Suitability of Superregenerative Nuclear Quadrupole Resonance Detector R. Cumby, N. Sullivan Abstract: Nuclear quadrupole resonance (NQR) results when quadrupolar nuclei are excited by an AC magnetic field at the NQR frequency, inducing transitions in quadrupole energy levels. The energy of the transition is proportional to the product of the nuclear quadrupole moment and the electric field gradient (EFG) at the site of the nucleus. Because EFGs are properties of their crystals, no external DC magnetic field is needed to detect resonance. A superregenerative detector was designed with hopes of being used in a laboratory setting for NQR. Unfortunately, the device proved too difficult to tune and too sensitive to be used in a general lab setting. I.
Introduction Nuclear quadrupole resonance (NQR) results when quadrupolar nuclei are excited by an AC magnetic field at the NQR frequency. As NQR is a material specific property, no external DC magnetic field is needed to detect NQR spectra. Because of this nuclear quadrupole resonance detection has a broad range of applications, from bomb detection to the discovery of oil wells.1 NQR detection has been performed with a number of different devices which irradiate a sample with an AC magnetic field at radio frequency (RF) and measure the response in some way.2 One major tool that may be suitable for this is a superregenerative receiver. Superregenerative receivers function by using an active component such as a transistor to simulate a negative impedance and thus amplify a signal across a tank circuit consisting of a capacitor and inductor in parallel.3,4,5 When the voltage across the tank reaches a certain cutoff, it must be “quenched” and allowed to return to zero in order to receive a signal.3,4,5 This can be accomplished by an external signal or by means of an internal signal such as a colpitt’s oscillator.3,6 A superregenerative device for NQR detection was developed earlier by Bruin and Khunaysir.6 However, they believed their device to be more useful as a classroom demonstration than as a laboratory tool.6 The purpose of the project was then to develop a supperregnerative NQR detector to be used in a lab environment, specifically to determine the resonances of Cl35 and Cl37 in DTN, an organic quantum magnet. The detector design developed by this lab proved to be too sensitive and difficult to tune to be of use for scientific purposes, and it has been determined that significant improvements must be made in the design and use of these receivers in order to get useful results from them in the future. II.
Methods The design used was inspired by the receiver design of Ranmuthu and Bruin and Khunaysir.6,7 The final design is shown in Fig 1. Fig 1: Circuit diagram of superregnerative detector The device was powered at 12V by a standard laboratory power supply. The sample was contained in a 3 turn coil of approximately 7/8 “ in diameter and 2” long. The length of the coil was adjusted to help tune the circuit. Quenching frequency was determined by R4 and C1. An MPF 102 Field Effect Transistor (FET) was selected as the active component of the circuit, although similar n-­‐channel FETs such as the J309 proved adequate. The variable capacitor across the collector and emitter was adjusted to both tune the circuit and cause the circuit to superregenerate. The circuit could also be tuned using C9, although too much adjustment of the capacitance frequently caused the circuit to stop displaying the superregenerative characteristics. The detection circuit consisted of a series of low pass filters connected to another FET, which acted as an amplifier. This was connected to the oscilloscope by means of s short length of BNC cable. The AC voltage from the transformer provided the AC magnetic field across the sample to induce transition in the quadrupole energy levels. In order to test a sample, the circuit was first tuned to the desired frequency. Tuning was achieved by adjusting R2, C1, the length of the sample coil, and/or the varicap across Q1. In order to test the frequency, a high-­‐accuracy RF generator was used. The RF frequency on the generator was moved until a response was viewed on the screen of the oscilloscope. The RF generator was then switched off and the sample placed in the coil. If a similar response on the screen of the oscilloscope was viewed, then the NQR frequency was the same as the frequency on the RF generator. Further confirmation that the observed effect was NQR could be observed by bringing a magnet near the sample. If the “dip” on the screen disappeared, then the frequency the receiver was tuned to was indeed the NQR frequency of the sample. III. Results and Discussion The detection circuit proves capable of picking up radio signals quite well. Fig 2 displays the oscilloscope screen for the de-­‐tuned circuit, while Fig 3 shows the results of tuning the circuit to its resonant frequency. When the correct frequency is selected, there is a noticeable drop off in the amount of noise in the signal. The introduction of AM modulation from the frequency generator shows very clearly. Fig 4 shows the introduction of I kHz wave to the signal, While Fig. 5 shows the introduction of a 400 Hz signal. In both cases, the modulation is visible and would clearly be audible were one to attach a speaker to the output. The detector functions as a very good radio receiver. Fig 2. Output of de-­‐tuned circuit on oscilloscope. Noise dominates the signal. The AC modulation is only slightly visible. Fig 3. Output of tuned circuit. AC modulation is now clearly visible and there is almost no noise in the signal. Fig 4. Output of tuned circuit with 1kHz modulation. Signal is now clearly modulates by the signal. Due to good clarity and relatively little noise reception is quite good. Fig 5. Output of tuned circuit with 400 Hz modulation. The AM modulation is clearly visible and has almost no noise obscuring the signal. The detector was unable to pick up NQR. The issue lies primarily in two areas. First, the lack of ability to tune the circuit accurately over a broad range of frequencies prevents one from adequately being able to dial in a very specific frequency as is needed in NQR detection. While some change in frequency is possible from adjusting either of the variable capacitors and the potentiometer, too much adjustment leads to the circuit no longer superregnerating. Second, the circuit is far too sensitive to external factors. The signal on the screen changes dramatically when a person moves towards the receiver and detects a host of other objects in the vicinity of the device. These signals are far greater than any signal that could be received from the sample, and completely overwhelm the desired signal. IV. Conclusion The device proved to be unsuitable for the purposes intended. The issues of tuning and sensitivity were too great for to be used to detect resonances that are too complicated to be calculated from first principles alone. In order to improve the design of the detector a better way of isolating the sample and the circuit must be made to accurately tune to a broader range of frequencies. It is possible that isolating the sample in some kind of box or shielding might be able to solve the sensitivity issue, although the issue of tuning might not be as easy to solve. However, the positive attributes of this device means that it may be a helpful tool in the future if its problems are addressed. Acknowledgement Lab and materials provided by University of Florida. This work was supported by the UF Material Physics REU Program through NSF grant DMR-­‐
1156737. Much help was given by Allen Majewski. References 1 A Garroway Nav. Rsch. Lab. (2003) 2 Taken from the lecture notes of N. Sullivan 3 E Insam, Electronics World (2002) 4 C Kitchin, AARL handbook (1998) 5 F. Xavier Moncunill-­‐Geniz, P. Pala-­‐Schonwalder, C. Dehollain, N. Joehl, and M. Declercq, IEEE Trans. Microw. Theory Techn. 55 6 (2007) 6 F. Bruin, and H. Khunaysir, Am. J Phys. 38 12 (1970) 7 N. Ranmuthu, qsl.net (1999)