Online Plots

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PREX Main

See Also: Alarm Documentation

Measuring, Controlling and Removing Helicity Correlated Noise in our Main Detectors

We assume that our main detectors are simultaneously measuring the helicity correlated motion and fluctuations of the electron beam from CEBAF, as well as physically interesting Parity Violating Asymmetry (Apv) of highly longitudinally-polarized electrons on an isotopically pure Lead 208 (Calcium 48) target of appreciable, but intentionally small scattering length.

The way that we can disentangle the beam position, angle, current and energy fluctuations due to helicity correlated beam (i.e. generated in the polarized source non-uniformly between helicity states and then damped or amplified farther along the beamline) are myriad.

Live and Online Analysis Result Plots

Critical Systems

List of Critical Systems and Descriptions

Beam Current Monitor

BCMs measure the current entering the hall. They should be reading less than 90 uA at all times. This is important to not destroy our target or generate excess ion chamber trips.

Hall A Arc, Energy Degree of Freedom-Sensitive BPM 12

This BPM is located in the BSY and measures beam position. It's also most sensitive to beam energy fluctuations. If the wires in the BPM saturate then its indicative of unacceptably high energy fluctuations. Therefore the readout must be kept below 50,000 at all times.

Main Detectors

List of kinds of plots we care to make from main detectors

Yield (signal in Volts/Microamps of Beam Current)

This measures our detector response as a function of beam current, with higher current generating stronger signal. Signals should be roughly the same for both left and right HRS's.

Small Angle Monitors (SAMs)

What are SAMs?

The SAMs consist of eight small pieces of scintillating quartz placed in the beamline roughly seven meters downstream of the target. Placed into the beamline, circularly at 45 degree intervals, these act as luminosity monitors, measuring both flux and scattering cross-section of the beam after hitting the lead target. PREX requires high luminosity to achieve the statistic necessary to observe the ppm-level asymmetries required.

Lists of kinds of plots

Detectors Yields (not-normalized to beam current)

SAM signal vs event number. Will read higher values at higher beam current. If signal is zero, check first whether or not SAM HV is off.

SAM Asymmetries

SAM asymmetries are plotted first by themselves, and then correlated with BCM asymmetry.

SAM beam profile/halo sensitivities around the circle/combining 8 Degrees of Freedom

SAM signals and asymmetries are plotted in a circle with the center plot indicating which SAM number corresponds to which SAM position as seen looking downstream along the beam line. Positional differences in the SAM readout may correspond to excess beam halo or give information about the beam profile. They may also act as a diagnostic for the detector expert crew if SAMs are behaving incorrectly.

Beam Position Monitors (BPMs)

What is a BPM and how does it work?

As the name suggests: a Beam Position Monitor monitors the beam position. A BPM consists of four wires arranged along the beam line

BPMs as multi-degree of freedom Beam-Position-Correlated noise regression monitors

Continuation of Measuring, Controlling and Removing Helicity Correlated Noise in our Main Detector Measurements

Using beam position monitors, to keep track of the momentum/energy fluctuations in the magnetically energy selecting Hall A arc, as well as the 4 degrees of X and Y position and angle onto the target information, allows us to measure helicity correlated beam fluctuations to fairly high precision and then calculate correlation slopes over various time scales and modes of beam motion (normal beam motion and intentionally large modulation induced beam motion) which are then subtracted, per event, from our main detectors to arrive at the remaining physics asymmetry.