- Email: [email protected]
Microwave detection of air showers with MIDAS
Nuclear Instruments and Methods in Physics Research A 662 (2012) S118–S123
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Microwave detection of air showers with MIDAS P. Facal San Luis a,, I. Alekotte b, J. Alvarez c, A. Berlin a, X. Bertou b, M. Bogdan a, M. Bohacova a, C. Bonifazi d, W.R. Carvalho c, J.R.T. de Mello Neto d, J.F. Genat a, E. Mills a, M. Monasor a, P. Privitera a, I.C. Reyes a, B. Rouille d’Orfeuil a, E.M. Santos d, S. Wayne a, C. Williams a, E. Zas c a
University of Chicago, Enrico Fermi Institue and Kavli Institute for Cosmological Physics, 5640 South Ellis Avenue, Chicago, IL 60637, USA ´mico Bariloche and Instituto Balseiro (CNEA-UNCuyo-CONICET), 8400 San Carlos de Bariloche, Rı´o Negro, Argentina Centro Ato c Universidad de Santiago de Compostela, Departamento de Fı´sica de Partı´culas, Campus Sur, E-15782 Santiago de Compostela, Spain d Univ. Federal do Rio de Janeiro (UFRJ), Instituto de Fı´sica, Cidade Universitaria, Caixa Postal 68528, 21945-970 Rio de Janeiro, RJ, Brazil b
a r t i c l e i n f o
a b s t r a c t
Available online 23 November 2010
MIDAS (MIcrowave Detector of Air Showers) is a prototype of a microwave telescope to detect extensive air showers: it images a 201 101 region of the sky with a 4.5 m parabolic reflector and 53 feeds in the focal plane. It has been commissioned in March 2010 and is currently taking data. We present the design, performance and first results of MIDAS. & 2010 Elsevier B.V. All rights reserved.
Keywords: Ultra-high energy cosmic rays Microwave detection New detectors
1. Introduction The measurement of high energy cosmic rays is based on two distinct and complementary techniques: surface detectors sample the shower particle content at ground; fluorescence detectors, in turn, observe the shower longitudinal development by means of the UV light emitted by the atmospheric nitrogen upon the shower passage. The Pierre Auger Observatory [1] and the Telescope Array [2] are hybrid detectors where both techniques are combined to benefit from their complementary advantages. Fluorescence detectors provide a near calorimetric measurement of the energy released by the air shower in the atmosphere and are sensitive to the mass of the primary cosmic ray through the observation of the position of the maximum of the particle cascade [3]. On the downside, UV light observation only takes place on clear moonless night, severely impacting the ‘duty cycle’, i.e. the time these detectors can operate. Recently it has been suggested [4] that radiation in the microwave region of the spectrum is likely to be emitted as a by-product of the energy released through ionization by the extensive air shower. A possible emission mechanism [4] is bremsstrahlung radiation produced when the free electrons in the plasma created by the passage of an extensive air shower interact with the neutral molecules in the atmosphere. This radiation should be isotropic and unpolarized, and could be the basis of a fluorescence-like detector, capable of observing the longitudinal development of the shower. Such a detector could be operated with 100% duty cycle. Also, atmospheric attenuation, one of the main sources of uncertainty in the fluorescence technique, is negligible at these frequencies. Corresponding author.
E-mail address: [email protected] (P. Facal San Luis). 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.11.020
MIDAS (MIcrowave Detector of Air Showers) is a prototype instrument designed with the goal of confirming and characterizing the molecular bremsstrahlung radiation as a viable technique to study ultra-high energy cosmic rays. Provided that the microwave signal is as strong as measured by Ref. [4], the MIDAS prototype is able of standalone detection of showers with a sizable aperture. MIDAS is currently installed at the University of Chicago Campus (Fig. 1).
2. The MIDAS prototype The MIDAS design is based on the successful concept of the fluorescence detectors [5]: the sky is imaged using a parabolic reflector dish into a wide field of view camera, with pixels of about 21 aperture. The pixel’s signal is then digitized by a Flash ADC at 20 MHz. This frequency is dictated by time of passage of the shower in the field of view of a single pixel; 50 ns time resolution is enough to fully characterize the shower passage from 5 km and further. A trigger system optimized for transient events is then able to identify shower candidates against the noise. MIDAS uses a 4.5 diameter parabolic dish (f/D ¼0.34) to focus the incoming radiation into a 53 pixel camera arranged in 7 rows of 7 or 8 pixels (Fig. 2). Rows are staggered to maximize the coverage. Each pixel covers approximately 21 21 for a total of 201 101. Pixels are instrumented with C-Band feeds operating in the 3.4– 4.2 GHz range. The feeds (DMX241, manufactured by WS International) are satellite television receivers (LNBF’s) that incorporate a low noise amplifier (13 K noise level and 70 dB amplification figure) and a frequency down-converter. Feeds are sensitive to both linear polarization components; the actual polarization can be selected by the value of the amplifier bias voltage.
P. Facal San Luis et al. / Nuclear Instruments and Methods in Physics Research A 662 (2012) S118–S123
Fig. 1. General view of the MIDAS prototype installed on the roof of the Karsten Physics Teaching Center at the University of Chicago. The 4.5 m dish has a 53 pixels camera mounted at the focus.
The down-converted signal in the 950–1750 MHz range is fed through approximately 30 m of RG-6U coaxial cable to the counting room where it is routed to a coaxial mounted power detector. The output of the power detector is a voltage level in the 2–0 V range that is inversely proportional to the logarithm of the radio frequency input power in the 55 to 0 dBm range. Its time response is at the level of 10 ns, well below the tens of microseconds transit time of the shower through the field of view of the pixel. The analog electronics for eight channels is contained in a rackmount case (Fig. 3), which provides low voltage distribution to the power detectors and the LNBFs. Each analog channel includes a power inserter (bias-tee) for the LNBF, a 75250 O impedance adapter and a power detector. A set of eight instrumented cases holds the 53 analog channels, receiving the RF signal cables from the camera and routing the voltage output signal of the power detectors to the digitizers. The output signal of the power detector is digitized by a 20 MHz Flash-ADC with 14 bit resolution. Due to the inverting regime of the power detectors output, lower ADC’s values correspond to higher RF power intensities. The FADC board, developed by the Electronics Design Group at the Enrico Fermi Institute of the University of Chicago, hosts 16 channels and has a VME interface. An on-board FPGA is used for digital signal processing and trigger.
2.1. Trigger and DAQ The trigger system uses the FPGAs of the Flash-ADC boards to select fast transients in the midst of the RF noise. The signals of the 53 channels are continuously digitized and the running sum of 10
S119
Fig. 2. Close view of the camera installed at the focus of the dish. The camera is made of 53 commercial feed-horns.
Fig. 3. One of the cases for the power detectors and ‘bias-tee’s’ for eight channels with the low-voltage distribution system. The connectors for the power supplies are at the front of the case, while input and output signal cables are routed through the lateral panels.
ADC values is calculated. Whenever the sum goes below a preset threshold a First Level Trigger (FLT) is issued for that pixel (Fig. 4). To account for the varying conditions in the RF background the threshold of each individual channel is adjusted every second so that the individual pixel FLT rate is kept at 100 Hz (Fig. 5).
S120
P. Facal San Luis et al. / Nuclear Instruments and Methods in Physics Research A 662 (2012) S118–S123
11400
ADC Counts
11200
11000
10800
10600
10400
10200 0
200
400
600
800 1000 1200 1400 1600 1800 Time (50 ns)
Fig. 4. Illustration of the First Level Trigger: in black, digitized trace of relative calibration pulse (see Section 3.1); superimposed, in blue, the running sum of 10 ADC values; when the running sum is smaller than a threshold (in red) a FLT for the pixel is issued. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Entries Mean RMS
1000
21601 107 45
800
Entries
Rate < 400Hz 92% 600
400
200
The operation of the detector in an urban environment, and particularly the proximity of an airport leads to interference from the navigational system of overflying planes, in the form of ‘noisebursts’, sudden increase of the RF background level for several seconds. In order to protect the DAQ and reduce the impact of the bursts in the threshold regulation algorithm, the whole trigger system is inhibited for the duration of the bursts. To further reduce the effect of the ‘noise-bursts’ a band-pass RF filter between 1050 and 1750 MHz has recently been installed at the output of each one of the feeds (Fig. 6). Once an SLT has been issued and the data are available, they are readout by the VME master and written into disk for further processing. The trigger signal is also sent to a GPS unit that tags the precise timing of the event; the time-stamp is read through VME and stored with the data. A second stream of data, for monitoring purposes, is recorded each second and consists, for each one of the channels, of (1) the average baseline over 10 ms, (2) the pixel FLT rate and (3) the value of the threshold. The data acquisition software steers both the event and the monitoring data readout. Data runs are restarted periodically and data is backed-up in a server for offline analysis. Programs have been developed for the control and monitoring of the antenna, power supplies and VME crate, allowing fully automatic and remote operation of the whole system. 2.2. Background events The data taking with the MIDAS prototype is dominated by background events. Of these, a fraction consists of random coincidences of First Level Triggers due to thermal noise fluctuations. These are characterized by small signals barely below the FLT threshold and by a timing that does not correspond to a trajectory within the camera, but is randomly sorted within the FLT window. The expected rate of thermal noise is 0.2 Hz with the 3 pixel trigger, becoming much smaller in the case of a 4 pixel trigger. In addition to the thermal noise, another background component has been identified within the data: events characterized by very short, simultaneous pulses usually covering the whole camera. As in the case of the thermal noise, the timing signature of these events makes them distinguishable from the expected signal of a shower crossing the field of view. The origin of these background events is unknown, but there is certainly not shortage of RF signal that can be present in an urban environment like Chicago. The rate of these events varies greatly with the varying conditions at the site, but in general the global trigger rate is below 1 Hz with a typical value of 0.5 Hz.
0 0
50
100
150
200 250 Rate (Hz)
300
350
400
Fig. 5. Distribution of the FLT rate of a pixel during 6 h run. A high-rate tail that extends above 400 Hz is due to ‘noise-bursts’ (see text).
Each FLT opens a 10 ms gate; the Second Level Trigger (SLT) logic searches for pre-defined patterns of FLTs that correspond to the expected topology of a cosmic ray shower (straight tracks across the camera). When an SLT is issued a stream of 100 ms of data (including 500 pre-trigger samples) is stored in memory for each of the 53 channels. The SLT is implemented independently on each FADC board, selecting track-like patterns of 3 pixels. In a future system upgrade, FLT triggers will be multiplexed and sent during each digitizer cycle to a Master Trigger Board where another FPGA will implement a global SLT looking for track-like patterns of 4 pixels.
3. Calibration and sensitivity During the detector commissioning, and periodically during operation, an extensive program of measurements dedicated to calibrate the detector and to investigate its sensitivity have been carried out. 3.1. Relative calibration A log-periodic antenna positioned at center of the reflector has been used to derive the detector relative calibration. The antenna is connected to an RF pulser which delivers a 4 GHz RF pulse of several microseconds duration, illuminating all the pixels at the same time. The mean value of the pulse power in ADC counts is calculated by averaging over many pulses. The antenna pulse power is varied in 5 dB steps between 60 and 0 dBm.
ADC counts
ADC counts
P. Facal San Luis et al. / Nuclear Instruments and Methods in Physics Research A 662 (2012) S118–S123
S121
11060 11055 11050 11045 11040 11035 11030 11025 11020 11015 11010 0
2000
4000
6000
8000
10000 12000 14000 16000 18000 20000 Seconds
0
2000
4000
6000
8000
10000 12000 14000 16000 18000 20000 Seconds
11845 11840 11835 11830 11825 11820 11815 11810 11805
Fig. 6. The ‘noise-bursts’ effect is clearly visible in the top plot, that shows the average baseline of a channel for each second during a 6 h run. The bottom plot shows the same run in a different pixel where a band-pass RF filter eliminates the bursts.
k = 173 ± 4 ADC/dB
Entries Mean RMS
20
11000 18
53 171.6 4.25
16
10000
9000
Entries
ADC Counts
14
8000
12 10 8 6
7000
4 6000
2 -70
-60
-50
-40 -30 -20 Pulse Power (dBm)
-10
0
Fig. 7. Relative calibration curve for one pixel. Dots are experimental measurements, superimposed is the result of the fit. The highest signal points are excluded from the fit due to the saturation.
The response of the pixel to a pulse of intensity Ppulse applied to the external antenna, can be parametrized as follows: Ppulse nADC ¼ nsys k log 1þ f ð1Þ Psys where nsys and Psys are the ADC counts and power without external signal (i.e. the system +sky temperature); f relates the power of the pulse applied to the antenna to the effective power received at the feed (accounting for signal losses, distance, polarization, etc.); and k is the calibration constant of the channel in ADC/dB. Fig. 7 shows
0 140
150
170 190 160 180 Calibration Constant (ADC/dB)
200
210
Fig. 8. Distribution of the calibration constants derived for the 53 pixels.
the data for one channel, fitting nsys, k and f/Psys in the expression above. Fig. 8 shows the distribution of the calibration constants of the 53 pixels. The calibration constants are uniform at a 3% level, quite remarkable for commercial units. A new antenna has been mounted in the center of the dish in order to provide a fixed calibration source. Regular relative calibration runs are foreseen during data taking. The same antenna is also used to monitor the stability of the system during data taking firing a set of 10 pulses with fixed power and duration every 15 min.
S122
P. Facal San Luis et al. / Nuclear Instruments and Methods in Physics Research A 662 (2012) S118–S123
3.2. Absolute calibration To perform an absolute calibration of the detector, the sun was used as a known source. Fig. 9 shows the signal recorded for a transit of the sun in the field of view of the central pixel. The flux of the sun at 4 GHz can be obtained from radio-observatory measurements [6]. For the transit in Fig. 9, Fsun ¼ 88 1022 W=m2 =Hz, thus providing an absolute calibration of the peak value. To determine the system sensitivity, we use the absolute calibration to calculate the system noise temperature. The value of the peak measured in Fig. 9 can be expressed in terms of the flux of the sun and of the equivalent system noise flux:
Dn ¼ k logð1 þ Fsun =Fsys Þ
ð2Þ
and since Fsun is known, from the measured Dn we obtain the equivalent noise flux and from it the system temperature as 1 Fsys Aeff ¼ kb Tsys 2
ð3Þ
where the factor 1/2 comes from the fact that we only measure one of the polarization components. Assuming an effective area Aeff C10 m2 we obtain Tsys C 120 K. This calibration has been repeated routinely giving compatible results. As a cross-check, the moon and the Crab nebulae have also been observed yielding consistent results.
3.3. Simulation With this system temperature we expect the maximum of a shower of 5 1018 eV at 10 km distance to yield around 2000 counts over the baseline if we scale the value in Ref. [4] quadratically with the shower energy, as that same measurements suggest; in case of linear scaling, a 1019 eV shower at the same distance would yield about 200 counts, compared to 1s fluctuations of the baseline smaller than 100 counts. An end to end Monte Carlo simulation of the MIDAS prototype, including the camera beam patterns and the absolute calibration, has been developed
11000
ADC counts
10500 10000 9500 9000 8500 7000
8000
9000
10000
11000 12000 Seconds
13000
14000
15000
16000
Fig. 9. The sun passing through the central pixel field of view, used for the absolute calibration of the detector. Plotted is the average baseline (over 10 ms) as a function of time (in 1 second steps, where 0 corresponds to the beginning of the data taking run).
Fig. 10. Event display of a Monte Carlo simulated event.
P. Facal San Luis et al. / Nuclear Instruments and Methods in Physics Research A 662 (2012) S118–S123
and can be used to calculate the expected sensitivity in realistic conditions as well as to study the characteristics of the expected events in the same format as the data (Fig. 10).
S123
in Ref. [4] to atmospheric air showers. Installation of the prototype at the Southern site of the Auger Observatory is also foreseen.
Acknowledgments 4. Outlook The MIDAS prototype has been recently installed and commissioned. The first months of operation have been dedicated to calibration activities and preliminary data taking. Several upgrades are underway in order to improve the quality of the data and the operation of the telescope, including the installation of band-pass filters to suppress the ‘noise-bursts’ caused by planes, the implementation of a global camera trigger and the installation of a fixed antenna for system monitoring. Once a stable operation has been achieved, we plan to run for several months. MIDAS has the sensitivity to detect cosmic ray showers or to place a strong limit on the scaling of the laboratory measurement
Support for this research at The University of Chicago, Kavli Institute for Cosmological Physics was provided by NSF Grant PHY-0551142. References [1] J.A. Abraham, et al., Nucl. Instr. and Meth. A 523 (2004) 50. [2] H. Tokuno, et al., AIP Conference Proceedings, vol. 1238, 2010, p. 365. [3] J. Abraham, et al., Pierre Auger Observatory Collaboration, Phys. Rev. Lett. 104 (2010) 091101 (arXiv:1002.0699 [astro-ph.HE]). [4] P.W. Gorham, et al., Phys. Rev. D 78 (2008) 032007 (arXiv:0705.2589 [astro-ph]). [5] J.A. Abraham, et al., Nucl. Instr. and Meth. A 620 (2010) 227, arXiv:0907.4282 [astro-ph.IM]. [6] Nakajima, et al., Publ. Astron. Soc. Jpn. 37 (1985) 163.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Microwave detection of air showers with MIDAS P. Facal San Luis a,, I. Alekotte b, J. Alvarez c, A. Berlin a, X. Bertou b, M. Bogdan a, M. Bohacova a, C. Bonifazi d, W.R. Carvalho c, J.R.T. de Mello Neto d, J.F. Genat a, E. Mills a, M. Monasor a, P. Privitera a, I.C. Reyes a, B. Rouille d’Orfeuil a, E.M. Santos d, S. Wayne a, C. Williams a, E. Zas c a
University of Chicago, Enrico Fermi Institue and Kavli Institute for Cosmological Physics, 5640 South Ellis Avenue, Chicago, IL 60637, USA ´mico Bariloche and Instituto Balseiro (CNEA-UNCuyo-CONICET), 8400 San Carlos de Bariloche, Rı´o Negro, Argentina Centro Ato c Universidad de Santiago de Compostela, Departamento de Fı´sica de Partı´culas, Campus Sur, E-15782 Santiago de Compostela, Spain d Univ. Federal do Rio de Janeiro (UFRJ), Instituto de Fı´sica, Cidade Universitaria, Caixa Postal 68528, 21945-970 Rio de Janeiro, RJ, Brazil b
a r t i c l e i n f o
a b s t r a c t
Available online 23 November 2010
MIDAS (MIcrowave Detector of Air Showers) is a prototype of a microwave telescope to detect extensive air showers: it images a 201 101 region of the sky with a 4.5 m parabolic reflector and 53 feeds in the focal plane. It has been commissioned in March 2010 and is currently taking data. We present the design, performance and first results of MIDAS. & 2010 Elsevier B.V. All rights reserved.
Keywords: Ultra-high energy cosmic rays Microwave detection New detectors
1. Introduction The measurement of high energy cosmic rays is based on two distinct and complementary techniques: surface detectors sample the shower particle content at ground; fluorescence detectors, in turn, observe the shower longitudinal development by means of the UV light emitted by the atmospheric nitrogen upon the shower passage. The Pierre Auger Observatory [1] and the Telescope Array [2] are hybrid detectors where both techniques are combined to benefit from their complementary advantages. Fluorescence detectors provide a near calorimetric measurement of the energy released by the air shower in the atmosphere and are sensitive to the mass of the primary cosmic ray through the observation of the position of the maximum of the particle cascade [3]. On the downside, UV light observation only takes place on clear moonless night, severely impacting the ‘duty cycle’, i.e. the time these detectors can operate. Recently it has been suggested [4] that radiation in the microwave region of the spectrum is likely to be emitted as a by-product of the energy released through ionization by the extensive air shower. A possible emission mechanism [4] is bremsstrahlung radiation produced when the free electrons in the plasma created by the passage of an extensive air shower interact with the neutral molecules in the atmosphere. This radiation should be isotropic and unpolarized, and could be the basis of a fluorescence-like detector, capable of observing the longitudinal development of the shower. Such a detector could be operated with 100% duty cycle. Also, atmospheric attenuation, one of the main sources of uncertainty in the fluorescence technique, is negligible at these frequencies. Corresponding author.
E-mail address: [email protected] (P. Facal San Luis). 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.11.020
MIDAS (MIcrowave Detector of Air Showers) is a prototype instrument designed with the goal of confirming and characterizing the molecular bremsstrahlung radiation as a viable technique to study ultra-high energy cosmic rays. Provided that the microwave signal is as strong as measured by Ref. [4], the MIDAS prototype is able of standalone detection of showers with a sizable aperture. MIDAS is currently installed at the University of Chicago Campus (Fig. 1).
2. The MIDAS prototype The MIDAS design is based on the successful concept of the fluorescence detectors [5]: the sky is imaged using a parabolic reflector dish into a wide field of view camera, with pixels of about 21 aperture. The pixel’s signal is then digitized by a Flash ADC at 20 MHz. This frequency is dictated by time of passage of the shower in the field of view of a single pixel; 50 ns time resolution is enough to fully characterize the shower passage from 5 km and further. A trigger system optimized for transient events is then able to identify shower candidates against the noise. MIDAS uses a 4.5 diameter parabolic dish (f/D ¼0.34) to focus the incoming radiation into a 53 pixel camera arranged in 7 rows of 7 or 8 pixels (Fig. 2). Rows are staggered to maximize the coverage. Each pixel covers approximately 21 21 for a total of 201 101. Pixels are instrumented with C-Band feeds operating in the 3.4– 4.2 GHz range. The feeds (DMX241, manufactured by WS International) are satellite television receivers (LNBF’s) that incorporate a low noise amplifier (13 K noise level and 70 dB amplification figure) and a frequency down-converter. Feeds are sensitive to both linear polarization components; the actual polarization can be selected by the value of the amplifier bias voltage.
P. Facal San Luis et al. / Nuclear Instruments and Methods in Physics Research A 662 (2012) S118–S123
Fig. 1. General view of the MIDAS prototype installed on the roof of the Karsten Physics Teaching Center at the University of Chicago. The 4.5 m dish has a 53 pixels camera mounted at the focus.
The down-converted signal in the 950–1750 MHz range is fed through approximately 30 m of RG-6U coaxial cable to the counting room where it is routed to a coaxial mounted power detector. The output of the power detector is a voltage level in the 2–0 V range that is inversely proportional to the logarithm of the radio frequency input power in the 55 to 0 dBm range. Its time response is at the level of 10 ns, well below the tens of microseconds transit time of the shower through the field of view of the pixel. The analog electronics for eight channels is contained in a rackmount case (Fig. 3), which provides low voltage distribution to the power detectors and the LNBFs. Each analog channel includes a power inserter (bias-tee) for the LNBF, a 75250 O impedance adapter and a power detector. A set of eight instrumented cases holds the 53 analog channels, receiving the RF signal cables from the camera and routing the voltage output signal of the power detectors to the digitizers. The output signal of the power detector is digitized by a 20 MHz Flash-ADC with 14 bit resolution. Due to the inverting regime of the power detectors output, lower ADC’s values correspond to higher RF power intensities. The FADC board, developed by the Electronics Design Group at the Enrico Fermi Institute of the University of Chicago, hosts 16 channels and has a VME interface. An on-board FPGA is used for digital signal processing and trigger.
2.1. Trigger and DAQ The trigger system uses the FPGAs of the Flash-ADC boards to select fast transients in the midst of the RF noise. The signals of the 53 channels are continuously digitized and the running sum of 10
S119
Fig. 2. Close view of the camera installed at the focus of the dish. The camera is made of 53 commercial feed-horns.
Fig. 3. One of the cases for the power detectors and ‘bias-tee’s’ for eight channels with the low-voltage distribution system. The connectors for the power supplies are at the front of the case, while input and output signal cables are routed through the lateral panels.
ADC values is calculated. Whenever the sum goes below a preset threshold a First Level Trigger (FLT) is issued for that pixel (Fig. 4). To account for the varying conditions in the RF background the threshold of each individual channel is adjusted every second so that the individual pixel FLT rate is kept at 100 Hz (Fig. 5).
S120
P. Facal San Luis et al. / Nuclear Instruments and Methods in Physics Research A 662 (2012) S118–S123
11400
ADC Counts
11200
11000
10800
10600
10400
10200 0
200
400
600
800 1000 1200 1400 1600 1800 Time (50 ns)
Fig. 4. Illustration of the First Level Trigger: in black, digitized trace of relative calibration pulse (see Section 3.1); superimposed, in blue, the running sum of 10 ADC values; when the running sum is smaller than a threshold (in red) a FLT for the pixel is issued. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Entries Mean RMS
1000
21601 107 45
800
Entries
Rate < 400Hz 92% 600
400
200
The operation of the detector in an urban environment, and particularly the proximity of an airport leads to interference from the navigational system of overflying planes, in the form of ‘noisebursts’, sudden increase of the RF background level for several seconds. In order to protect the DAQ and reduce the impact of the bursts in the threshold regulation algorithm, the whole trigger system is inhibited for the duration of the bursts. To further reduce the effect of the ‘noise-bursts’ a band-pass RF filter between 1050 and 1750 MHz has recently been installed at the output of each one of the feeds (Fig. 6). Once an SLT has been issued and the data are available, they are readout by the VME master and written into disk for further processing. The trigger signal is also sent to a GPS unit that tags the precise timing of the event; the time-stamp is read through VME and stored with the data. A second stream of data, for monitoring purposes, is recorded each second and consists, for each one of the channels, of (1) the average baseline over 10 ms, (2) the pixel FLT rate and (3) the value of the threshold. The data acquisition software steers both the event and the monitoring data readout. Data runs are restarted periodically and data is backed-up in a server for offline analysis. Programs have been developed for the control and monitoring of the antenna, power supplies and VME crate, allowing fully automatic and remote operation of the whole system. 2.2. Background events The data taking with the MIDAS prototype is dominated by background events. Of these, a fraction consists of random coincidences of First Level Triggers due to thermal noise fluctuations. These are characterized by small signals barely below the FLT threshold and by a timing that does not correspond to a trajectory within the camera, but is randomly sorted within the FLT window. The expected rate of thermal noise is 0.2 Hz with the 3 pixel trigger, becoming much smaller in the case of a 4 pixel trigger. In addition to the thermal noise, another background component has been identified within the data: events characterized by very short, simultaneous pulses usually covering the whole camera. As in the case of the thermal noise, the timing signature of these events makes them distinguishable from the expected signal of a shower crossing the field of view. The origin of these background events is unknown, but there is certainly not shortage of RF signal that can be present in an urban environment like Chicago. The rate of these events varies greatly with the varying conditions at the site, but in general the global trigger rate is below 1 Hz with a typical value of 0.5 Hz.
0 0
50
100
150
200 250 Rate (Hz)
300
350
400
Fig. 5. Distribution of the FLT rate of a pixel during 6 h run. A high-rate tail that extends above 400 Hz is due to ‘noise-bursts’ (see text).
Each FLT opens a 10 ms gate; the Second Level Trigger (SLT) logic searches for pre-defined patterns of FLTs that correspond to the expected topology of a cosmic ray shower (straight tracks across the camera). When an SLT is issued a stream of 100 ms of data (including 500 pre-trigger samples) is stored in memory for each of the 53 channels. The SLT is implemented independently on each FADC board, selecting track-like patterns of 3 pixels. In a future system upgrade, FLT triggers will be multiplexed and sent during each digitizer cycle to a Master Trigger Board where another FPGA will implement a global SLT looking for track-like patterns of 4 pixels.
3. Calibration and sensitivity During the detector commissioning, and periodically during operation, an extensive program of measurements dedicated to calibrate the detector and to investigate its sensitivity have been carried out. 3.1. Relative calibration A log-periodic antenna positioned at center of the reflector has been used to derive the detector relative calibration. The antenna is connected to an RF pulser which delivers a 4 GHz RF pulse of several microseconds duration, illuminating all the pixels at the same time. The mean value of the pulse power in ADC counts is calculated by averaging over many pulses. The antenna pulse power is varied in 5 dB steps between 60 and 0 dBm.
ADC counts
ADC counts
P. Facal San Luis et al. / Nuclear Instruments and Methods in Physics Research A 662 (2012) S118–S123
S121
11060 11055 11050 11045 11040 11035 11030 11025 11020 11015 11010 0
2000
4000
6000
8000
10000 12000 14000 16000 18000 20000 Seconds
0
2000
4000
6000
8000
10000 12000 14000 16000 18000 20000 Seconds
11845 11840 11835 11830 11825 11820 11815 11810 11805
Fig. 6. The ‘noise-bursts’ effect is clearly visible in the top plot, that shows the average baseline of a channel for each second during a 6 h run. The bottom plot shows the same run in a different pixel where a band-pass RF filter eliminates the bursts.
k = 173 ± 4 ADC/dB
Entries Mean RMS
20
11000 18
53 171.6 4.25
16
10000
9000
Entries
ADC Counts
14
8000
12 10 8 6
7000
4 6000
2 -70
-60
-50
-40 -30 -20 Pulse Power (dBm)
-10
0
Fig. 7. Relative calibration curve for one pixel. Dots are experimental measurements, superimposed is the result of the fit. The highest signal points are excluded from the fit due to the saturation.
The response of the pixel to a pulse of intensity Ppulse applied to the external antenna, can be parametrized as follows: Ppulse nADC ¼ nsys k log 1þ f ð1Þ Psys where nsys and Psys are the ADC counts and power without external signal (i.e. the system +sky temperature); f relates the power of the pulse applied to the antenna to the effective power received at the feed (accounting for signal losses, distance, polarization, etc.); and k is the calibration constant of the channel in ADC/dB. Fig. 7 shows
0 140
150
170 190 160 180 Calibration Constant (ADC/dB)
200
210
Fig. 8. Distribution of the calibration constants derived for the 53 pixels.
the data for one channel, fitting nsys, k and f/Psys in the expression above. Fig. 8 shows the distribution of the calibration constants of the 53 pixels. The calibration constants are uniform at a 3% level, quite remarkable for commercial units. A new antenna has been mounted in the center of the dish in order to provide a fixed calibration source. Regular relative calibration runs are foreseen during data taking. The same antenna is also used to monitor the stability of the system during data taking firing a set of 10 pulses with fixed power and duration every 15 min.
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3.2. Absolute calibration To perform an absolute calibration of the detector, the sun was used as a known source. Fig. 9 shows the signal recorded for a transit of the sun in the field of view of the central pixel. The flux of the sun at 4 GHz can be obtained from radio-observatory measurements [6]. For the transit in Fig. 9, Fsun ¼ 88 1022 W=m2 =Hz, thus providing an absolute calibration of the peak value. To determine the system sensitivity, we use the absolute calibration to calculate the system noise temperature. The value of the peak measured in Fig. 9 can be expressed in terms of the flux of the sun and of the equivalent system noise flux:
Dn ¼ k logð1 þ Fsun =Fsys Þ
ð2Þ
and since Fsun is known, from the measured Dn we obtain the equivalent noise flux and from it the system temperature as 1 Fsys Aeff ¼ kb Tsys 2
ð3Þ
where the factor 1/2 comes from the fact that we only measure one of the polarization components. Assuming an effective area Aeff C10 m2 we obtain Tsys C 120 K. This calibration has been repeated routinely giving compatible results. As a cross-check, the moon and the Crab nebulae have also been observed yielding consistent results.
3.3. Simulation With this system temperature we expect the maximum of a shower of 5 1018 eV at 10 km distance to yield around 2000 counts over the baseline if we scale the value in Ref. [4] quadratically with the shower energy, as that same measurements suggest; in case of linear scaling, a 1019 eV shower at the same distance would yield about 200 counts, compared to 1s fluctuations of the baseline smaller than 100 counts. An end to end Monte Carlo simulation of the MIDAS prototype, including the camera beam patterns and the absolute calibration, has been developed
11000
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10500 10000 9500 9000 8500 7000
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Fig. 9. The sun passing through the central pixel field of view, used for the absolute calibration of the detector. Plotted is the average baseline (over 10 ms) as a function of time (in 1 second steps, where 0 corresponds to the beginning of the data taking run).
Fig. 10. Event display of a Monte Carlo simulated event.
P. Facal San Luis et al. / Nuclear Instruments and Methods in Physics Research A 662 (2012) S118–S123
and can be used to calculate the expected sensitivity in realistic conditions as well as to study the characteristics of the expected events in the same format as the data (Fig. 10).
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in Ref. [4] to atmospheric air showers. Installation of the prototype at the Southern site of the Auger Observatory is also foreseen.
Acknowledgments 4. Outlook The MIDAS prototype has been recently installed and commissioned. The first months of operation have been dedicated to calibration activities and preliminary data taking. Several upgrades are underway in order to improve the quality of the data and the operation of the telescope, including the installation of band-pass filters to suppress the ‘noise-bursts’ caused by planes, the implementation of a global camera trigger and the installation of a fixed antenna for system monitoring. Once a stable operation has been achieved, we plan to run for several months. MIDAS has the sensitivity to detect cosmic ray showers or to place a strong limit on the scaling of the laboratory measurement
Support for this research at The University of Chicago, Kavli Institute for Cosmological Physics was provided by NSF Grant PHY-0551142. References [1] J.A. Abraham, et al., Nucl. Instr. and Meth. A 523 (2004) 50. [2] H. Tokuno, et al., AIP Conference Proceedings, vol. 1238, 2010, p. 365. [3] J. Abraham, et al., Pierre Auger Observatory Collaboration, Phys. Rev. Lett. 104 (2010) 091101 (arXiv:1002.0699 [astro-ph.HE]). [4] P.W. Gorham, et al., Phys. Rev. D 78 (2008) 032007 (arXiv:0705.2589 [astro-ph]). [5] J.A. Abraham, et al., Nucl. Instr. and Meth. A 620 (2010) 227, arXiv:0907.4282 [astro-ph.IM]. [6] Nakajima, et al., Publ. Astron. Soc. Jpn. 37 (1985) 163.