A Low-Noise Preamplifier for Mössbauer Spectroscopy

Transcription

1 Ibersensor 010, 911 November 010, Lisbon, Portugal A LowNoise Preamplifier for Mössbauer Spectroscopy J. Alves (1), G. Evans (1), () and L. P. Ferreira () Department of Physics (1) Condensed Matter Physics Centre () Faculdade de Ciências da Universidade de Lisboa Edifício C8, Lisbon PORTUGAL gevans@fc.ul.pt Abstract We describe a lownoise preamplifier to be used with generic gas flow proportional detectors in a Mössbauer spectroscopy setup. This preamplifier is responsible for providing the detector high voltage bias, for converting the positive or negative charge pulse of the detector to a voltage signal, for signal amplification and pulse shaping and finally, for the matching between the high impedance detector output and the low impedance of the coaxial cables. To test the preamplifier in a real Mössbauer spectroscopy setup a printed circuit board prototype was made. A detailed circuit analysis and the most relevant simulation results, together with some experimental ones, will be presented. Keywords: lownoise preamplifier, compact preamplifier, gas flow proportional detectors, Mössbauer spectroscopy. Introduction This paper describes a lownoise preamplifier to be used with generic gas flow proportional detectors in a Mössbauer spectroscopy setup. This preamplifier is responsible for providing the detector high voltage bias (up to 3 K), for converting the positive or negative charge pulse of the detector (up to 5x10 9 Coulomb) to a voltage signal [1], for signal amplification and pulse shaping and finally, for matching the high impedance detector output to the low impedance of the coaxial cables. In Mössbauer spectroscopy the number of photoelectrons is small, and they should be detected individually [1]. This fact dictates that the pulses at the preamplifier output must be as short as possible. The easiest solution to deal with short pulses and to minimize the noise and maximize the linearamplification range is to use a combination of discrete bipolar transistors for the input stages of the preamplifier and a lownoise highfrequency operational amplifier for the output stage [,3,4]. Due to its low output impedance, this operational amplifier can be connected to a transmitting 50 Ω cable without the occurrence of an appreciable loss in the pulse amplitude. The main characteristics of the proposed preamplifier are: very low equivalent noise at the output, high count rate capability (1x10 5 Coulomb/s), fast rise time (between 10 to 60 ns), slow fall time (between 1 to 50 µs), an amplification factor between 3 and 350 (the output pulses are inverted) and matched termination for the transmitting 50 Ω cables. To test the preamplifier in a real Mössbauer spectroscopy setup a printed circuit board prototype was made. A detailed circuit analysis and the most relevant simulation results, together with some experimental ones, will be presented. Proposed Preamplifier The proposed preamplifier is depicted in Fig. 1. The general requirements for the implemented circuit include more than just the preamplifier function. If the proportional detector used in the Mössbauer spectroscopy setup does not have a high voltage bias, the input H should be used (but only up to 3 K). Due to that high voltage, ceramic capacitors should be used in this part of the circuit and in the preamplifier input line. The values available in the market for these capacitors are small, the maximum value used in our circuit was 4.7 nf. For this reason we need to used some of them in parallel (C1) to achieve the output signal fall time range without significant loss of preamplifier bandwidth. This range is obtained with the voltage divider consisting in the potentiometer R1 and the input resistance of the preamplifier. The preamplifier amplification factor takes this voltage division into account. This circuit can also be tested without the detector input. For that, an input line (TEST) is

2 Ibersensor 010, 911 November 010, Lisbon, Portugal R15 R16 50M,0.5W 50M,0.5W C10.n,3k DET R1 00 C1 18.8n,3k D1 H (up to 3K) R17 33M,0.5W C11.n,3k D R R4.k Q1 R3 6.8k C 10n D3 R5 00 C3 15p R6 1.k R7 3.3k C4 100n Q R8 56 R9 4.7k OP1 R10 1k OP C5 10n L1 10u C6 10u C7 10u C8 10n D5 L 10u R11 1k R1 10k D4 C1 10u C9 10n R13 33k C14 4.7u 1 AGND PB 1 C13 10u OUTPUT R14 51 R18 51 TEST Figure 1 The proposed preamplifier. added. The others notable features are the preamplifier voltage bias, PB, (±1 ) and the small size of the printed circuit board. The preamplifier itself consists of two cascaded stages. The input stage uses two highfrequency discrete bipolar transistors (N3904 and N3906) and serves as a current to voltage converter. This stage is also responsible for the low output noise and for part of the preamplifier gain (9.6 db). The diodes D1 and D at the preamplifier input prevent the input stage transistors from breakdown in case of high voltage arising at the input. The capacitor C increases the gain at high frequencies, and its value (15 pf) was selected experimentally. As mentioned before, the preamplifier should convert the positive or negative charge pulse of the detector to a voltage signal but the output pulses should have the opposite polarity of the input pulse. This is done at the preamplifier output stage. This output stage consists in an inverted voltage amplifier designed with some resistors and with a lownoise and fast operational amplifier (LT108 two circuits are available in the package). This voltage amplifier is responsible for a preamplifier factor gain between 1 and 11. To match the referred stages (and to reduce the size of the printed circuit board) the second operational amplifier available is used as a buffer. Finally, for a transmitting 50 Ω cable, the adequate termination was made at the output. The custom techniques to reduce noise are implemented in the biasing circuitry and in the printed circuit board prototype design. Circuit and Noise Analysis The noise of the preamplifier can be one limiting factor in Mössbauer spectroscopy, thus there is the need to characterize the noise performance of the proposed circuit. For noise analysis we need to use circuit values and so these two analyses will be made together. The circuit of Fig. was used to analyse the noise of the input stage of the preamplifier. The major source of noise in resistors is thermal noise. This source of noise could be modelled as voltage source, R(f), in series with the resistor or as a current source, I R(f), in parallel. Depending on the noise analysis, one of the following expressions could be used [6]: R ( f ) = 4 k T R (1) k T I R ( f ) = 4 () R where k is the Boltzmann constant and T is the temperature in Kelvin. R R R4 R4 I Q1 Q1 Q1 R6 R6 Q I Q Figure Circuit equivalent for input stage noise analysis. The simplified inputreferred spectral density noise of each transistor of Fig. can be represented by ([5] and [6]): ( f ) = ( f ) I ( f ) r ( f ) in _ Q Q Q b rb (3) the first and the second term of this equation are due, respectively, to collectorcurrent and basecurrent shot noise (the dominant noise sources) and the last term is due to the baseresistor thermal noise. If we expand these terms, eq. (3) is given by: k T q Ic r ( ) b ( f ) = in _ Q q Ic hfe (4) 4 k T rb where q is the charge of the electron. For uncorrelated noise sources, the total inputreferred noise spectral density is the sum of each input of referred noise voltage. In a multistage amplifier cascade the output noise spectral density is given by [7]: in _ in _ i on ( f ) = in _1... (5) A1 A1... Ai 1 R7 Q R8

3 Ibersensor 010, 911 November 010, Lisbon, Portugal where in_i and A i are respectively the inputreferred noise spectral density and the gain of the stage i. So in a multistage amplifier the noise spectral density at the output is mainly due to the first amplifier stage. This was the principal reason for using bipolar transistors in the input stage. If in_q1(f) and in_q(f) are, respectively, the inputreferred spectral density noise due to transistors Q1 and Q, the output noise spectral density of the preamplifier input stage is: ( ) ( ) f f in _ R in _ R4 ( f ) = on _ in _ stg ( f ) in _ Q1 (6) hie 1 ( ( f ) ( f )) Q in _ R6 in _ Q 1 6 hfeq R To analyse the noise of the output stage of the preamplifier the circuit shown on Fig. 3 was used. OP1 OP1 R10 I R10 I OP OP Req = R11R1 OP I Req Figure 3 Circuit equivalent for output stage noise analysis. The simplified noise spectral density at the output of each operational amplifier in Fig. 3 can be represented by ([5] and [6]): ( f ) = ( f ) I ( f ) R on _ OP in _ OP in _ OP (7) I ( f ) R in _ OP the second and third term of this equation are null if there are not resistors at the inverter and noninverter operational amplifier inputs. Using superposition, and assuming all noise sources are uncorrelated, the output noise spectral density of the preamplifier output stage is given by: ( f ) in _ Op1 Re q ( f ) in _ Op R10 ( ) = _ 10 ( ) I f on out stg f in R _Re ( ) Re I f q (8) in q I ( f ) in _ OP. hie 1 Q 1 6 hfeq R The integrated preamplifier output noise is: on = ( on _ in _ stg ( f ) on _ out _ stg ( f )) f (9) where f is the preamplifier noise bandwidth. The noise bandwidth is calculated for the brickwall response approach. The frequency limits are obtained by: f 3dB fl = π (10) The calculated preamplifier output noise values are presented in table 1. The parameter values needed to calculate those noise values are taken from the transistors and the operational amplifier datasheets. As expected, the output noise in mainly due to the preamplifier input stage contribution. This could be reduced for higher gain of the input stage. The noise of the output stage of the preamplifier could be reduced in applications where inverted polarity is not need. In these cases, a non inverter voltage configuration could be used, and the first operational amplifier is not necessary. Simulated and Experimental Results The proposed circuit was simulated using the Multsim simulation program. Several simulations indicated the optimal values for the capacitor C1 and for the potentiometer R1. The value of C1 affects the low frequency limit and the output signal fall time. The value of R1 also changes the preamplifier gain. A compromise between the minimum gain, the output signal fall time and the bandwidth are needed to define these values. The simulation results obtained for the extreme values of potentiometers R1 and R1 are resumed in table 1. Table I Preamplifier simulated performance. R1=00 Ω R1=0 Ω R1=00 Ω R1=10 kω R1=0 Ω R1=0 Ω R1=0 Ω R1=10 kω Gain [db] Fall Time [µs] Bandwidth 38.0 khz to 4.0 MHz 38.0 khz to 4.5 MHz khz to.5 MHz khz to 5.3 MHz on [µ] Some of the simulated results are presented in Figs. 4 and 5. Fig. 4 shows the working bandwidth for a preamplifier gain of 5.1 db. The high frequency limit and, the output noise are highly dependant of the output stage gain. The transient characteristics for an input square signal are shown in Fig. 5. In this figure

4 Ibersensor 010, 911 November 010, Lisbon, Portugal the preamplifier gain factor is 5.1 db also, and the output signal rise and fall time are, respectively, 0 ns and 30 µs. In all the simulations, the rise times are in the pretended range. The output signal fall time values are slower and sit in the range 4 to 30 µs. Figure 4 Simulated amplitudefrequency response. probe points (with matched terminations at both ends). Due to that, the measured output signal amplitude and noise level are reduced by a factor of two. Fig. 7 shows a more detailed photograph of one of the experimental results for a preamplifier gain of 3.6 db. The output signal fall time is 9.7 µs, which is quite near of the expected one (10 µs). The noise visible in the input signal shown in Fig. 7, and when the preamplifier supply voltage was switched off, shows that the oscilloscope output noise is of the same order of magnitude of the output preamplifier noise. These two sources of noise are not correlated, and so in the measured output signal the oscilloscope noise is added to the preamplifier noise. This could be minimized with suitable oscilloscope probes. Figure 5 Simulated transient characteristics for an input square signal with amplitude of 5 m and period of 60 µs. To test the proposed circuit a printed circuit board prototype was made (Fig. 6). The circuit was tested with a square input signal with several amplitude and frequency values, generated by a function generator (Agilent 330A, 0 MHz). Due to practical reasons, the amplitude of the experimental square signal at the input is larger than the simulated ones. Figure 6 Experimental setup and the preamplifier prototype. In the first tests, the TEST input line was used, but in the performance tests the input signal was applied to the DET input. The signals were measured with an oscilloscope (Tektronix TDS101, 100 MHz, 1 Gs/s) and with standard Figure 7 Experimental transient characteristic for an input square signal (amplitude of 40 m and frequency of 50 khz). The implemented prototype board does not support the detector high voltage bias, and so we could not test the circuit in a Mössbauer spectroscopy setup. To verify if the proposed circuit was able to detect the charge pulse of a gas flow proportional detector, we use as an input signal the output of a commercial preamplifier applied to a detector. Fig. 8 is a photograph of one of the results obtained. The gain of the proposed preamplifier was adjusted to 15 db. The large fall time signal is the input signal. This photograph shows that the output noise of the commercial preamplifier is of the same order of magnitude of the proposed one. Another two conclusions can be drawn from this experience: the proposed preamplifier is able to discriminate small pulses from the noise level, and its bandwidth is enough for Mössbauer spectroscopy.

5 Ibersensor 010, 911 November 010, Lisbon, Portugal Figure 8 Experimental transient characteristics, where the input signal to our preamplifier is the output of a commercial preamplifier coupled to a gas flow proportional detector. Conclusions A lownoise preamplifier with variable gain and output signal fall time, targeting generic gas flow proportional detectors was presented. The results obtained confirm the expected performance of the preamplifier and its suitability to the goal. After a redesign, which will include the use of lower noise and higher bandwidth transistors and operational amplifiers, and a better board type, the preamplifier will be tested in a real Mössbauer spectroscopy setup. Due to the small number of components, the prototype has dimensions of just 13.5 x 7.5 cm. These dimensions could be smaller if a two layer printed circuit board is used. Lower output noise should be obtained with a two layer printed circuit board and with an adequate ground metal layer. References [1] Sobir M. Irkaev, Trends in Mössbauer Spectrometer Designs, Mössbauer Effect Reference and Data Journal, olume 8, Number 10, December 005. [] D. Yu. Akimov, Yu. K. Akimov, A. A. Bogdzel, A. G. Kovalenko and D.. Matveev, A LowNoise Fast Eight Channel Preamplifier, Instruments and Experimental Techniques, ol. 45, No., pp , 00. [3] P. Horowitz and W. Hill, The Art of Electronics, nd ed., Cambridge University Press, [4] Proportional Counter Preamplifier PEA6, Wissel Manual. [5] P. R. Gray, P. J. Hurst, S. H. Lewis e R. G. Meyer, Analysis and design of analog integrated circuits, John Wiley & Sons, Inc., 4ª Edition, 001. [6] D. A. Johns e K. Martin, Analog integrated circuit design, John Wiley & Sons, Inc., [7] F. N. H. Robinson, Noise in electrical circuits, Oxford Library of the Physical Sciences, Oxford University Press, 196.