High Speed Continuous Self Calibrated Density Measuring System Based on a Gamma Source

Introduction

As a part of mass production processes of various materials, quality assurance becomes a challenge, as the product inspected rate rises (tens per second). Integrating a quality assurance system into production lines decreases the risk of providing a defective product. This work presents a density measuring system, which produces an analog signal proportional to the material under test (MUT) density. The developed system comprises a detector unit and a control unit (Fig. 1a). The detector unit includes a scintillator, a photomultiplier tube (PMT), custom electronics and two optical sensors, which are used to indicate that the MUT is in front of the detector (Fig. 1b). The detector unit generates a signal proportional to the MUT density. The signal height is sampled by a peak detector and held by a Sample and Hold circuit for further evaluation.

Method

At steady state, when a MUT is not located in front of the detector, the detector`s PMT is exposed to the direct gamma rays of a ¹³⁷Cs isotope. At this steady state the PMT generates a steady DC current which is compared (by subtraction) to a Gain-set current (see Fig. 2). The feedback loop reacts to a difference between currents by adjusting the high voltage of the PMT, which sets the PMT to produce the required steady state DC current. When a MUT passes in front of the detector unit, the optical sensors break the feedback loop. The working point is held constant by a large capacitor over the feedback amplifier. At the measuring phase, the MUT attenuates ¹³⁷Cs gamma ray intensity read by the detector as a function of density. Consequently, PMT current signal is reduced. The signal is shaped and amplified generating an analog signal proportional to the density of the MUT. Subsequently, in the Control unit, the analog signal is sampled by a Sample and Hold circuit developed as a low leakage, adjustable logic, enabling use of relatively slow sampling controllers.

Results and Conclusion

Electrical and radiological tests were executed to verify the system`s reliability. Fig. 3 presents typical waveforms obtained by injected signals from a pulse generator, simulating PMT signals.

As shown in Fig. 3a, the Analog Sig, which is proportional to the MUT density, has a wide amplitude range. Fine tuning of the detection unit gain for specific density range can provide high dynamic range and improve the resolution. Number of events at the detection unit, while a MUT was present, was analyzed, in order to evaluate measurement accuracy. For the examined configuration accuracy of 1% with a probability of more than 99% was obtained. Fig. 3b presents the sampled Analog Sig (Peak and Hold), which drops less than 0.01% from its peak voltage. The system is designed to avoid stressing working points of the components, with emphasis on component quality and durability. Moreover, such system design combined with system`s adjusting capability to various collimators and decreasing source activity enables reliable performance and long life utilization.