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Fast Josephson cryodetector for time of flight mass spectrometry
Physica C 372–376 (2002) 423–426 www.elsevier.com/locate/physc
Fast Josephson cryodetector for time of flight mass spectrometry E. Esposito a
a,b,* ,
R. Cristiano a,b, S. Pagano D. Twerenbold c
a,b
, D. Perez de Lara
a,b
,
Istituto di Cibernetica ‘‘E.R. Caianiello’’ del CNR, Bldg. 70, via Campi Flegrei 34, I-80078 Pozzuoli (Napoli), Italy b INFN Sezione di Napoli, Ed.G, Complesso Univ M.S. Angelo, via Cintia, 80100 Naples, Italy c Institut de Phyique de l’Universit e Neuchatel, Rue A.L. Breguet 1, CH-2000 Neuchatel, Switzerland
Abstract A fast current discriminator, based on Josephson junctions, has been developed to boost the performance of novel type of superconductive detectors in time-of-flight mass spectrometers. The Josephson discriminator could greatly improve the mass resolution in such machines, and it is technologically compatible with the proposed superconductive tunnel junction detector. The effect of intrinsic thermally induced fluctuations in such a device has been investigated to identify the possible operating ranges, in terms of temperature, geometry and materials concerned. Experimental results on the feasibility of the proposed device are also reported. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Josephson; Particle detector; MALDI; Mass spectrometry
1. Introduction A new superconductive electronic readout for improving mass resolution in bio-molecule time of flight mass spectrometry (TOF-MS) [1] is discussed. This readout, in addition of providing energy information, uses a Josephson junction as a fast current discriminator for the accurate determination of the bio-molecule arrival time.
*
Corresponding author. Address: Istituto di Cibernetica ‘‘E.R. Caianiello’’ del CNR, Bldg. 70, via Campi Flegrei 34, I80078 Pozzuoli (Napoli), Italy. Tel.: +39-081-8675054; fax: +39-081-8042604/2519. E-mail address: [email protected] (E. Esposito).
In a TOF-MS the mass resolution DM is directly related to the time of flight resolution Dttof : DM Dttof ¼2 : M ttof
ð1Þ
Conventional TOF-MSs use micro-channel plates (MCPs) to measure the arrival times of molecular ions. MCPs are fast and easy to operate, but suffer from a considerable decrease in sensitivity for ion masses above a few tens of kDa (1 Da ¼ 1 a.m.u.) [2–4]. Recently [5–8] cryogenic detectors, i.e. superconducting tunnel junctions (STJ) and transition-edge micro-calorimeters, have been successfully employed to detect the ion impact. Superconductive detectors, being sensitive to the total energy deposited and not to the ion speed, do
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 0 7 1 4 - 1
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E. Esposito et al. / Physica C 372–376 (2002) 423–426
not suffer from the limitations of the MCPs. The main drawback of cryogenic detectors is their relatively slow speed compared to MCPs. The risetime scale of an STJ detector signal is related to the non-equilibrium phenomena and depends on the material-dependent quasiparticle-phonon scattering time. In the standard STJ detection scheme a charge amplifier is normally used. This choice, however, limits the time resolution achievable to about 100 ns [6], corresponding to a mass determination error of about 10 Da for a 10 kDa macromolecule. Recent experiments [9] indicate that the rise-time is made of different components, with values ranging from 20 to 100 ns, although the lower measured value was limited by the bandwidth of the fast current amplifier. The electronic readout scheme proposed here uses a Josephson tunnel junction (JTJ) as a fast current switching device to directly discriminate the current pulse generated by a STJ detector. The goal is to better time resolve the molecule impact in order to approach a mass resolution of 1 Da for 100 kDa molecules.
2. Device operation and noise analysis The proposed detector scheme consists of a conventional STJ coupled to a Josephson junction. The STJ operates in the Giaever mode [10] and generates the primary signal after a molecule impact. The STJ sends its signal both to a charge amplifier, to measure the total generated charge, and to a JTJ. The JTJ operates in the Josephson mode and discriminates the fast signal generated in the STJ, to accurately measure the arrival time. The intrinsic switching time of a Josephson junction, being ultimately related to quanto-mechanical aspects, can be fast as few ps. However, the JTJ must switch only when a signal is generated by the STJ. External or intrinsic noise can mimic the arrival of a molecule signal. To avoid this possibility, the JTJ has to be properly shielded and biased. Even in absence of external noise, a Josephson junction biased in the zero voltage state has a finite probability of switching out of this state due to intrinsic thermal fluctuations [11], given by the following expression:
P ¼ 1 expðtw =sÞ
ð2Þ
where the lifetime s is: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2p/0 C=Ic /0 Ic 2 1 ð I=Ic Þ s ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi exp 4 pKB T 2 1 ð I=Ic Þ ð I=Ic Þ cos1 ð I=Ic Þ :
ð3Þ
Here U0 is the flux quantum, KB the Boltzmann constant, I the bias current, Ic the Josephson critical current, C the junction capacitance, and tw is the waiting time. To operate the proposed device, the JTJ has to be biased in the zero voltage state at a current value lower than the junction critical one. The bias current must be close to Ic to be sure that the current pulse generated by the STJ (Isign 1 lA) is sufficient to induce a switching of the junction. To determine the possible bias current values for given P, tw , Ic , and C Eq. (2) can be solved for the activation temperature as: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 1 ðI=Ic Þ ðI=Ic Þ cos1 ðI=Ic Þ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi T ¼a ; 4 2 ln b 1 ðI=Ic Þ
ð4Þ
where a¼
/0 Ic ; pKB
b¼
tw 1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi : ln ð1 P Þ 2p/0 C=Ic
ð5Þ
As an example of the results obtained, in Fig. 1 the dependence of the bias current on the junction temperature is shown for a Josephson junction with C ¼ 7 pF and Ic ¼ 5 lA, and thermal switching probabilities of 99% and 1% (curves I99% and I1% respectively). The curve I1% is computed for tw ¼ 1 ms, a reasonable value in TOF experiments, and the curve I99% is computed for tw ¼ 100 ns, a reasonable value for the signal pulse width. The curve I99% in Fig. 1 represents the value of the bias current for which there is a 99% probability that the junctions switches during the signal pulse width. In order to operate the JTJ as a discriminator the bias current level must be lower than I99% of an amount not larger than the STJ pulse amplitude Isign . However, if such bias value
E. Esposito et al. / Physica C 372–376 (2002) 423–426
Fig. 1. Temperature dependence of the switching threshold bias current, curves I1% , I99% and left axis, and of the bias current fluctuation region, curve DI ¼ I99% I1% and right axis. The first three curves correspond to switching probabilities P ¼ 1% and 99%. The junction parameters are C ¼ 7 pF and Ic ¼ 5 lA, and the waiting time ttof is 1 ms.
becomes larger than I1% , there will be a finite probability for unwanted switching. Therefore the following expression must hold: I99% Isign < I < I1% :
425
and an amplitude that can be chosen in order to satisfy the relation (6). The experiment has been performed using a 100 lm2 Nb–AlOx –Nb junction at a temperature of 300 mK, and in presence of an external dc magnetic field to reduce the junction critical current to about 5 lA. The experimental set-up allows the detection of the JTJ switch. By sending a sequence of n (typically 5000) waveforms to the JTJ and recording the number of switches occurred, it is possible to compute the noise-induced switching frequency (due to intrinsic and external sources). This procedure has been repeated for different bias amplitudes obtaining the circles shown in Fig. 2. In the same figure is shown, as full curve, the switching probability induced by the intrinsic noise only, as from Eq. (2). It is worth noting that the experimental distribution is much wider due to the presence of external noise. To simulate the STJ signal a fast (1 ls wide) current pulse has been added to the previous waveform at t ¼ 0:5 ms. By repeating the above procedure we have obtained the squares and
ð6Þ
The dependence of DI I99% I1 % on the JTJ temperature is also shown in Fig. 1. These results indicate that, for a typical small size Josephson junction and a 1 lA current pulse, once the external noise is properly reduced, there are wide operational margins for the JTJ discriminator in terms of bias current, temperature and JTJ properties.
3. Experimental results In order to perform tests of the proposed detection scheme, the response of a JTJ to a bias waveform simulating the TOF-MS operation and the effect of the STJ signal has been experimentally investigated. This has been achieved by implementing a trapezoidal current bias waveform. The waveform is characterised by a rise-time of 100 ls, a fall time of 100 ls, a duration of 1 ms (corresponding to the waiting time tw ), a period of 2 ms,
Fig. 2. Experimental bias current dependence of the switching probability. The circles are values in absence of signal pulse (only external and intrinsic noise), the squares and triangles correspond to superimposed signal pulses of 1 and 2 lA respectively. The theoretical dependence, for intrinsic noise only, is shown as full curve. The arrows show the operational bias range for which the detector can discriminate 2 lA pulses with 100% efficiency.
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E. Esposito et al. / Physica C 372–376 (2002) 423–426
triangles shown in Fig. 2, corresponding to a pulse amplitude of 1 and 2 lA respectively. Even in the presence of the relatively large noise of the experimental set-up, there is a reasonable operational bias range, shown by the arrows in Fig. 2, for which the detector can discriminate 2 lA pulses with 100% efficiency. In comparison with the simulation results a smaller discrimination sensitivity has been measured. However, by reducing the external noise, it should be possible to fully recover such discrepancy. Moreover, the reported calculations indicate that the operating temperature of the JTJ discriminator can be substantially increased, while still keeping good signal sensitivity. This relaxes the cryogenic requirements for this type of detector.
4. Concluding remarks A new detection scheme for achieving simultaneous time and energy detection of macromolecules impacts has been proposed. The time resolution achievable is ultimately limited by the STJ intrinsic non-equilibrium characteristics, rather than the JTJ discriminator, and should be of the order of few ns. The noise analysis indicates
that the JTJ discriminator can operate at a temperature somewhat higher than that of the STJ detector, giving more flexibility in the cryogenic set-up, and opening the possibilities to use more complex (digital) Josephson circuitry. Preliminary experimental tests show that the discrimination principle works, although it is necessary to improve the noise shielding performances. More accurate measurements are in progress.
References [1] M.W. Senko, F.W. McLafferty, Annu. Rev. Biophys. Biomol. Struc. 23 (1994) 763. [2] R.J. Beuhler, L. Friedman, Nucl. Instr. Meth. 170 (1980) 309. [3] G. Westmacott et al., Nucl. Instr. Meth. B 108 (1996) 282. [4] A. Brunelle et al., Rapid. Commun. Mass Spectrom. 11 (1997) 353. [5] D. Twerenbold, Nucl. Instr. Meth. A 370 (1996) 253–255. [6] D. Twerenbold et al., Appl. Phys. Lett. 68 (1996) 3503. [7] D. Twerenbold, US patent no. 5460010, 1997. [8] G.C. Hilton et al., Nature 391 (1998) 672. [9] M. Kishimoto et al., Nucl. Instr. Meth. A 444 (2000) 124– 128. [10] D. Twerenbold, Nucl. Instr. Meth. A 273 (1988) 575. [11] A. Barone, G. Patern o, Physics and Applications of Josephson Effect, John Wiley, NY, 1980.
Fast Josephson cryodetector for time of flight mass spectrometry E. Esposito a
a,b,* ,
R. Cristiano a,b, S. Pagano D. Twerenbold c
a,b
, D. Perez de Lara
a,b
,
Istituto di Cibernetica ‘‘E.R. Caianiello’’ del CNR, Bldg. 70, via Campi Flegrei 34, I-80078 Pozzuoli (Napoli), Italy b INFN Sezione di Napoli, Ed.G, Complesso Univ M.S. Angelo, via Cintia, 80100 Naples, Italy c Institut de Phyique de l’Universit e Neuchatel, Rue A.L. Breguet 1, CH-2000 Neuchatel, Switzerland
Abstract A fast current discriminator, based on Josephson junctions, has been developed to boost the performance of novel type of superconductive detectors in time-of-flight mass spectrometers. The Josephson discriminator could greatly improve the mass resolution in such machines, and it is technologically compatible with the proposed superconductive tunnel junction detector. The effect of intrinsic thermally induced fluctuations in such a device has been investigated to identify the possible operating ranges, in terms of temperature, geometry and materials concerned. Experimental results on the feasibility of the proposed device are also reported. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Josephson; Particle detector; MALDI; Mass spectrometry
1. Introduction A new superconductive electronic readout for improving mass resolution in bio-molecule time of flight mass spectrometry (TOF-MS) [1] is discussed. This readout, in addition of providing energy information, uses a Josephson junction as a fast current discriminator for the accurate determination of the bio-molecule arrival time.
*
Corresponding author. Address: Istituto di Cibernetica ‘‘E.R. Caianiello’’ del CNR, Bldg. 70, via Campi Flegrei 34, I80078 Pozzuoli (Napoli), Italy. Tel.: +39-081-8675054; fax: +39-081-8042604/2519. E-mail address: [email protected] (E. Esposito).
In a TOF-MS the mass resolution DM is directly related to the time of flight resolution Dttof : DM Dttof ¼2 : M ttof
ð1Þ
Conventional TOF-MSs use micro-channel plates (MCPs) to measure the arrival times of molecular ions. MCPs are fast and easy to operate, but suffer from a considerable decrease in sensitivity for ion masses above a few tens of kDa (1 Da ¼ 1 a.m.u.) [2–4]. Recently [5–8] cryogenic detectors, i.e. superconducting tunnel junctions (STJ) and transition-edge micro-calorimeters, have been successfully employed to detect the ion impact. Superconductive detectors, being sensitive to the total energy deposited and not to the ion speed, do
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 0 7 1 4 - 1
424
E. Esposito et al. / Physica C 372–376 (2002) 423–426
not suffer from the limitations of the MCPs. The main drawback of cryogenic detectors is their relatively slow speed compared to MCPs. The risetime scale of an STJ detector signal is related to the non-equilibrium phenomena and depends on the material-dependent quasiparticle-phonon scattering time. In the standard STJ detection scheme a charge amplifier is normally used. This choice, however, limits the time resolution achievable to about 100 ns [6], corresponding to a mass determination error of about 10 Da for a 10 kDa macromolecule. Recent experiments [9] indicate that the rise-time is made of different components, with values ranging from 20 to 100 ns, although the lower measured value was limited by the bandwidth of the fast current amplifier. The electronic readout scheme proposed here uses a Josephson tunnel junction (JTJ) as a fast current switching device to directly discriminate the current pulse generated by a STJ detector. The goal is to better time resolve the molecule impact in order to approach a mass resolution of 1 Da for 100 kDa molecules.
2. Device operation and noise analysis The proposed detector scheme consists of a conventional STJ coupled to a Josephson junction. The STJ operates in the Giaever mode [10] and generates the primary signal after a molecule impact. The STJ sends its signal both to a charge amplifier, to measure the total generated charge, and to a JTJ. The JTJ operates in the Josephson mode and discriminates the fast signal generated in the STJ, to accurately measure the arrival time. The intrinsic switching time of a Josephson junction, being ultimately related to quanto-mechanical aspects, can be fast as few ps. However, the JTJ must switch only when a signal is generated by the STJ. External or intrinsic noise can mimic the arrival of a molecule signal. To avoid this possibility, the JTJ has to be properly shielded and biased. Even in absence of external noise, a Josephson junction biased in the zero voltage state has a finite probability of switching out of this state due to intrinsic thermal fluctuations [11], given by the following expression:
P ¼ 1 expðtw =sÞ
ð2Þ
where the lifetime s is: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2p/0 C=Ic /0 Ic 2 1 ð I=Ic Þ s ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi exp 4 pKB T 2 1 ð I=Ic Þ ð I=Ic Þ cos1 ð I=Ic Þ :
ð3Þ
Here U0 is the flux quantum, KB the Boltzmann constant, I the bias current, Ic the Josephson critical current, C the junction capacitance, and tw is the waiting time. To operate the proposed device, the JTJ has to be biased in the zero voltage state at a current value lower than the junction critical one. The bias current must be close to Ic to be sure that the current pulse generated by the STJ (Isign 1 lA) is sufficient to induce a switching of the junction. To determine the possible bias current values for given P, tw , Ic , and C Eq. (2) can be solved for the activation temperature as: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 1 ðI=Ic Þ ðI=Ic Þ cos1 ðI=Ic Þ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi T ¼a ; 4 2 ln b 1 ðI=Ic Þ
ð4Þ
where a¼
/0 Ic ; pKB
b¼
tw 1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi : ln ð1 P Þ 2p/0 C=Ic
ð5Þ
As an example of the results obtained, in Fig. 1 the dependence of the bias current on the junction temperature is shown for a Josephson junction with C ¼ 7 pF and Ic ¼ 5 lA, and thermal switching probabilities of 99% and 1% (curves I99% and I1% respectively). The curve I1% is computed for tw ¼ 1 ms, a reasonable value in TOF experiments, and the curve I99% is computed for tw ¼ 100 ns, a reasonable value for the signal pulse width. The curve I99% in Fig. 1 represents the value of the bias current for which there is a 99% probability that the junctions switches during the signal pulse width. In order to operate the JTJ as a discriminator the bias current level must be lower than I99% of an amount not larger than the STJ pulse amplitude Isign . However, if such bias value
E. Esposito et al. / Physica C 372–376 (2002) 423–426
Fig. 1. Temperature dependence of the switching threshold bias current, curves I1% , I99% and left axis, and of the bias current fluctuation region, curve DI ¼ I99% I1% and right axis. The first three curves correspond to switching probabilities P ¼ 1% and 99%. The junction parameters are C ¼ 7 pF and Ic ¼ 5 lA, and the waiting time ttof is 1 ms.
becomes larger than I1% , there will be a finite probability for unwanted switching. Therefore the following expression must hold: I99% Isign < I < I1% :
425
and an amplitude that can be chosen in order to satisfy the relation (6). The experiment has been performed using a 100 lm2 Nb–AlOx –Nb junction at a temperature of 300 mK, and in presence of an external dc magnetic field to reduce the junction critical current to about 5 lA. The experimental set-up allows the detection of the JTJ switch. By sending a sequence of n (typically 5000) waveforms to the JTJ and recording the number of switches occurred, it is possible to compute the noise-induced switching frequency (due to intrinsic and external sources). This procedure has been repeated for different bias amplitudes obtaining the circles shown in Fig. 2. In the same figure is shown, as full curve, the switching probability induced by the intrinsic noise only, as from Eq. (2). It is worth noting that the experimental distribution is much wider due to the presence of external noise. To simulate the STJ signal a fast (1 ls wide) current pulse has been added to the previous waveform at t ¼ 0:5 ms. By repeating the above procedure we have obtained the squares and
ð6Þ
The dependence of DI I99% I1 % on the JTJ temperature is also shown in Fig. 1. These results indicate that, for a typical small size Josephson junction and a 1 lA current pulse, once the external noise is properly reduced, there are wide operational margins for the JTJ discriminator in terms of bias current, temperature and JTJ properties.
3. Experimental results In order to perform tests of the proposed detection scheme, the response of a JTJ to a bias waveform simulating the TOF-MS operation and the effect of the STJ signal has been experimentally investigated. This has been achieved by implementing a trapezoidal current bias waveform. The waveform is characterised by a rise-time of 100 ls, a fall time of 100 ls, a duration of 1 ms (corresponding to the waiting time tw ), a period of 2 ms,
Fig. 2. Experimental bias current dependence of the switching probability. The circles are values in absence of signal pulse (only external and intrinsic noise), the squares and triangles correspond to superimposed signal pulses of 1 and 2 lA respectively. The theoretical dependence, for intrinsic noise only, is shown as full curve. The arrows show the operational bias range for which the detector can discriminate 2 lA pulses with 100% efficiency.
426
E. Esposito et al. / Physica C 372–376 (2002) 423–426
triangles shown in Fig. 2, corresponding to a pulse amplitude of 1 and 2 lA respectively. Even in the presence of the relatively large noise of the experimental set-up, there is a reasonable operational bias range, shown by the arrows in Fig. 2, for which the detector can discriminate 2 lA pulses with 100% efficiency. In comparison with the simulation results a smaller discrimination sensitivity has been measured. However, by reducing the external noise, it should be possible to fully recover such discrepancy. Moreover, the reported calculations indicate that the operating temperature of the JTJ discriminator can be substantially increased, while still keeping good signal sensitivity. This relaxes the cryogenic requirements for this type of detector.
4. Concluding remarks A new detection scheme for achieving simultaneous time and energy detection of macromolecules impacts has been proposed. The time resolution achievable is ultimately limited by the STJ intrinsic non-equilibrium characteristics, rather than the JTJ discriminator, and should be of the order of few ns. The noise analysis indicates
that the JTJ discriminator can operate at a temperature somewhat higher than that of the STJ detector, giving more flexibility in the cryogenic set-up, and opening the possibilities to use more complex (digital) Josephson circuitry. Preliminary experimental tests show that the discrimination principle works, although it is necessary to improve the noise shielding performances. More accurate measurements are in progress.
References [1] M.W. Senko, F.W. McLafferty, Annu. Rev. Biophys. Biomol. Struc. 23 (1994) 763. [2] R.J. Beuhler, L. Friedman, Nucl. Instr. Meth. 170 (1980) 309. [3] G. Westmacott et al., Nucl. Instr. Meth. B 108 (1996) 282. [4] A. Brunelle et al., Rapid. Commun. Mass Spectrom. 11 (1997) 353. [5] D. Twerenbold, Nucl. Instr. Meth. A 370 (1996) 253–255. [6] D. Twerenbold et al., Appl. Phys. Lett. 68 (1996) 3503. [7] D. Twerenbold, US patent no. 5460010, 1997. [8] G.C. Hilton et al., Nature 391 (1998) 672. [9] M. Kishimoto et al., Nucl. Instr. Meth. A 444 (2000) 124– 128. [10] D. Twerenbold, Nucl. Instr. Meth. A 273 (1988) 575. [11] A. Barone, G. Patern o, Physics and Applications of Josephson Effect, John Wiley, NY, 1980.