Microwave measurement of the mass of frozen hydrogen pellets

A nondestructive method for measuring the mass of high-speed, frozen hydrogen pellets is described. The measurement technique is an improvement on previous methods based on the perturbation to a resonant cavity caused by a dielectric pellet passing through the cavity. In the new method, an oscillator circuit is formed with a resonant cavity in the positive feedback loop of a microwave power amplifier. An injected pellet perturbs the resonance characteristics of the cavity causing a shift in the operating frequency of the oscillator proportional to the ratio of the pellet volume to the volume of the cavity. Through digital measurement of the frequency shift the size of the pellet is determined automatically. In a ‘‘proof-of-principle’’ experiment it was shown that polyethylene pellets, with an average volume of 0.26 mm3 and velocity ≊500 m/s, caused a shift in the operating frequency of the oscillator circuit that was within 10% of the predicted value. The hydrogen pellets (of interest to us) will have 4–10× the volume of the plastic pellets. Thus with a dielectric constant of about 1.26 for frozen hydrogen, the signals in the actual experiment would be 1–2× those described here.