Re: Update to Response to NuSAG Question #9

From: Mike Shaevitz (
Date: Sun Jun 26 2005 - 09:52:15 CDT

Hi Josh,

Thanks for the update; it looks very good. The one thing we might add
is a paragraph about temperature. To me, we will being doing the
moving operations only a few times and will install and move detectors
only when the weather is good (not -10 degreesF). We might also talk
about the engineering study that Jon reported concerning the
maintenance of temperature.

This all would then be backed up with statements that 1) the changes
in calibrations due to temperature will be calibrated out and 2) the
changes in density will be determined by monitoring the volume and by
calculations (it might be good to have some quantitative estimates
here if possible.)


Josh R Klein wrote:

> Braidwooders, I have attached an update to our response to Question
> #9. The update
is just the
> addition of slightly more specific plans for the things which might
change. It may
> be "too much information" however, especially since we don't really
have a lot of
> detailed simulation to back things up. It is hard to estimate
exactly what we can
> tolerate in terms of changes since we'll re-measure it all
anyway---the only time we
> run into problems if things are disasterously different, in which
case we're
> probably out of business anyway.

> Josh
> ------------------------------------------------------------------------
> \documentclass[11pt]{article}
> \usepackage{epsfig}
> \usepackage{graphics}
> \usepackage{amsmath}%
> \usepackage{amsfonts}%
> \usepackage{amssymb}%
> \usepackage{graphicx}
> \setlength{\textwidth}{6.5in}
> \renewcommand{\baselinestretch}{1.1}
> \setlength{\oddsidemargin}{0pt}
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> \begin{document}
> \section{Question 9}
> {\it What detector parameters (volume? PMT gain? energy calibration? other?) must
> remain constant, and to what level, when the detectors are moved for this
> cross-calibration to work?} \\
> ``Cross-calibration'' of the detectors by moving them is intended as a
> bottom-line test of Braidwood's entire data acquisition, calibration, and analysis
> chain. The requirements for constancy of detector parameters is minimal, however,
> because nearly all the parameters will be re-measured before and after any move.
> The most important part of the detector which must remain the same is therefore the
> calibration system itself, in particular its geometry and its positioning accuracy,
> as these can affect the calibration we will do at each position. As we hope to have
> a positioning accuracy of the calibration system to be better than 2~cm, we would
> not like this to change significantly more than that when we move. Even here,
> however, we will be able to check for changes in the system, for example by
> comparing the expected point at which a source just touches the inner vessel to the
> point when it actually does. We are not concerned with changes in the calibration
> sources themselves, as these can be moved from one detector to another at any
> time. We will also check our relative efficiencies to a precision of $\sim$0.5\%
> using the $\beta$-decays from $^{12}$B. The $^{12}$B decays will be produced via
> muon spallation throughout the volume of each detector, and at a nearly identical
> rate near and far because of Braidwood's uniform overburden. We outline below the
> calibration plans for the various configurations, and then some of the explicit
> detector parameters whose changes we have considered.
> \subsection{Two Modules Near Site (Initial Running)}
> We will use the initial configuration, in which two modules are both
> installed in the near site, to shakedown and commission the detectors as well
> as to understand the relationship between the detector parameters (attenuation and
> scattering lengths, PMT gains, electronics channel efficiencies, neutron capture
> efficiencies, etc.) and the overall detection efficiency. During this time, we will
> use both the embedded optical sources (LEDs on the outer sphere) as well as the
> calibration system itself to deploy $\gamma$, electron, neutron, and optical sources
> throughout the volumes (the same radioactive sources can be deployed in both
> detectors). The goal of these source deployments is to build a complete detector
> model which will allow us to predict the relative detector efficiencies to much
> better than 1\%, and to verify that our model also correctly predicts the dependence
> of the relative efficiency on any parameters which might change during the move to
> the far location. At the end of this initial period we will understand the sources
> of difference (if any) between the responses of the two detectors, and the
> bottom-line test of this will be in comparing the antineutrino fluxes measured by
> each detector. The measurements of the fluxes should agree within the systematic
> uncertainties we have derived from the joint calibration program.
> \subsection{Two Modules Near, Two Far}
> In the second configuration, in which one of the near detectors has been
> moved to the far location and an additional module added to both the near and far
> locations, we will begin by re-calibrating the moved detector and comparing its
> parameters to those we measured at the near location. These calibrations will
> include measurements of the optical parameters (embedded LEDs and diffuse
> deployed source), PMT gains and channel efficiencies (LED's and deployed
> source), neutron capture efficiency (AmBe source), the overall energy scale of
> the detector (AmBe and elecron source), and the volume of the scintillator
> (direct measurement). With these parameters we will then update our detector
> model and predict what the relative response and efficiency of the detector is
> after the move. We will test our prediction by moving one or more radioactive
> sources between the near and far detectors, as well as by comparing the number of
> $^{12}$B $\beta$-decays we measure in the new location relative to the initial
> location. One of the advantages of the Braidwood configuration is that the
> production rate of $^{12}$B is nearly identical at the two sites (near and far), and
> we can therefore make use the comparison of the rates as an explicit test of our new
> response prediction.
> During this stage, we will also perform the same comparison between
> the two modules at each location as we did in the initial configuration.
> Again, we will be measuring the detector parameters upon which our model
> depends, and comparing the prediction to radioactive source runs in each
> detector at various positions. The final comparison will be the measurements of the
> neutrino fluxes between each of the modules at the same location---they should agree
> to well within the systematic uncertainties derived from the calibration.
> \subsection{Swapping of Two Modules}
> As a final check, towards the end of the experiments we will exchange one of
> the near detectors with one from the far site. Once again, we will re-calibrate the
> detectors, and can perform a final comparison of our predicted response and the true
> response, by comparing the total rates of events between the detectors at each
> location.
> \subsection{Handling of Explicit Parameter Changes}
> We describe here how we will deal with the changes in detector parameters
> that can be affected by moving.
> \subsubsection{Volume}
> A change in detector volume will result in a change in the effective number
> of hydrogen targets, which in turn will appear to be a loss of efficiency. We will
> measure the volume at both locations using the known expansion coefficients,
> temperature, and height of the scintillator in the neck region, as well as with the
> $^{12}$B spallation products. Nevertheless, the analysis is simpler if the volume
> stays constant at a level small compared to 0.1\% ($\sim 75$kg).
> \subsubsection{Vessel Shape}
> Our simulations have shown that a change in the sphericity of the acrylic
> vessel holding the scintillator is a small effect even if the result is that the
> major and minor axes differ by as much as 20\%. We will explicitly check this,
> however, by deploying sources near the edges of the active volume and comparing the
> response there to what we expect based on our calibrations of the optical, PMT,
> and electronics responses made before and after the move.
> \subsection{Neutron Capture Efficiency}
> The neutron capture efficiency depends primarily on the Gd fraction within
> the scintillator. We will measure this before and after the move both through
> Americium-Beryllium (AmBe) source deployments and by the capture time profile of the
> neutrons produced by antineutrino interactions. These measurements are independent
> of any other changes in the detector parameters, and therefore we can tolerate large
> variations, although we do not anticipate any changes at all to this mixture during
> the move.
> \subsubsection{PMT Gains and Efficiencies}
> The most likely change during the move will be to some fraction of the PMTs
> and associated electronics. The number of tubes (if any) which fail during the move
> will be easily determined through the measurements by the embedded LED sources, and
> can be accounted for in the detector model to allow us to predict the response after
> the move. Changes in gains will likewise be measured with the single
> photoelectron spectrum created by the light sources. Changes in the tube-by-tube
> (and related electronics) efficiencies will be measured using a normalized light
> source deployed at the center of the detector, and checked by the depolyment of a
> radioactive source (probably AmBe). The local magnetic field differences between
> the two sites will also be known through explicit field measurements, and our
> knowledge of the PMT responses as a function of field strength and direction will
> have to be incorporated in the detector model. As a final comparison, the mean and
> width of the Gd capture peak from antineutrino reactions will be used to check for
> changes before and after the move.
> We are very insensitive to changes in these efficiencies and the consequent change
> in the energy response because our analysis thresholds are low compared to the
> Gd capture peak and the positron annihilation edge. Only catastrophically large
> changes---perhaps 50\% or more---leading to a substantial broadening of the energy
> response will make it difficult to accurately re-calibrate the energy response.
> \subsubsection{Optical Parameters}
> Extinction and scattering lengths at wavelengths spanning the scintillator
> response will be re-measured through the use of both the embedded sources and a
> diffuse optical source deployed inside the active volume. Our measurements and
> simulations currently show that we are very insensitive to these parameters, in part
> because the relevant lengths are large compared to the size of the detector itself,
> and in part because our energy thresholds are low. Again, we would not like to see a
> dramatic change in these lengths (less than a factor of two) because then the
> requirements on the precision of the new measurements will be far higher. The same
> is true of the reflection coefficients of the vessel and the PMTs.
> \subsubsection{Scintillation Light Output}
> A change in the light output or quenching of the scintillator must be
> re-measured after the move through a combination of radioactive source deployments
> and the mean and width of the Gd capture peak. Our energy response depends directly
> on the number of photons/MeV produced in the scintillator, and therefore small
> changes can have a big effect. The fact that the Gd peak integrates over positions
> and that the radioactive sources can be deployed at many locations within the vessel
> will allow us to break the covariance between these changes and those associated
> with optical parameters and PMT efficiencies.
> \end{document}
> \bye

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