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PREx Lead target Hall A Design of Cherenkov Detectors Quartz blocks Tungsten Plates Construction Drawing for Stacked Detector Construction Drawing for Thin Quartz Holder Detector Parameters & Considerations: Optimization: -- cone length (mirror angle) -- quartz thickness

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Lead target

Hall A

Design of Cherenkov Detectors

Quartz blocks

Tungsten Plates

Construction Drawing for Stacked Detector

Construction Drawing for Thin Quartz Holder

Detector Parameters & Considerations:


-- cone length (mirror angle)

-- quartz thickness

-- quartz position in the cone

-- PM diameter (2” and 3”)

Design, Construction, and Performance of Cherenkov Detectors for PREx Experiment at Thomas Jefferson National Accelerator Facility (JLab) in Virginia

(sponsored by the National Science Foundation RUI Grant “Parity-violating Electron Scattering Experiments at JLab”)

Katie Su ’11 and Shorena Kalandarishvili ’11 with Piotr Decowski


The goal of the 208Pb Radius Experiment (PREx) is to measure the “neutron skin” in the 208Pb nucleus with high precision of ~1%. Due to the high ratio of neutrons to protons (126 neutrons versus 82 protons), the radius of neutron distribution in 208Pb is slightly larger than the radius of its proton distribution. This difference in radii is very sensitive to model parameters, especially to the “symmetry term” in nuclear potential; good knowledge of these parameters is required in the broad range of calculations, from those used in nuclear physics to those used in astrophysics (neutron stars). Neutrons, unlike protons, are electrically neutral, and therefore cannot be probed by electromagnetic interactions. On the other hand, neutrons respond to “weak interactions” much more than protons, as neutrons have a much larger “weak charge” than protons. In the forthcoming PREx experiment at JLab, the neutron distribution will be measured using weak interactions extracted from the parity-violating asymmetry in the scattering of longitudinally polarized electrons.

Two different prototypes of detectors for this experiment were designed and constructed at Smith College in collaboration with UMass. One of them consisted of a stack of quartz blocks interleaved with tungsten plates, while another of a single quartz block surrounded by a conical mirror. Both versions were tested in January 2008 using electron beam from the Continuous Electron Beam Accelerator Facility (CEBAF) at JLab.


Overview of the CEBAF electron accelerator at JLab Setup of PREx

Layout of PREx Experiment at JLab

Computer Simulations of Thin Detector Performance

Electrons scattered from lead target, pass through the quartz blocks surrounded by a conical mirror, and mounted in the detector placed in the focal plane of the high resolution magnetic spectrometer. While passing through the quartz, the electrons release photons (Cherenkov radiation), which propagate in the direction of a photomultiplier (PM).

Left panels in Fig.1 show the result of the computer simulations describing relationship between the number of photons reaching PM, and the length of the conical mirror for two diameters of the PM cathode (2 inch and 3 inch). The simulation was run using different cone lengths (which resulted in different numbers of photons), and the data is shown on the curved plots. Different plots illustrate changing distribution of photons caused by changing the distance d between the mouth of the cone and the quartz. Based on this data, the cone length that maximizes the number of photons passing through PM can be chosen.

These computer simulations show that a maximum number of photons reaches the PM when the cone has length of 6cm and the quartz is placed 2mm away from the mouth of the cone.





2 inch PM





6 cm

6 cm

Relative width of distributions

Number of photons reaching PM

Cone length (cm)

Cone length (cm)


3 inch PM

Side View


7 cm

7 cm

Right panels in Fig.1 show the relationship between the relative width of distribution of number of photons reaching PM and the cone length.

The relative width of distribution is calculated by dividing the normal distribution width by its mean. The goal of the project is to find the smallest relative width which provides the best precision. The graph indicates that the cone length of 6cm and distance of 2mm between the quartz and the mouth of the cone yields the best resolution of about 16.2%.

Figure 1: Results of Simulations of Detector Performance

Top View

Assembly of Thin Detector

Quartz Holder of Thin Detector Quartz Holder Fully Mounted in Detector

Fig.2 presents the distribution of magnitude of pulses from the PM measured during test run at JLab in January 2008. The resolution of ~27% represents the combined resolution of the detector and of the photomultiplier, and agrees with the simulation data.

Figure 2: Response of Thin Detector to 1 GeV Electrons.


Based on the test run performed at Jlab this past January, the detectors were designed appropriately. The detectors met the parameters required for the PREx experiment.