- Email: [email protected]
Composition near the knee: results from the CACTI experiment
Nuclear Physics B (Proc. Suppl.) 122 (2003) 239-242
PROCEEDINGS SUPPLEMENTS www.clscvicr.com/locatc/npc
Composition near the knee: results from the CACTI experiment * Gaurang B. Yodh a aDepartment University
of Physics and Astronomy of California
Irvine
Irvine, CA, USA,92717 This paper presents a summary of the results the CACTI experiment, which studied the shape of Cherenkov lateral distributions of showers triggering the CYGNUS II air shower array at Los Alamos National Laboratory, to investigate the composition of cosmic rays in the PeV energy range.
1. THE
METHOD
An experiment was carried out to measure the shape of the Cherenkov lateral distribution(CLD) for high energy air shower events that triggered the CYGNUS II array, using six judiciously placed Cherenkov detectors, called the CACTI detectors. The shape of the lateral distribution of Cherenkov light is sensitive to the height of shower maximum, X,,,, which in turn is sensitive
to the
nuclear
species
of the
cosmic
rays
triggering the array. This sensitivity arises because showers induced by Iow mass primary reach shower maximum lower in the atmosphere resulting in a steeper CLD close to the shower core while in showers induced by high mass primaries the expected CLD is much flatter in the same region. At larger lateral distances, the Cherenkov light intensity is less sensitive to X,,, (and hence to nuclear species) and is closely related to total shower energy [1,2] The basic Cherenkov parameters measured by the experiment were the Cherenkov photon densities at 40 m and 140 m from the shower core, called here C40 and C140. The value of Cl40 provides an estimate of the Cherenkov intensity far from the core and is used to estimate shower energy (E 0; C140°.g7 ). The ratio C4O/C140 provides a measure of the CLD steepness and estimates the average depth of maximum (X,,, oc log(C4O/C140). The air shower array is used to *Work supported in part by the National tion and the Department of Energy
Science Founda-
determine the core location to about 5 m for each shower
2. EXPERIMENTAL The CYGNUS II array was located an altitude of 2310m(780 gm cm-*), consisted of 96 scintillator detectors dispersed over 6x lo4 m* and a 70 m* muon detector with an energy threshold of 2 GeV located near the array center[3]. The CACTI experiment was located towards the center of the CYGNUS II array, and consisted of six wide-angle Cherenkov detectors positioned in two distinct groups of three separated by about 150m as shown in figure 1. The design of the CACTI Cherenkov detectors and the data acquisition signal path is shown in figure 2. Each detector consisted of an 8” hemispherical Hamamatsu R1408 photomultiplier tube mounted in a steel housing with a 0.31 sr aperture. The PMT signals were amplified at the detector to reduce the effects of electronic pickup before travelling 150 m of cable. After further amplification and stretching out to about 30 ns, the signals were split to provide a trigger and the other was waveform digitized recording the previous 8 microseconds of each signal waveform in 10 bit 25 ns samples. The energy threshold of the array was 0.3 PeV and the range of energies over which recorded events were clear of trigger biases and waveform digitizer saturation was 1 to 10 PeV. The experiment was operated during dark periods in 1995.
0920-5632/03/$ - see front matter 0 2003 Published by Elsevier Science B.V. doi: lO.l016/SO920-5632(03)02016-4
240
G.B.
Yodh/Nuclear
Physics
B (Pmt.
Suppl.)
122 (2003)
239-242
.Ll6Onl QbC
T Figure 1. A plan view of the CYGNUS and the CACTI Cherenkov array
II array
Over 70,000 events were recorded in 100 hours of observation under good sky conditions (duty cycle of 6 percent). Final selection required that the Cherenkov samples covered a wide range of core distances and that the angle of incidence was within the full view of aperture of the detector. 3. DATA
ANALYSIS
AND
RESULTS
Monte-Carlo shower simulations[4] were used to investigate the performance of the experiment and develop techniques used in data analysis to determine the cosmic ray mass composition between 1 and 10 PeV. Showers of varying primary energy, incident angle and mass were generated and the response of CYGNUS II air shower detectors, the muon detector and the CACTI Cherenkov detectors were modelled, including the effects of core location error, Cherenkov signal measurements and Night Sky Background noise. 3.1.
Comparision of CLD Data and Monte-Carlo
Figure 2. Schematic of the Chereknov detector and data acquisition signal path
distributions
of
For both the CACTI data and simulated data, the individual CLDs were determined for each event by fitting a parametrised function approximating the shape of the CLD distribution to the six recorded Cherenkov signals. From the fit-
ted function the expected Cherenkov signals at 40m (C40) and at 140m (C140) from the shower core were determined with an accuracy of 20% and 10% respectively(limited principally by corelocation error and NSB noise). Figure 3 shows average CLDS for recorded data and simulated data of different primaries with energy between 1 and 3.2 PeV. The observed CLD matches most closely with simulations for iron nuclei with no significant variation observed with energy across the knee. The CLDs for each event have been normalized to an energy of 3.2 PeV(determined from Cl40 and an average zenith angle of 12’. CACTI CLD is significantly flatter than CLDs of simulated primary masses lighter than iron.
G.B. Yodh/Nuclear Physics B (Proc. Suppl.) 122 (2003) 239-242
241
4 0 .
t
. . . ..I
I la
lo2 Core Distance(m)
Figure 4. Comparision of Data and simulations for the C40/140 ratio distributions. Figure 3. Average Cherenkov lateral distributions for simulations and CACTI data normalized to 3.2 PeV
3.2.
Comparision of Steepness C4O/C140 distributions with simulations; mass resolution The average value of the ratio of C40 and Cl40 for CACTI data also agrees with simulations or iron as expected. However the observed spread in the ratio C4O/C140 is significantly larger than would be expected for a pure iron composition, and is consistent with a lighter or mixed composition. Figure 4 shows the distributions of C4O/C140 ratios for CACTI and simulated data for events between 1 and 3.2 PeV again normalized to 3.2 PeV and an average incident angle of 12”. 3.3. Variation of X,,, with energy Figure 5 shows the variation in estimated value of &naz for the CACTI data as a function of energy in comparision to a variety of simulation models and also the previous measurements of Fly’s Eye and Yakutsk at higher energies. Our X maz data appear to agree well with a continuation of higher energy data to lower energies. Our data are in reasonable agreement with HEGRA
data, however BLANCA data and DICE data have systmatically larger X,,, values than ours (these points are not shown in the figure). There is a considerable spread in the predictions of different simulations and the actual conclusions about the mass composition is highly model dependent. 4. acknowledge The work reported here is on behalf of the CYGNUS-CACTI Collaboration. The CYGNUS collaboration consisted of physicists from UC Irvine, UC Santa Cruz, UC Riverside, UC Berkeley, Los Alamos National Lab., University of Maryland and George Mason University. The CACTI detector and simulations were done by Sean Paling and Michael Hillas from the University of Leeds. The work was supported in part by the National Science Foundation and the US Department of Energy. REFERENCES 1. J.R. Patterson and A.M.Hillas, Nucl. Phys. 9(1983) 1433. 2. Sean Paling and A.M.Hillas,
J. Phys.G: Proc.
24th
G.B. Yodh/Nuclcur
Physics B (Pmt. Suppl.) 122 (2003) 239-242
Figure 5. Depth of shower maximum versus primary energy for recorded data and various simulation models. HEGRA, BLANCA and DICE results are not shown. They are systematically larger than for this experiment in the lowest energy range, 1 to 10 PeV
Int. Cosmic Ray Conference, (Rome), 3(1995)508.. 3. R.C.Allen, et al., Nucl. Inst. and Methods, A311(1992)350. 4. A.M.Hillas, Nuclear Physics B(Porc. Supp1.)52B(1997)29.
PROCEEDINGS SUPPLEMENTS www.clscvicr.com/locatc/npc
Composition near the knee: results from the CACTI experiment * Gaurang B. Yodh a aDepartment University
of Physics and Astronomy of California
Irvine
Irvine, CA, USA,92717 This paper presents a summary of the results the CACTI experiment, which studied the shape of Cherenkov lateral distributions of showers triggering the CYGNUS II air shower array at Los Alamos National Laboratory, to investigate the composition of cosmic rays in the PeV energy range.
1. THE
METHOD
An experiment was carried out to measure the shape of the Cherenkov lateral distribution(CLD) for high energy air shower events that triggered the CYGNUS II array, using six judiciously placed Cherenkov detectors, called the CACTI detectors. The shape of the lateral distribution of Cherenkov light is sensitive to the height of shower maximum, X,,,, which in turn is sensitive
to the
nuclear
species
of the
cosmic
rays
triggering the array. This sensitivity arises because showers induced by Iow mass primary reach shower maximum lower in the atmosphere resulting in a steeper CLD close to the shower core while in showers induced by high mass primaries the expected CLD is much flatter in the same region. At larger lateral distances, the Cherenkov light intensity is less sensitive to X,,, (and hence to nuclear species) and is closely related to total shower energy [1,2] The basic Cherenkov parameters measured by the experiment were the Cherenkov photon densities at 40 m and 140 m from the shower core, called here C40 and C140. The value of Cl40 provides an estimate of the Cherenkov intensity far from the core and is used to estimate shower energy (E 0; C140°.g7 ). The ratio C4O/C140 provides a measure of the CLD steepness and estimates the average depth of maximum (X,,, oc log(C4O/C140). The air shower array is used to *Work supported in part by the National tion and the Department of Energy
Science Founda-
determine the core location to about 5 m for each shower
2. EXPERIMENTAL The CYGNUS II array was located an altitude of 2310m(780 gm cm-*), consisted of 96 scintillator detectors dispersed over 6x lo4 m* and a 70 m* muon detector with an energy threshold of 2 GeV located near the array center[3]. The CACTI experiment was located towards the center of the CYGNUS II array, and consisted of six wide-angle Cherenkov detectors positioned in two distinct groups of three separated by about 150m as shown in figure 1. The design of the CACTI Cherenkov detectors and the data acquisition signal path is shown in figure 2. Each detector consisted of an 8” hemispherical Hamamatsu R1408 photomultiplier tube mounted in a steel housing with a 0.31 sr aperture. The PMT signals were amplified at the detector to reduce the effects of electronic pickup before travelling 150 m of cable. After further amplification and stretching out to about 30 ns, the signals were split to provide a trigger and the other was waveform digitized recording the previous 8 microseconds of each signal waveform in 10 bit 25 ns samples. The energy threshold of the array was 0.3 PeV and the range of energies over which recorded events were clear of trigger biases and waveform digitizer saturation was 1 to 10 PeV. The experiment was operated during dark periods in 1995.
0920-5632/03/$ - see front matter 0 2003 Published by Elsevier Science B.V. doi: lO.l016/SO920-5632(03)02016-4
240
G.B.
Yodh/Nuclear
Physics
B (Pmt.
Suppl.)
122 (2003)
239-242
.Ll6Onl QbC
T Figure 1. A plan view of the CYGNUS and the CACTI Cherenkov array
II array
Over 70,000 events were recorded in 100 hours of observation under good sky conditions (duty cycle of 6 percent). Final selection required that the Cherenkov samples covered a wide range of core distances and that the angle of incidence was within the full view of aperture of the detector. 3. DATA
ANALYSIS
AND
RESULTS
Monte-Carlo shower simulations[4] were used to investigate the performance of the experiment and develop techniques used in data analysis to determine the cosmic ray mass composition between 1 and 10 PeV. Showers of varying primary energy, incident angle and mass were generated and the response of CYGNUS II air shower detectors, the muon detector and the CACTI Cherenkov detectors were modelled, including the effects of core location error, Cherenkov signal measurements and Night Sky Background noise. 3.1.
Comparision of CLD Data and Monte-Carlo
Figure 2. Schematic of the Chereknov detector and data acquisition signal path
distributions
of
For both the CACTI data and simulated data, the individual CLDs were determined for each event by fitting a parametrised function approximating the shape of the CLD distribution to the six recorded Cherenkov signals. From the fit-
ted function the expected Cherenkov signals at 40m (C40) and at 140m (C140) from the shower core were determined with an accuracy of 20% and 10% respectively(limited principally by corelocation error and NSB noise). Figure 3 shows average CLDS for recorded data and simulated data of different primaries with energy between 1 and 3.2 PeV. The observed CLD matches most closely with simulations for iron nuclei with no significant variation observed with energy across the knee. The CLDs for each event have been normalized to an energy of 3.2 PeV(determined from Cl40 and an average zenith angle of 12’. CACTI CLD is significantly flatter than CLDs of simulated primary masses lighter than iron.
G.B. Yodh/Nuclear Physics B (Proc. Suppl.) 122 (2003) 239-242
241
4 0 .
t
. . . ..I
I la
lo2 Core Distance(m)
Figure 4. Comparision of Data and simulations for the C40/140 ratio distributions. Figure 3. Average Cherenkov lateral distributions for simulations and CACTI data normalized to 3.2 PeV
3.2.
Comparision of Steepness C4O/C140 distributions with simulations; mass resolution The average value of the ratio of C40 and Cl40 for CACTI data also agrees with simulations or iron as expected. However the observed spread in the ratio C4O/C140 is significantly larger than would be expected for a pure iron composition, and is consistent with a lighter or mixed composition. Figure 4 shows the distributions of C4O/C140 ratios for CACTI and simulated data for events between 1 and 3.2 PeV again normalized to 3.2 PeV and an average incident angle of 12”. 3.3. Variation of X,,, with energy Figure 5 shows the variation in estimated value of &naz for the CACTI data as a function of energy in comparision to a variety of simulation models and also the previous measurements of Fly’s Eye and Yakutsk at higher energies. Our X maz data appear to agree well with a continuation of higher energy data to lower energies. Our data are in reasonable agreement with HEGRA
data, however BLANCA data and DICE data have systmatically larger X,,, values than ours (these points are not shown in the figure). There is a considerable spread in the predictions of different simulations and the actual conclusions about the mass composition is highly model dependent. 4. acknowledge The work reported here is on behalf of the CYGNUS-CACTI Collaboration. The CYGNUS collaboration consisted of physicists from UC Irvine, UC Santa Cruz, UC Riverside, UC Berkeley, Los Alamos National Lab., University of Maryland and George Mason University. The CACTI detector and simulations were done by Sean Paling and Michael Hillas from the University of Leeds. The work was supported in part by the National Science Foundation and the US Department of Energy. REFERENCES 1. J.R. Patterson and A.M.Hillas, Nucl. Phys. 9(1983) 1433. 2. Sean Paling and A.M.Hillas,
J. Phys.G: Proc.
24th
G.B. Yodh/Nuclcur
Physics B (Pmt. Suppl.) 122 (2003) 239-242
Figure 5. Depth of shower maximum versus primary energy for recorded data and various simulation models. HEGRA, BLANCA and DICE results are not shown. They are systematically larger than for this experiment in the lowest energy range, 1 to 10 PeV
Int. Cosmic Ray Conference, (Rome), 3(1995)508.. 3. R.C.Allen, et al., Nucl. Inst. and Methods, A311(1992)350. 4. A.M.Hillas, Nuclear Physics B(Porc. Supp1.)52B(1997)29.