MainPage:Nuclear:Summer2015:ChristianPbWO4

Overview

At the Jefferson Lab (JLab) in Newport News, VA there is an electron accelerator which launches electrons into different halls. Each hall has its own specific targets for the electrons and detecting equipment for results of the collision. In Hall C a high momentum spectrometer and a variety of detectors are used. However, since there is no detector for high energy neutral particles, The JLab must create the neutral particle spectrometer (NPS). One of the major components of the NPS will be PbWO4, a crystal which scintillates, releasing photo-electrons, when hit by particles. These photo-electrons can then be picked up by a Photomultiplier Tube (PMT) which uses high voltage to allow for the detection of individual photons. It is imperative to the precision of the detector that the characteristics between the crystals remains uniform.

Lead Tungstate

Goals

1. Collect data on the light yield, transmittance, and refractive index of multiple crystals.

2. Check the data to confirm that the crystals meet the parameters needed for the NPS.

3. Compare the data between different crystals to analyze their uniformity.

Conventions

Due to the growth methods of the crystals, there are variances within a given crystal between position on the crystal and orientation of the crystal. In order to keep track of this, I developed several conventions. First, I added dots to the corners of the crystals to denote the orientation. The third orientation was not marked but was always the orientation through the 20cm length. To keep track of position, I always measured starting from the side OPPOSITE the dots moving toward them.

Refractive Index

The refractive index of a material is defined as the ratio of speed of light in a vacuum to the speed of light in the material. As the light changes speed within a material it also bends. Snell's law relates the angle of incidence, refractive indices, and angle of refractive between two materials.

An equation

Using Snell's law:

n1\sinΘ1 = n2\sinΘ2

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Some geometry.

and some geometry, I, with the help of Marco, was able to create an equation which solved for the refractive index (n) as a function of the width of the crystal(L), the displacement of the laser(Δx), and the angle of incidence (Θ). Since the crystal reflects light well, the angle of incidence can be calculated by measuring the angle of reflectance and dividing that by two.

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Measurements

To take measurements, I used calipers and image analyzation software. The calipers were used to measure the width of the crystal, while the analyzation software was used to measure the angle.

Uncertainty

One concern with the equation is that it is very sensitive to fractions of a millimeter. This makes diligence in measurements very important. Unfortunately, I did not discover the my best method until near the end of my work. Therefore, for most of the refractive index measurements, the uncertainty is fairly high, around ±.25. I was able to reduce this number closer to ±.15 for my final measurements.

These uncertainties were found by taking each of the measurements numerous times to discover the uncertainty in those measurements. For the displacement it turned out to be ± .3mm for the width it was ±.05mm and for the angle it was ± .3°. Then, using excel, the highest and lowest refractive indexes based on those uncertainties were calculated. The difference between these numbers and the calculated refractive index is the uncertainty.

Results & Conclusions

With my measurements, I calculated the refractive index of 5 crystals. There was about a XY% varience between them. Due to the high error, however, we cannot make any conclusions about any one crystal being inherently different. The calculated refractive indices of the crystals agree with literature, which gives confidence to the quality of the crystals.

Transmittance

The transmittance of a material is the the fraction of incident electromagnetic power that is transmitted through a sample. Knowing the transmittance of the crystals tells us: a) if the crystal is of the right characteristics for the detector. The detector requires a crystal which transmits at least 60% of light at a wavelength of 420nm. b) if there is uniform characteristics between the crystals. Further, using Fresnel's equations, it can be used to double check the refractive index.

Measurements

Measurements were taken from 250nm to 800nm using a PerkinElmer Lambda 750 UV/VIS/NIR spectrometer. For each crystal, a measurement was taken along the longest side from 1in-7in by 1in increments (2.54cm) with an uncertainty of ± 1/8in for each position. The same was done for the second orientation.

Light Yield

Light yield is a property of scintillators which is used to calculate which particle hit the scintillator. Light yield is defined as the number of photoelectrons emitted per amount of energy striking the scintillator. In a detector, one can work backwards from knowing the number of photoelectrons detected, to then knowing the energy of the particle which struck it, to then knowing what the particle is. In our specific case, a PMT is attached to the PWO crystal to measure the light yield when a photon emitted from a Na-22 radioactive source strikes the crystal. This can be used to check that the yield meets the parameters required for the Neutral Particle Spectrometer.

Set Up

It is known that the light yield of lead tungstate varies with temperature. In order to correct this, we modified a freezer that could house the Na-22 source, a Hamamatsu R4125 PMT, a collimator, a scintillator for triggering, and an ADC-based readout. Inside the freezer was also a heater, a stepper motor to control the position of the crystal, and a dehumidifier to stop ice from forming on the walls of the freezer. The heater was used to keep the temperature at ~50 degrees and to stop the freezer from displaying an on and off cycle (which saves the motor and would not have an adverse effect on food) at low temperatures.

The stepper motor was controlled remotely using a computer. Each 'step' equaled 3.175 micrometers. Therefore 3150 steps caused to motor to move the crystal 1 centimeter and 6300 for 2. However, due to a thoughtless math error, for some of the measurements 7300 steps were used which equates to 2.3 centimeters.

Keeping the temperature constant proved to be difficult. When left on overnight, sometimes the freezer would shut off or slow down and the temperature would rise into the 100 degrees fahrenheit. While here, keeping the temperature within ~1 degree was also difficult. The fight between the heater and freezer proved it could keep the temperature constant, but if ever knocked out of equilibrium, sometimes the temperature would rise, sometimes fall. The difficulty could arise from opening the freezer for a few seconds, or sometimes come up without notice.

VSL Presentation

File:PWO Presentation.pdf

The Cube

A Chinese company gave us a small 2x2x2cm sample cube of lead tungstate. I used this cube to become acquainted with measurement techniques for measuring the refractive index and transmittance. However, my results contrasted with literature (cite). I measured a much lower refractive index of 1.9 and higher transmittance. This caused confusion and I hypothesized that potentially the crystal was grown with a less perfected method (there was data from studies(cite) that showed older methods had a lower refractive index). Older crystals, contrastingly, displayed a lower transmittance. Therefore we sent the cube to be analyzed by an X-ray floriencience (XRF) machine to measure the chemical makeup of a material. The machine uses x-rays to displaces electrons causing a measurable burst of energy characteristic to each element.