.} \label{fig:hrrl_val}
\end{figure}
The physics target will be placed downstream of the pair spectrometer. When polarimetry experiments are being performed, this target will consist of $D_2O$ and $H_2O$ (for background subtraction) and during production runs will generally be a solid actinide target. A plastic scintillator neutron detector will be placed at an angle of 90 degrees with respect to the beam, at a 45 degree angle from the horizontal plane. This detector will measure the neutron yields as a function of the orientation of the polarization. A second detector will be placed at a 90 degree angle with respect to the beam in the horizontal plane. The purpose of this detector will be to monitor the setup for false asymmetries, and to provide a relative photon flux monitor which is redundant with the pair spectrometer. Neutrons will be identified via the time of flight method, with the start signal coming from the accelerator beam pulse.
\begin{figure}[ht] \centering \psfig{figure=ps-val.eps,height=4.5in,width=5.5in} \caption{A schematic of the proposed pair spectrometer luminosity monitor (red). Photons entering the experimental hall impinge upon a thin ($10^{-3}$ radiation length) pair converter. Electrons and positrons are momentum analyzed in a magnet, and detected in plastic scintillator detectors. Target (white) and neutron detectors (blue) are also shown.} \label{fig:ps-val} \end{figure}
\vspace*{0.5cm}
\noindent {\bf Milestones}\\
This is a proposed three year project. Key milestones are noted below.\\
\noindent 12 months:\\
Acquire and assemble instrumentation: vacuum pipe for
electron beam dump, beam dump magnet, exit window/pair converter, radiator, photomultiplier tubes and bases, octal discriminators, 4-fold logic unit.\\
\noindent 24 months:\\
Commission experimental setup, begin measurements.\\
\noindent 36 months:\\
Report on fundamental measurements and provide written technical assessment of potential for applications.\\
\noindent {\bf Timeline and Deliverables}\\
The work plan for this proposed project is as follows:
\begin{itemize}
\item{} month 6: Develop beam diagnostics including two view screens for monitoring the position and angle of the electron beam, and a pair spectrometer photon flux monitor consisting of a converter target, dipole magnet, plastic scintillation detector and associated electronics.
\item{} month 12: Establish beam line for linearly polarized photons. This will consist of the installation of a new vacuum system, a remotely removable bremsstrahlung radiator, coil deflector magnets to deflect the beam up and down two degrees to vary the photon beam polarization, and an off axis photon beam collimator.
\item{}month 20: Verify the production of linearly polarized beam using the high analyzing power of the photodisintegration of the deuteron\cite{gpnp} ($\vec \gamma + d \rightarrow n + p$). Here, we will measure the azimuthal dependence of the neutron yields at $\theta_n = 90^{\circ}$ from deuterated water targets. Backgrounds will be directly measured using an identical setup except with an $H_2O$ target.
\item{}month 24: Measure neutron azimuthal asymmetries for actinides as
a function of target, bremsstrahlung endpoint energy, and neutron energy.
\item{}month 30: Measure neutron azimuthal asymmetries for a variety of other targets to investigate the possibility of false positives.
\item{}month 34: Perform an overall assessment of the effectiveness of the technique for safeguards applications.
\item{}month 36: Publish results in peer reviewed journals.
\end{itemize}
\noindent {\bf Roles of Participants and Team Qualifications}\\
Participant A - Project Manager, specialist in photonuclear physics\\
Participant A has published extensively in the area of photonuclear physics in energy ranges which cover the the giant resonance region to several $GeV$. This work has been accelerator based and has utilized polarized and unpolarized photons and electrons. He has previously built beam line instrumentation similar to that proposed here, and has been involved in a number of detector development and construction projects.
Participant B - Data analysis coordinator\\
Participant C - Data acquisition manager, beam line design\\
Participant D - Beam line instrumentation\\
Three of the above participants have extensive experience in accelerator based experimental physics and nuclear instrumentation. One of the above participants is a new investigator. She has recently joined the group and was involved in previous photofission measurements. In addition to the above personnel, there are two Ph.D. students and one M.S. student currently available and ready to immediately begin their graduate studies on this project. In addition, we plan to recruit another M.S. student for this project. \\
\noindent
{\bf Work scope challenges and expected innovations:}\\
This project poses a number of technical challenges. It involves the improvement of our existing electron beam line in which the beam can be steered up and down two degrees in order to change the photon polarization state. This will involve a set of coil magnets placed around the beam pipe upstream of the bremsstrahlung radiator. Since we are proposing to measure a yield asymmetry, a critical aspect of these measurements will involve the relative normalization of the photon flux between the two polarizations states. We propose the construction of a pair spectrometer, which will measure the coincidence between momentum analyzed electrons and positrons which are produced in a thin converter. This will ensure a high precision relative photon flux measurement at the photon energy of interest. Before making measurements on actinide targets, we will unambiguously ascertain that we are producing polarized photons in a reliable fashion by measuring the known asymmetry arising from the photodisintegration of the deuteron\cite{gpnp}.\\
The possibilities of using the azimuthal neutron asymmetries for materials control and accountability hinges upon the results of the fundamental measurements proposed here. If the asymmetries are indeed favorable, one advantage of a set up where the position of the electron beam is alternated with respect to a fixed photon collimator is that target absorption and neutron attenuation and scattering effects are the same for the two polarization states, as the photon beam - target - neutron detector geometry is the same. In the experimental configuration proposed, photons will be produced with two polarizations states, each at 45 degrees with respect to the horizontal. One means of checking for false asymmetries will be provided by a second neutron detector which will be placed in the horizontal plane where no asymmetry is expected.\\
The technique proposed here seeks to exploit the unique kinematics of photofission in the development of a signature for actinides. Should this prove to be a viable technique, we propose to perform a systematic study of a variety of targets of interest to determine the possibility of neutron asymmetries arising from other mechanisms which could potentially cause false positive signals. Here, an investigation of the dependence of the polar angle dependence of the asymmetry (see figure \ref{fig:w}) may prove useful as well.\\
\noindent {\bf Quality assurance}\\
The procedures used to carry out this research and the results obtained will be documented in refereed journals. In addition, full documentation will be recorded in graduate student theses and dissertations. Laboratory notebooks will be maintained throughout the project and will be available for inspection by the funding agency at any time.\\
Bibliography
\begin{thebibliography}{99}
\bibitem{Win1956}Winhold and Halpern, Phys. Rev., 103, no. 4, 990, (1956).
\bibitem{Geissen1982}R. Ratzek, {\em et al.} Z. Phys. A -- Atoms and Nuclei 308, 63-71 (1982).
\bibitem{Geissen1987}U.E.P. Berg and Ulrich Kneissl, Ann. Rev. Nucl. Part. Sci. 37, 33-69 (1987).
\bibitem{Ploeg95}H. van der Ploeg {\em et al.}, Phys. Rev. C, 52, no. 4 (1995).
\bibitem{gpnp}M.P. De Pascale, {\em et al.}, Phys. Rev. C, 32, no. 6 (1985).
\end{thebibliography}
R&D resources and Capabilities (10%)
The research and development resources to support this proposal include an in house electron accelerator, several particle detector systems, and signal processing electronics. The linac shown in Figure~\ref{fig:hrrl) is capable of accelerating electrons up to energies of 16 MeV. The electrons are transported in pulse trains having a minimum width of 25 ns and a maximum repetition rate of 1 kHz. The accelerator can produce 80 mA peak currents which can intersect a radiator target to produce bremsstrahlung photons. The bremsstrahlung photons enter an experimental cell through a collimator configured to select off axis polarized photons. A target, in the experimental cell, is positioned in the path of the polarized photons.
Particle detection systems are placed at predetermined locations in the experimental cell to measure the reactants emanating from the target. The authors of this proposal performed a previous photofission experiment using two neutron sensitive scintillators place at 90 degrees to the incident photon beam and in the same plane as a 20 cm long target of heavy water (D2O). A NaI crystal optically coupled to a photomultiplier tube (PMT) was placed off the beam line axis at the end of the experimental cell to monitor the incident photon flux. The photomultiplier signals were amplified by Ortec Model 672 amplifiers and discriminated using a CAEN constant fraction discrimiator model N842. The NaI detector output was sent directly to a sampling ADC manufactured by FAST~\cite{FAST7070ADC} while the neutron detector outputs were send to a time to amplitude converter (ORTEC 567, 566). The TAC output is proportional to the neutron time of flight and was measured with the ADCs manufactured by FAST~\cite{FAST7070ADC}. A data acquisition system based on MPA-3 ~\cite{MPA-3} recorded the measurements for analysis. Figure~\ref{fig:n_ToF} shows a sample Time of Flight (ToF) spectrum from the neutron sensitive detectors. The spectrum illustrates the apparatus' ability to identify particles with flight times consistent with photons and distinguish them from particles with flight times consistent with neutrons.
\bibitem{MPA-3} Fast ComTec GmbH, Grunwalder Weg 28a, D-82041 Oberhaching Germany
\bibitem{FAST7070ADC} http://www.fastcomtec.com/products/product-lines/nim-modules/7070.html
\bibitem{ORTEC567TAC}http://www.ortec-online.com/electronics/tac/567.htm
\bibitem{ORTEC672Amp}http://www.ortec-online.com/electronics/amp/672.htm
\begin{figure}[ht] \centering \psfig{figure=hrrl.eps,height=5.in,width=7.in} \caption{} \label{fig:hrrl} \end{figure}
\begin{figure}[ht]
\centering
\psfig{figure=countroom.eps,height=5.in,width=7.in}
\caption{}
\label{fig:countroom}
\end{figure}
(V.S.- pictures + T.Forest - description)
CUT ALL THIS, NO? (DD) Yes! or Move (TF)
Idaho State University and the Idaho Accelerator Center have a number of electron linacs capable of generating
milliamp beams of electrons with energies up to 40 MeV.
Our proposed beamline for the production of linearly
polarized
bremsstrahlung
photons will be modeled after that of the Geissen group\cite{Geissen87}.
We propose to
construct such a linearly polarized bremsstrahlung beamline and a
polarimeter
to measure its performance,
and to measure the angular dependence of
the photofission
neutron yields as a function of both the bremsstrahlung endpoint
energy and the neutron energy (as determined by time of flight)
to evaluate the sensitivity of this technique to the
presence of photofission events.
Budget form
Materials and supplies: 26k
4 PMT - 5k beam pipe - 15k collimator - 5k scintillators - 1k
Proposal Graphs
