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Sonoluminescence and Blackbody Radiation

Sonoluminescence is the process by which a gas bubble trapped at the antinode of an ultrasonic standing wave emits visible radiation. This strange phenomenon will be the platform on which 3-dimensional standing waves and black-body radiation will be investigated.

Introduction

Single bubble sonoluminescence, hereafter abbreviated as SL, was discovered in the late 1980's and has received a great deal of attention. This remarkable process involves the trapping of gas bubble at the antinode of a ultrasonic standing wave in a liquid.

3D Standing Waves

A plane wave in the z</hat> direction incident on a flat wall will reflect into the -z</hat> direction, and interfere with the incoming wave based on the superposition principle. at a particular distance from the wall,

In order for the trapping of gas bubbles, a 3-dimensional ultrasonic standing wave need to be attained. The resonant frequency of a the standing wave will occur if each of the 3 dimensions has a length which is equal to an integer number of half wavelengths of the acoustic wave. One will notice that the shape of the liquid cell is rectangular, having the length and width being equal. The height of the cell, and hence a value of great importance for this experiment, is set by the volume of water added to the cell.

The Ultrasonic Horn

The ultrasonic horn is used to deliver acoustic power to the volume of water. Internally, the horn contains a series of annular shaped disc transducers which are bolted into its base. The basic structure and shape of the horn is designed to efficiently couple the pressure waves generated from the transducers to the narrow stem of the horn. All of the transducers are the same. They consist of a ceramic material which has been prepared in such a manner as to have a permanent polarization. In other words,there are specialized capacitors. As a charged is placed across this capacitor there is a force generated across the ends, and the capacitor wants to separate. Since the transducers are compressed, this repulsive force does not physically expand the disc but does produce dynamic pressure. As the charge across the transducer is reversed, there is now an attractive force which results in a negative pressure amplitude. The capacitance of the particular tranducers used is 11 nF. In order to efficiently couple electrical power into the transducers, it is advantageous to connect an inductor in series to achieve electrical resonance as given by the formula:

The Cell

The cell is a plastic container onto which is epoxied a small ceramic transducer that serves as a microphone. Since this transducer is not compressed small fluctuations in its diameter produce a measurable signal. BY attaching this transducer to the bottom of the cell, one can easily detect when the pressure is the cell in in resonance by looking for large amplitude sine waves on a oscilloscope at the ultrasonic frequency. The cell is provided with a black shroud which serves which has a circular opening on one side to allow for light from the SL to enter the photomultiplier tube.

The Photomultiplier Tube

A photomultiplier tube (PMT) is a device which coverts one incident photon into a large pulse of electrons which can then be read out on an oscilloscope. The photon passes through the glass enclosure of the PMT an liberates an electron from a coated surface, this electron is then accelerated by static electric fields into dynode 1, and this initial electron free many more electrons. This bunch of electron then accelerates into dynode 2 freeing a larger bunch of electrons. This process continues many times such that even one incident photon can produce a measurable number of electrons at the output. A PMT is a very sensitive optical detector. Due to the high sensitivity, having too many photons entering the PMT can damage the device, and this should be avoided. Even with the black shroud covering the cell, you should not apply power to the PMT without also shutting of the room lights.

The Fluke 412B power supply will provide the high voltage required to bias the dynodes of the PMT. Typically, the PMT should be run at XXXX V. The PMT will output a negative going pulse with a sharp rise time and a relatively slow fall time. The amplitude of this pulse is related to the number of photons hitting the PMT during a particular pulse. In this experiment, the effect of Sonoluminescence only occurs once during an acoustic wave period, hence, the SL bubbles is flashing at around 27 kHz. The PMT will output pulses synchronized with the acoustic frequency whose amplitude is related to the brightness of the sonoluminescence.

The particular ultrasonic horn used has components which make it tunes for a frequency from 25kHz to 27kHz. Outside of those ranges, the acoustic energy available in the horn is

Procedure

Required Components

Flask
1. De-ionized Water
2. Vacuum Pump
3. HP33258 Function Generator
4. TDS2014B Digital Oscilloscope
5. SL100B Sonoluminescence Control Box
6. Ultrasonic Horn
7. 1P28 PhotoMultiplier Tube
8. Various Optical Lenses
9. HV Power Supply

Degas the water

In order for the water to support a sonoluminescence bubble it is necessary for the water to be partially degassed. An effective way to do this is to starting will deionized water in a flask, place a rubber stopper in the top of the flask with a tube passing though it, then attach a roughing pump to end of the tube. The boiling point of water is reduced to below room temperature if the pressure in the flask can be reduced below about 25 torr. The vacuum pump you are using can easily achieve that. You will notice rapid bubbling of the water inside the flask after the pump is turned on. Allow to pump to work for 30 minutes, gently shaking the flask while pumping will help ensure more thorough degassing. In addition, during the degassing procedure, have the flask sitting in a bath of ice water, as the sonoluminescence effect is greater when the liquid is at lower temperatures.

Figure XX - Degassing the water.

Once the degassing procedure is complete, turn off the vacuum pump, and slowly open the pressure relief wave. Then, gently pour the water into the cell being careful not to introduce more bubbles. Fill the cell to the correct amount to create standing waves of the desired order.

Electrical Connections

The basic setup for the experiment is shown below. The acoustic frequency is provided by a 1 Vpp sine wave from the HP3325A synthesizer/function generator. The appropriate resonance frequency, as determined by cell geometry is around 27kHz. This sine wave is then amplifier by the Sonolumiscence control box, and fed into the ultrasonic horn. To electronically view the effects of sonoluminescence, the output of cell transducer is put to one input of the oscilloscope (you could trigger off of this), and the second input should be the High Freq. Output. This High Freq. Ouput filters the cell transducer signal with 150 kHz high-pass filter since the effect of a trapped bubble is to cause a high frequency response on the cell transducer. There are two connections made from the Sonoluminescence SL100B to the cell box- a multi-pin power/control cable, and cell tranducer input using coaxial connector.

When using the Photomulitplier

Observe trapped bubbles and Sonoluminescence

Once the water is added, place the black shroud around the cell. Lower the ultrasonic horn into the cell such that the tip of the horn is 5mm - 10mm below the surface of the water.

Detect the Sonoluminescence using a PMT

With the HV power supply still switched off, adjust the height of the PMT and bring it close to the opening in the black shroud. Once you have a trapped, sonoluminescence bubble, close the front viewing flap. Turn on a small desk lamp, and shut off the main room light. Turn on the HV power supply to about 800V and look on the oscilloscope for pulses synchronous with the acoustic radiation. Sketch the result. Determine the rise time and decay time of the PMT pulses. Characterize the peak height.

Determine the Temperature of the SL

Using the varoius