S16_RadiationDamage

Abstract

We use a photomultiplier tube to measure light output from plastic scintillator exposed to a Cesium 137 radiation source. Measurements are made for scintillators previously damaged with 10, 1.7, and 0 MRads of radiation. We compare intensity spectra of the three scintillators to observe the effect of radiation damage. The spectra are approximated as exponentials, and least square fits of the logs of the spectra are performed in order to measure the exponential decay constants. These constants are compared in an attempt to quantify the reduction in intensity. The reduction in light output is found to be non-uniform across all intensities, such that a finding quantitative number for the intensity reduction using this apparatus is not meaningful. The apparatus is found to be sufficient for qualitatively comparing relative radiation damage.

Introduction

When plastic scintillator is exposed to ionizing radiation, the incident particles ionize the molecules in the base plastic. The base molecules then re-absorb electrons with energies above the ground state. The electrons de-excite, and their energy transfers radiatively or radiationlessly to compounds suspended in the plastic known as “fluors”. The fluors are used to shift the light to higher wavelengths, where it can pass through the scintillator with less absorption. Typical scintillators emit light with wavelengths of about 410-440 nm. The light can then be used to observe when a ionizing particle passes through the scintillator. Scintillator is commonly used in high energy physics, as the light is emitted within several nanoseconds of radiation passing through the plastic, and is useful for timing of events.

When plastic scintillator is exposed to high doses of ionizing radiation for long periods of time, some of the molecules in the plastic can break down, reducing the light output. This reduction in light intensity can greatly reduce the effectiveness of the detector. When the intensity of the output decreases, more sensitivity is required to detect the output light, and some events may decrease below the noise threshold of the measurement apparatus and become lost. This damage is caused by the breakdown of the base polymer molecules in the plastic. When exposed to large amounts of radiation, the hydrogen bonds of the base polymers break, releasing hydrogen which diffuses out of the scintillator. The remaining carbon rich molecules have absorption spectra in the range used by the scintillator, and create competing absorption processes which prevent some of the light from exiting the scintillator. This effect can also be seen as a yellowing of the initially clear plastic.

In our experiment, we seek to measure the reduction in light output by measuring the intensity of light emitted by irradiated scintillators and comparing their spectra. If successful, this method may be used to compare the rate of radiation damage to scintillators irradiated under different conditions.

Figure 1: A sample irradiated with 10 MRads (top) and 0 MRads (bottom)

Apparatus

The experimental apparatus consisted of the scintillators, a photomultiplier tube, and an oscilloscope (Figure 2). The Cesium 137 was placed on top of the scintillator at a distance of 1 cm from the photomultiplier tube. When beta and gamma rays created by radioactive decays in the source pass through the scintillator, they generate pulses of light which are converted into a current by the photomultiplier tube. With this setup, each decay event through the scintillator appears as a sharp pulse of voltage on the oscilloscope. During measurement, a threshold amplitude voltage was set, and the pulse areas of all events above the threshold were measured. Histograms were made of these areas to determine the distribution of event areas. The thresholds chosen were at the tail end of the event distribution, and thus rapidly decreased with increasing voltage.

Figure 2: The experimental apparatus. In addition to pulse areas, event rates were measured by replacing the oscilloscope with a discriminator and a counter.

Figure 3: A sample spectrum from the unirradiated scintillator using a threshold voltage of 80 mV. The soft peak is caused by the trigger cutoff, and is removed for analysis.

Results

While the relative radiation damage can be easily observed by comparing the pulse distributions for damaged and undamaged samples , quantifying this damage with a uniform reduction in light intensity for all input pulse areas is not possible. When comparing full spectra for different scintillators, we find that the lower pulse areas are reduced in intensity proportionally more than the high pulse area events (Figure 4).

Figure 4: The spectra the undamaged and 1.7 MRad sample. The separation observed for the lower pulse areas is significantly larger than for the high pulse areas. The relative damage can be observed as a reduction in the count rates

for the irradiated sample at all measured areas.

Conclusion

The apparatus studied in this experiment is sufficient for measuring relative radiation damage between two different scintillators, but is not sufficient to quantify the damage as a single reduction constant. The excess damage observed at low pulse areas may be due to the geometry of light passing through the scintillator, or due to a non-uniform conversion of light to output voltage. The is sufficient to measure relative damage, and will be able to be used to compare samples irradiated under differing conditions.