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Fast cryodetector and SQUID read-out for mass spectrometry
Physica C 367 (2002) 295±297
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Fast cryodetector and SQUID read-out for mass spectrometry bst, S. Rutzinger, W. Seidel S.V. Uchaikin *, P. Christ, F. Pro Max-Planck-Institut fur Physik, Fohringer Ring 6, Munich 80805, Germany
Abstract We report a cryogenic detector with a superconducting phase transition thermometer and SQUID read-out for time of ¯ight (TOF) mass spectrometry of biomolecules. Cryogenic detectors combine 100% detection eciency independent of the mass of the molecule, an excellent energy resolution and a low threshold. Implementation of the cryogenic detectors into TOF mass spectrometers is expected to improve the eciency of the detection of large biomolecules. To reach a high time resolution, we have developed two new read-outs based on a double stage SQUID and a cooled ampli®er. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: SQUID; Mass spectrometry; Cryogenic detector
1. Cryogenic detector and TOF mass spectrometry Our group is developing cryogenic detectors for detection of small deposited energies [1]. The detector consists of a sapphire absorber and a superconducting strip evaporated onto it (Fig. 1). The ®lm serves as a superconducting phase transition thermometer (SPT). The detector is operating within the superconducting to normal conducting phase transition of the thermometer, where a small temperature rise of the thermometer causes a relatively large rise of its resistance. If a particle interacts in the crystal absorber, the high frequency phonons are produced that ®nally causes a temperature rise of the strip. This temperature rise re¯ects the energy deposited by the particle. The SQUID ampli®er is ideally suitable for our cryogenic detector with the low impedance about
* Corresponding author. Tel.: +49-89-32354376; fax: +49-893226704. E-mail address: [email protected] (S.V. Uchaikin).
0.05 X. To measure the resistance rise DR corresponding to the temperature rise, a SQUID based circuit shown in Fig. 2, is used. We are going to implement our cryogenic detector into a standard time of ¯ight (TOF) mass spectrometer. It allows one to identify biomolecules by measuring their mass. The biomolecules are ionized in the probe and accelerated in a high electric ®eld where all of them get the same kinetic energy. The TOF of the molecules over the spectrometer body depends on their mass. In conventional TOF mass spectrometers the micro-channel plate detectors (MCP) are used to detect molecules. The operation of the MCP is based on the ionization initiated by the incident molecule. The ionization eciency depends on the velocity of the molecules and drops dramatically for velocities below tens of kilometers per second. This is the reason why the molecules with masses above 100 000 a.u. are hard registered with ionization detectors. The operation of the cryogenic detectors does not include on the ionization processes. The
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 1 ) 0 1 0 1 7 - 6
296
S.V. Uchaikin et al. / Physica C 367 (2002) 295±297
Fig. 1. The cryogenic detector with SPT thermometer. Fig. 3. A typical pulse resulting from the absorption of a 6 keV X-ray with a detector.
Fig. 2. The read-out circuit: R T Юlm resistance, RREF Ðreference resistor, IB Ðbias current, LIN ÐSQUID input coil. The change of the ®lm resistance R T aects the branching of the dc bias current IB . The change of the current through the SQUID input coil LIN is measured with a SQUID.
cryogenic detectors have a low threshold and an excellent energy resolution. We are going to implement these detectors into TOF mass spectrometers to improve the eciency of the large mass molecules detection. For this reason we have been performing a development of fast detectors based of the iridium±gold proximity eect thermometer [1]. At the moment our fastest detectors (3 3 mm2 ) have a rise time of 4 ls (Fig. 3).
using a double stage SQUID concept [3] and with SQUIDs placed close to the detector to reduce the input circuit time constant. In this con®guration the input SQUID de®nes the lower limits of the system noise. The second SQUID is used as a preampli®er for an input SQUID (Fig. 4). The measured ¯ux noise of the double stage SQUID read-out system of about 4.5 lU0 /Hz1=2 , is translated to an input current noise of about 5 pA/Hz1=2 with a current sensitivity of the input coil of 1.2 lA/U0 . The small signal bandwidth is 2 MHz. The measured slew rate is 2 105 U0 /s. The parameters we have obtained are sucient for our cryogenic detector. But as for the double SQUID ampli®er, there are possibilities for ¯ux losses for
2. Double stage SQUID read-out To get the required time resolution the read-out bandwidth of TOF mass spectrometer should be, at least, about 1 MHz with the highest possible slew rate. Recently [2] we have reported a read-out
Fig. 4. LIN , LS are the input coils of the ®rst and second stage SQUIDs; M1 and M2 are the mutual inductances between the input coils and SQUIDs; RS is the current limit resistor; IB1 and IB2 are SQUID bias currents; VOUT is the output voltage of the double stage SQUID.
S.V. Uchaikin et al. / Physica C 367 (2002) 295±297
297
both of the SQUIDs. It looks as a base line jumps on the read-out output. In contrasts with the usual single stage SQUID ¯ux losses which can be easily extracted because the known one step value the double stage SQUID ¯ux losses steps is combined with a number of the steps combination coming from the ®rst and second SQUIDs. 3. Read-out with cooled preampli®er To avoid the shortcoming of the double stage SQUID systems and to exclude the in¯uence of the cryostat wiring on the read-out dynamic, we have developed another read-out system. Its feature is a cooled preampli®er using a commercial operational ampli®er (OA). The OAs produced with bipolar transistors technology do not operate at the helium temperature because of freeze-out of the thermally activated charge carriers. In comparison to the OAs produced with complementary metaloxide-semiconductor (CMOS) transistors, the charge carriers can be created by the ®eld eect at this temperature range. A possible problem in using the commercial CMOS OAs at helium temperature is a potential hill in the source and/or drain [4]. The potential hill appears due to incomplete overlap of the gate with a source and drain or because of the increased isolation thickness used in the overlap region to reduce parasitic capacitances. To exclude the potential hill in¯uence, the author [5] increased the supply voltage of the OA. The circuit diagram is shown in Fig. 5. The SQUID is used in the current bias mode and its output voltage is sensed. We have used a CMOS OA ICL7611 for our circuit. The quiescent current of the OA has been set more than 1 mA to get maximal AC characteristics. The OA is connected as a non-inverting ampli®er. To increase the supply voltage for functioning at the helium temperature, a simple circuit is used. The circuit contains a Zener diode Z1 and Z2 serves to switch the supply voltage between 8 and 13 V. At the room temperature the Zener diodes operate as normal and their Zener breakdown voltage de®nes the supply voltage of the OA. At the cryogenic temperature the Zener diode does not operate and the OA supply voltages are equal to 13.
Fig. 5. A preampli®er based on the OA ICL7611.
A ¯ux feedback has been created with the circuit RLF . The preampli®er was unstable for the feedback coecients exceeding 20 dB. We attribute this to the OA parameters degradation and nonlinear responses of OA components. Also we had stability problems related with the temperature deviation of the OA. To increase the stability, the second feedback loop R1R2 has been added into the circuit. The preampli®er has been tested with the dc SQUID produced by JeSEF [6]. The white noise level was obtained about 5 10 5 U0 /Hz1=2 , which is approximately by 10 times worse than the SQUID noise. The noise parameter degradation could be explained by decreasing of the voltage swing of the SQUID as the additional noise of the ampli®er. Further experiments are planned to be carried out later. To decrease the in¯uence of the low frequency noise of the OA, we are planning to add a bias current feedback circuit [6]. Testing other types of the OAs is in progress. References [1] F. Pr obst et al., J. Low Temp. Phys. 100 (1995) 69. [2] S.V. Uchaikin, F. Pr obst, W. Seidel, Physica C 350 (2001) 177±179. [3] Yu. Maslennikov, A. Beljaev, O. Snigirev, O. Kaplunenko, R. Mezzena, IEEE Trans. Appl. Supercond. 5 (1995) 3241± 3243. [4] S.H. Wu, R.L. Anderson, Solid-State Electron. 17 (1974) 1136. [5] K.-W. Ng, Physica B 194±196 (1994) 157±158. [6] Jena Superconductive Electronics Foundry, Jena, Germany.
www.elsevier.com/locate/physc
Fast cryodetector and SQUID read-out for mass spectrometry bst, S. Rutzinger, W. Seidel S.V. Uchaikin *, P. Christ, F. Pro Max-Planck-Institut fur Physik, Fohringer Ring 6, Munich 80805, Germany
Abstract We report a cryogenic detector with a superconducting phase transition thermometer and SQUID read-out for time of ¯ight (TOF) mass spectrometry of biomolecules. Cryogenic detectors combine 100% detection eciency independent of the mass of the molecule, an excellent energy resolution and a low threshold. Implementation of the cryogenic detectors into TOF mass spectrometers is expected to improve the eciency of the detection of large biomolecules. To reach a high time resolution, we have developed two new read-outs based on a double stage SQUID and a cooled ampli®er. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: SQUID; Mass spectrometry; Cryogenic detector
1. Cryogenic detector and TOF mass spectrometry Our group is developing cryogenic detectors for detection of small deposited energies [1]. The detector consists of a sapphire absorber and a superconducting strip evaporated onto it (Fig. 1). The ®lm serves as a superconducting phase transition thermometer (SPT). The detector is operating within the superconducting to normal conducting phase transition of the thermometer, where a small temperature rise of the thermometer causes a relatively large rise of its resistance. If a particle interacts in the crystal absorber, the high frequency phonons are produced that ®nally causes a temperature rise of the strip. This temperature rise re¯ects the energy deposited by the particle. The SQUID ampli®er is ideally suitable for our cryogenic detector with the low impedance about
* Corresponding author. Tel.: +49-89-32354376; fax: +49-893226704. E-mail address: [email protected] (S.V. Uchaikin).
0.05 X. To measure the resistance rise DR corresponding to the temperature rise, a SQUID based circuit shown in Fig. 2, is used. We are going to implement our cryogenic detector into a standard time of ¯ight (TOF) mass spectrometer. It allows one to identify biomolecules by measuring their mass. The biomolecules are ionized in the probe and accelerated in a high electric ®eld where all of them get the same kinetic energy. The TOF of the molecules over the spectrometer body depends on their mass. In conventional TOF mass spectrometers the micro-channel plate detectors (MCP) are used to detect molecules. The operation of the MCP is based on the ionization initiated by the incident molecule. The ionization eciency depends on the velocity of the molecules and drops dramatically for velocities below tens of kilometers per second. This is the reason why the molecules with masses above 100 000 a.u. are hard registered with ionization detectors. The operation of the cryogenic detectors does not include on the ionization processes. The
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 1 ) 0 1 0 1 7 - 6
296
S.V. Uchaikin et al. / Physica C 367 (2002) 295±297
Fig. 1. The cryogenic detector with SPT thermometer. Fig. 3. A typical pulse resulting from the absorption of a 6 keV X-ray with a detector.
Fig. 2. The read-out circuit: R T Юlm resistance, RREF Ðreference resistor, IB Ðbias current, LIN ÐSQUID input coil. The change of the ®lm resistance R T aects the branching of the dc bias current IB . The change of the current through the SQUID input coil LIN is measured with a SQUID.
cryogenic detectors have a low threshold and an excellent energy resolution. We are going to implement these detectors into TOF mass spectrometers to improve the eciency of the large mass molecules detection. For this reason we have been performing a development of fast detectors based of the iridium±gold proximity eect thermometer [1]. At the moment our fastest detectors (3 3 mm2 ) have a rise time of 4 ls (Fig. 3).
using a double stage SQUID concept [3] and with SQUIDs placed close to the detector to reduce the input circuit time constant. In this con®guration the input SQUID de®nes the lower limits of the system noise. The second SQUID is used as a preampli®er for an input SQUID (Fig. 4). The measured ¯ux noise of the double stage SQUID read-out system of about 4.5 lU0 /Hz1=2 , is translated to an input current noise of about 5 pA/Hz1=2 with a current sensitivity of the input coil of 1.2 lA/U0 . The small signal bandwidth is 2 MHz. The measured slew rate is 2 105 U0 /s. The parameters we have obtained are sucient for our cryogenic detector. But as for the double SQUID ampli®er, there are possibilities for ¯ux losses for
2. Double stage SQUID read-out To get the required time resolution the read-out bandwidth of TOF mass spectrometer should be, at least, about 1 MHz with the highest possible slew rate. Recently [2] we have reported a read-out
Fig. 4. LIN , LS are the input coils of the ®rst and second stage SQUIDs; M1 and M2 are the mutual inductances between the input coils and SQUIDs; RS is the current limit resistor; IB1 and IB2 are SQUID bias currents; VOUT is the output voltage of the double stage SQUID.
S.V. Uchaikin et al. / Physica C 367 (2002) 295±297
297
both of the SQUIDs. It looks as a base line jumps on the read-out output. In contrasts with the usual single stage SQUID ¯ux losses which can be easily extracted because the known one step value the double stage SQUID ¯ux losses steps is combined with a number of the steps combination coming from the ®rst and second SQUIDs. 3. Read-out with cooled preampli®er To avoid the shortcoming of the double stage SQUID systems and to exclude the in¯uence of the cryostat wiring on the read-out dynamic, we have developed another read-out system. Its feature is a cooled preampli®er using a commercial operational ampli®er (OA). The OAs produced with bipolar transistors technology do not operate at the helium temperature because of freeze-out of the thermally activated charge carriers. In comparison to the OAs produced with complementary metaloxide-semiconductor (CMOS) transistors, the charge carriers can be created by the ®eld eect at this temperature range. A possible problem in using the commercial CMOS OAs at helium temperature is a potential hill in the source and/or drain [4]. The potential hill appears due to incomplete overlap of the gate with a source and drain or because of the increased isolation thickness used in the overlap region to reduce parasitic capacitances. To exclude the potential hill in¯uence, the author [5] increased the supply voltage of the OA. The circuit diagram is shown in Fig. 5. The SQUID is used in the current bias mode and its output voltage is sensed. We have used a CMOS OA ICL7611 for our circuit. The quiescent current of the OA has been set more than 1 mA to get maximal AC characteristics. The OA is connected as a non-inverting ampli®er. To increase the supply voltage for functioning at the helium temperature, a simple circuit is used. The circuit contains a Zener diode Z1 and Z2 serves to switch the supply voltage between 8 and 13 V. At the room temperature the Zener diodes operate as normal and their Zener breakdown voltage de®nes the supply voltage of the OA. At the cryogenic temperature the Zener diode does not operate and the OA supply voltages are equal to 13.
Fig. 5. A preampli®er based on the OA ICL7611.
A ¯ux feedback has been created with the circuit RLF . The preampli®er was unstable for the feedback coecients exceeding 20 dB. We attribute this to the OA parameters degradation and nonlinear responses of OA components. Also we had stability problems related with the temperature deviation of the OA. To increase the stability, the second feedback loop R1R2 has been added into the circuit. The preampli®er has been tested with the dc SQUID produced by JeSEF [6]. The white noise level was obtained about 5 10 5 U0 /Hz1=2 , which is approximately by 10 times worse than the SQUID noise. The noise parameter degradation could be explained by decreasing of the voltage swing of the SQUID as the additional noise of the ampli®er. Further experiments are planned to be carried out later. To decrease the in¯uence of the low frequency noise of the OA, we are planning to add a bias current feedback circuit [6]. Testing other types of the OAs is in progress. References [1] F. Pr obst et al., J. Low Temp. Phys. 100 (1995) 69. [2] S.V. Uchaikin, F. Pr obst, W. Seidel, Physica C 350 (2001) 177±179. [3] Yu. Maslennikov, A. Beljaev, O. Snigirev, O. Kaplunenko, R. Mezzena, IEEE Trans. Appl. Supercond. 5 (1995) 3241± 3243. [4] S.H. Wu, R.L. Anderson, Solid-State Electron. 17 (1974) 1136. [5] K.-W. Ng, Physica B 194±196 (1994) 157±158. [6] Jena Superconductive Electronics Foundry, Jena, Germany.