- Email: [email protected]
An experiment to search for ultra high energy γ-ray sources from the south pole
622
Nuclear Instruments and Methods in Physics Research A276 (1989) 622 627 North-Holland, Amsterdam
AN EXPERIMENT TO SEARCH FROM THE SOUTH POLE
FOR ULTRA
HIGH
ENERGY
-/-RAY SOURCES
N.J.T. SMITH
Amundsen- Scott Station, South Pole, Antarctica
J.C. P E R R E T T
and M.A. POMERANTZ
Bartol Research Institute, University of Delaware, Newark, DE 19716, USA A.M. HILLAS,
P.A. O G D E N ,
M. PATEL, R.J.O. REID
and A.A. WATSON
Department of Physics, University of Leeds, Leeds, LS2 9JT, UK Received 7 November 1988
We describe the construction and performance characteristics of an extensive air shower array which has been established at the geographic South Pole. The experiment has been designed to search for sources for cosmic rays with primary energies above 50 TeV with an angular resolution of about 1 °. The array has an enclosed area of 6235 m 2 and is at an altitude of 2835 m (695 g c m - 2 ). The unique advantage of the site is the circumpolar nature of all candidate sources, including SN1987A, which lie at a constant zenith angle.
1. Introduction There is considerable c o n t e m p o r a r y interest in the construction a n d operation of extensive air shower arrays for the detection of emission of ultra high energy "y-rays from compact astrophysical objects such as X-ray b i n a r y systems. W e have followed the suggestion of Hillas [1] a n d established a n air shower array at the geographic South Pole for the detection of cosmic rays with primary energies above 50 TeV. The u n i q u e advantages of the location are the altitude of the site, 2835 m (695 g c m - 2 ) , the circumpolar nature of all candidate sources, all of which lie at a c o n s t a n t zenith angle, a n d the n u m e r o u s X-ray b i n a r y systems which lie within 50 o of the polar zenith. In addition the recent supernova, SN1987A, which is a candidate 50 TeV source [2], lies at a c o n s t a n t zenith angle of a b o u t 2l °. In this p a p e r we describe details of the South Pole Air Shower Experiment (SPASE) together with the initial assessment of its performance, particularly with respect to the angular resolution of the instrument.
2. Array description and detector features The experiment, which is a collaboration between the Bartol Research Institute at the University of Delaware U S A a n d the Leeds University Physics Depart0 1 6 8 - 9 0 0 2 / 8 9 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
ment U K , has been established on the A n t a r c t i c Plateau at the US A m u n d s e n - S c o t t South Pole Station. The layout of the 16 scintillation detectors is shown in fig. 1. The detectors are disposed on a 30 m triangular grid enclosing an area of 6235 m 2. A schematic diagram of a detector is given in fig. 2. Each detector m o d u l e contains four 0.25 m 2 blocks of 10 cm thick plastic scintil-
•
•
4
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2
7~8
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~
•3
RECORDING HUT ~J • 14
SOUTHPOLE~ t~, 20Ore) GREENWICH MERIDIAN
• 12
91~10
_.
16
15
30m
~.
13
Fig. 1. The 16 detectors in the SPASE array are disposed on a 30 m triangular grid at 14 unique positions. There are two pairs of adjacent detectors used to investigate fluctuations in time and density measurements. The recording electronics is kept at room temperature in a hut at the centre of the array. The signal, EHT and fibre optic cable from each detector to the recording hut is buried under about 0.3 m of snow to keep any temperature variations to a minimum,
N.J. T. Smith et al. / Search for ultra high energy "f -ray sources
ALUMINIZEDMYLAR SHEET •
Im ,-
~l
BLACK ~ , ~ INTERIOR ~U-METAL FIBRE IELD, , OPTIC
7 r-,
....0o.,° EGS
'
Fig. 2. The detector sits on four extendable legs so that as the snow level rises, due to snow accumulation, the detectors can be raised. The 3 in. PMT views the scintillator from below to remove the possibility of occasional prepulses due to Cherenkov emission in the glass face of the PMT. The inside of the detector is painted black, with the exception of a sheet of aluminized Mylar above the scintillator, so that only prompt light reaches the PMT and the delayed light due to multiple reflections is absorbed.
lator made in the early 1960s by Prof. J. Linsley using the method described by Clark et al. [3]: the scintillators were loaned to us by Linsley for this experiment. The scintillator is viewed, from below, by a 3 in. EMI 9281B photomultiplier (PMT) with a bialkali photocathode surrounded by a mu-metal shield (EMI type PS7B). The PMTs, which have a rise time of 2.5 ns, where evaluated for their performance down to - 8 0 ° C [4]. The mean monthly temperature ranges from - 20 o C to - 65 o C. The boxes, housing the scintillator and PMT, are mounted on four extendable tubular legs so that they can be raised when show accumulation ( - 0 . 3 m per year) requires it. At the start of the 1988 Austral winter the scintillator in each detector was approximately 2 m above the show level. After the first year of operation each detector was be covered with 6 m m of lead to take advantage of the multiplication of energetic electrons in the shower and of the conversion of photons above 1 MeV to improve the angular resolution of the telescope [5,61.
623
3. System electronics The electronics system used for SPASE is based on the design that had been successfully implemented at Haverah Park of the G R E X "t-ray telescope [7] and will only be described in outline here. All signal processing is done in a building held at room temperature at the array centre. At each detector there is a headunit with a dynode chain and PMT. E H T is distributed to the detectors from the centre along E H T cable (Suhner GX3272) and signals are transmitted to the processing electronics via high bandwidth cable (Delta Enfield TR233). All cables are buried under about 0.3 m of show to keep temperature variations to a minimum. Signals received from the detectors are split three ways. One is input to a low level discriminator, D1, set at a third of a particle: a particle is defined as the signal detected from a fully traversing relativistic particle. The output from the D1 discriminator is input into LeCroy 4208 TDCs. These have 1 ns timing resolution with the capability to measure signals positive and negative with respect to a c o m m o n signal. The second signal is input to a high level discriminator, D2, set at 1 particle. If five D2 signals occur in 1 I~s then a shower trigger is produced. The relative arrival times at each detector are measured at the D1 level. The third split signal is input into an l l - b i t charge integrating LeCroy 4300 A D C giving pulse height information. The density recording system saturates at - 150 m -2. When a shower trigger is produced the event time is latched. The system clock has a period of 100 ~s and is synchronised every hour by a signal from a rubidium clock housed in the main South Pole building complex 300 m away. These data along with the pulse height and time information are transmitted to the control computer ( U M A N 1000 32-bit computer running under O S / 9 ) , via an inhouse built fast transfer (FT) module [8] in 1 ms. The FT module controls the sequencing of data and commands to the LeCroy modules via a C A M A C data bus without the direct intervention of the computer. It dumps data into an inhouse designed G P I B deep fifo memory which has the capacity to store nearly 90 events. The computer can then interrogate this information while the FT module waits for the next event. In this way events that are separated by a millisecond can be recorded even though it takes the computer about 0.5 s to process an event. These data and a simple on-line analysis, arrival direction using a plane shower front fit and a simple centre of mass core location, are transferred to magnetic tape (Thorn E M I 0.5 in. 9900 streamer tape deck) in a compressed format. The SPASE system records about 0.6 million events, just over 70 Mbytes of data, a week. In addition to the event data the D1 and D2 rates for all channels along with timing calibration data, described below, are dumped to tape every 100 min.
624
N.J.T. Smith et al. / Search for ultra high energy 7 -r~(v source.s
4. System calibrations Before data from the array can be analysed three i m p o r t a n t m e a s u r e m e n t s / c a l i b r a t i o n s must be made. These are: the coordinates of the detectors with the orientation of the axes with respect to the G r e e n w i c h meridian, the relative system delays between detector channels and the value of the single particle used in the analysis to calculate the density at a detector. The coordinates of each detector were d e t e r m i n e d using a theodolite and a distance meter (Hewl e t t - P a c k a r d 3808A). The relative distances between detectors were measured to better t h a n 1 cm. The orientation of the array was d e t e r m i n e d with respect to the Greenwich meridian using a sun-dial technique in the early s u m m e r a n d using the transit of the Sun with lines of detectors as it set towards the beginning of winter. The o r i e n t a t i o n is accurate to within 0.2 o. The Antarctic Plateau is k n o w n to drift by a b o u t 10 m per year so it will be necessary for the array to be resurveyed annually to look for any changes in detector separation or changes of orientation with respect to the Greenwich meridian. The most critical part of the shower m e a s u r e m e n t with this i n s t r u m e n t is the d e t e r m i n a t i o n of the relative arrival times of the shower front at each detector. It is therefore necessary to determine the total relative system delay for each channel. The total delay has four c o m p o n e n t s : the electronic cable lengths, tube transit time and p r o p a g a t i o n delays through the T D C modules a n d discriminator electronics, respectively. The cable lengths were measured to 0.03 ns by measuring the frequency of standing waves w h e n the signal cable was shorted at one end [9]. The tube transit times were measured with respect to a s t a n d a r d tube using one of the scintillator boxes which was fitted with two tubes looking at the same scintillator blocks. These times were measured using the SPASE recording system. The variation in transit time as a function of P M T E H T was found to be - 1 . 1 8 n s / 1 0 0 V: this figure was used to adjust the time delays whenever a n E H T adjustment was required. The T D C and discriminator delays were similarly measured. As a further check on the stability of the timing channels the array was equipped with a 0.5 m W green He Ne laser ( M e l l e s - G r i o t ) as first i m p l e m e n t e d in the Haverah Park G R E X telescope [10]. The o u t p u t from the laser is m o d u l a t e d by a POckel Cell triggered by a 5 kV step generator (Optronics SPG5000). The resulting light pulse has a risetime of 1.5 ns a n d a width of 10 ns. This is transmitted by 5 0 / 1 2 5 graded index fibre optic cable to each detector. The relative arrival times of this light pulse at each detector can be used to look for drifts in overall inherent delays. With a similar system
at H a v e r a h Park drifts as small as 0.25 n s / m o m h wouM have been detected. A single particle peak was not clearly visible in the ungated spectrum from the detectors so d e t e r m i n i n g the b a c k g r o u n d rate above a specific density a n d setting thresholds for discriminators was not straight forward. The b a c k g r o u n d rate was determined from measurem e n t s with a 10 × 10 cm 2 block of scintillator in close proximity to a PMT. The single particle peak stood away from the noise in this setup. The D2 discriminators were then adjusted to give the same rate per unit area as the small block. This rate was approximately three times that at sea level for a similar setup. The D1 discriminators were then set to 1 / 3 the signal size of the D2 threshold. All D1 a n d D2 discriminators were set to the same voltage level for each detector channel. The D1 a n d D2 rates were 724 Hz _+8% a n d 271 Hz + 8 % respectively, The rates have remained stable throughout the first eight m o n t h s of operation to within this precision. Using two small scintilation detectors (100 cm 2 scintillator) a " M u o n Telescope" was constructed and used to gate the signals from a detector so that the single particle peak could be measured at various positions across the scintillator. The detector response to fully traversing particles was f o u n d to be uniform to better t h a n 20%. The weighted single particle, as would be observed if a good ungated spectrum was observed, agreed with that deduced from the b a c k g r o u n d rate of the small block.
5. Prelimina~ results The array started recorded on 21st D e c e m b e r 1987 with approximately 40% on time until late J a n u a r y while modifications a n d calibrations were being made to the array. The South Pole station was closed for the winter on the 16th F e b r u a r y 1988 and from then ran with > 95% o n t i m e t h r o u g h o u t the winter. Approximately 2 million events, which were recorded before station closing, having been analysed [11]. These d a t a have been used to check various properties of the array, including its angular resolution, time a n d density error functions a n d the sensitivity to different source regions. Checks can be m a d e t h r o u g h o u t the year as a b o u t 1-2% of the data are t r a n s m i t t e d via the c o m p u t e r and satellite link every week. In this way the winterover scientist gets input from the rest of the SPASE collaboration a b o u t the p e r f o r m a n c e of the array a n d can correct any problems that arise with a 2 3 week turn around. In practice the level of p e r f o r m ance has been so high that this facility has so far been little used.
N.J.T. Smith et al. / Search for ultra high energy "y-ray sources
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The zenith angle, 0, distribution of analysed events is shown in fig, 3. At the South Pole the declination of a star is 0 - 90 o and so on the same diagram we indicate the position of some interesting sources. SN1987A is in an ideal position for observation. When two adjacent detectors, e.g. 7 and 8, are compared from the viewpoint of time differences, the observed spreads reflect the angular distribution of the showers triggering the detectors and the thickness of the shower disc (which is a function of core distance and zenith angle) as well as the instrumental performance. The error in time measurement, as a function of density, deduced from detectors 7 and 8 is shown in fig. 4. The arrival time error along with the density error deduced from 7 and 8 is then used to weight detector times and densities in the full analysis.
We have used two independent fitting programs to assign shower directions, In both cases the basic strategy is to locate the shower core and to find a best fit of the times recorded by the detectors to a curved shower front by an iterative procedure, with the shower direction (zenith and azimuth angle) as the free parameters. Only showers with cores inside the array are included in the analysis because their directions can be more reliably determined. The Leeds analysis uses shower curvature determined from a full Monte Carlo simulation of photon showers. The Bartol analysis uses a curvature determined experimentally by studying the residuals of fits to plane shower fronts. The two programs also use different algorithms to assign weights to the times of the various detectors depending on their pulse heights, Given an algorithm for assigning shower direction, it is possible to explore the angular resolution experimentally by dividing the array into two independent subarrays and then constructing the distribution of differences in space angle, ,P. Fig. 5 shows this distribution for the subarrays 4, 7, 9, 14 and 5, 8, 11, 12. The value of q'~s measured from this distribution is 2.6 °. It is generally supposed that, to find the resolution of the whole array, one should divide the width, 'Prms, of the difference distribution by 2. One factor of ~ - is due to the increased number of times available and another factor of v~- to decreased Poissonian fluctuations. This would make the angular resolution 1.3 ° for showers satisfying the selection criterion given in fig 5. Simulations give useful insight into this question. With simulated showers, where the true direction is known, one can compare the result of the subarray analysis (as applied to simulated showers) with the angular resolution of the array determined by using the same direc-
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Fig. 5. The space angle (~/') in the arrival direction deduced from two independent subarrays (4, 7, 9, 14 and 5. 8. 11 and 12). The showers used in this analysis have 0 _<30, r ~<45 m and all 8 detectors had to trigger at >_l rn 2. The rms value of ~P is 2.6°. Thus the angular resolution for this geometry of showers at a median energy of 100 TeV is 1.3 °.
626
N.J. 72 Smith et a L / Search for ultra high energy y-rav sources I
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The array operated satisfactorily during its first nine months and for half that time the air temperature was less than - 6 0 o C, close to the minimum temperature which normally occurs towards the end of the Austral winter. We therefore feel confident that the experiment will run through the next few years without any major difficulties and that the problems of running an air shower experiment in the Antarctic have been solved. The majority of the data arrived at Leeds and Bartol at the end of 1988 when the main analysis and a serious search for cosmic ray point sources was started. Initial results show that the angular resolution is of the order of a degree and that the array is behaving in a stable manner despite the harsh Antarctic conditions.
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tion-finding algorithm on the whole array. Fig. 6 shows the results of such a comparison made with the Leeds analysis. Two points are apparent. First, the subarray analysis indeed give a difference approximately equal to twice the angular resolution of the whole array. Second, the resolution steadily deteriorates as the primary energy decreases: the calculated value of ~rm~ is approximately proportional to $30°5, where $30 is the density recorded at 30 m from the shower core. Analysis of simulated showers with the Leeds scheme, coupled with checks between the consistency of subarrays using both real and simulated shows, indicates that the rms error in space angle averaged over the expected size distribution of photon showers falling within 45 m of the center is expected to be 1.1 ° for photon showers. Internal consistency checks of the Bartol analysis using subarrays indicates an rms error in space angle of 1.3 ° for ordinary hadronic showers, which is quite consistent with 1.1 ° for photon showers. Although the conclusion of both the Bartol and the Leeds analysis schemes is that the overall angular resolution of the array for photon showers is = 1.1 °, there are still differences of comparable magnitude between the directions assigned by the two schemes to individual showers. It is expected that the differences can be greatly reduced when the structure of the shower front has been more fully investigated, as this determines the weight given to each timing measurement. The value of having two analyses starting from different assumptions is clear to us.
Acknowledgements Many people have helped in the setup phase of the experiment. The SPASE collaboration are indebted to the following: The N S F support under grant DPP8613231; I T T / A n t a r c t i c Services for constructing the detectors and providing invaluable help and assistance in setting up the experiment at the South Pole and especially R. Burdie for his help in making the detector modules light tight and E. Siefka for help in setting up our computer networks and transmission of data of Leeds and Bartol; Staff at Leeds University with special thanks to S.D. Bloomer acting as intermediary between the South Pole scientists and our Computer Manufacturer, J. MacMillan for doing environmental cold tests on our equipment. D. Pearce, A. Price and G. Wright for construction of components of the experiment. Staff at the BarIol Research Institute and in particular R. Pfeiffer for design of the detector and L. Shulman for help and advice on suitability of components of the experiment. Finally, we express our deep appreciation to John Linsley for making available to us the large number of scintillator blocks that the experiment requires. This was, indeed, crucial for implementing SPASE. We also thank Derek Swinson for carefully checking the scintillators and preparing them for shipment to Antarctica.
References [1] A.M. Hillas, Proc. Vulcano Workship on H E - U H E Behaviour of Accreting X-Ray Sources (1986) p. 331. [2] T. Gaisser, A. Harding and T. Stanev, Nature 329 (1987) 314. [3] Clark et al., Rev. Sci. Instr. 28 (1959) 43. [4] J.E. MacMillan and R.J.O. Reid, submitted to J. Phys. E.
N.J. T. Smith et al. / Search for ultra high energy T -ray sources
[5] J. Linsley, private communication (1986); and Proc, 20th Int. Cosmic Ray Conf., Moscow, Vol. 2 (1987) p. 442. [6] S.D. Bloomer, J. Linsley and A.A. Watson, J. Phys. G14 (1988) 645. [7] Eames et al., in preparation; Brooke et al., Proc. 19th Int. Cosmic Ray Conf., La Jolla vol. 3 (1985) p. 426.
[8] [9] [10] [11] [12]
627
Patel et al,, in preparation. G. Fidercaro, Suppl. Nuovo Cim. 2 (1960) 254. Patel et al,, in preparation. Gaisser et al., submitted to Phys. Rev. Lett, Bloomer et al., in preparation; Eames et al. Proc. 20th Int. Cosmic Ray Conf., Moscow, vol. 2 (1987) p. 449.
Nuclear Instruments and Methods in Physics Research A276 (1989) 622 627 North-Holland, Amsterdam
AN EXPERIMENT TO SEARCH FROM THE SOUTH POLE
FOR ULTRA
HIGH
ENERGY
-/-RAY SOURCES
N.J.T. SMITH
Amundsen- Scott Station, South Pole, Antarctica
J.C. P E R R E T T
and M.A. POMERANTZ
Bartol Research Institute, University of Delaware, Newark, DE 19716, USA A.M. HILLAS,
P.A. O G D E N ,
M. PATEL, R.J.O. REID
and A.A. WATSON
Department of Physics, University of Leeds, Leeds, LS2 9JT, UK Received 7 November 1988
We describe the construction and performance characteristics of an extensive air shower array which has been established at the geographic South Pole. The experiment has been designed to search for sources for cosmic rays with primary energies above 50 TeV with an angular resolution of about 1 °. The array has an enclosed area of 6235 m 2 and is at an altitude of 2835 m (695 g c m - 2 ). The unique advantage of the site is the circumpolar nature of all candidate sources, including SN1987A, which lie at a constant zenith angle.
1. Introduction There is considerable c o n t e m p o r a r y interest in the construction a n d operation of extensive air shower arrays for the detection of emission of ultra high energy "y-rays from compact astrophysical objects such as X-ray b i n a r y systems. W e have followed the suggestion of Hillas [1] a n d established a n air shower array at the geographic South Pole for the detection of cosmic rays with primary energies above 50 TeV. The u n i q u e advantages of the location are the altitude of the site, 2835 m (695 g c m - 2 ) , the circumpolar nature of all candidate sources, all of which lie at a c o n s t a n t zenith angle, a n d the n u m e r o u s X-ray b i n a r y systems which lie within 50 o of the polar zenith. In addition the recent supernova, SN1987A, which is a candidate 50 TeV source [2], lies at a c o n s t a n t zenith angle of a b o u t 2l °. In this p a p e r we describe details of the South Pole Air Shower Experiment (SPASE) together with the initial assessment of its performance, particularly with respect to the angular resolution of the instrument.
2. Array description and detector features The experiment, which is a collaboration between the Bartol Research Institute at the University of Delaware U S A a n d the Leeds University Physics Depart0 1 6 8 - 9 0 0 2 / 8 9 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
ment U K , has been established on the A n t a r c t i c Plateau at the US A m u n d s e n - S c o t t South Pole Station. The layout of the 16 scintillation detectors is shown in fig. 1. The detectors are disposed on a 30 m triangular grid enclosing an area of 6235 m 2. A schematic diagram of a detector is given in fig. 2. Each detector m o d u l e contains four 0.25 m 2 blocks of 10 cm thick plastic scintil-
•
•
4
II
2
7~8
I
~
•3
RECORDING HUT ~J • 14
SOUTHPOLE~ t~, 20Ore) GREENWICH MERIDIAN
• 12
91~10
_.
16
15
30m
~.
13
Fig. 1. The 16 detectors in the SPASE array are disposed on a 30 m triangular grid at 14 unique positions. There are two pairs of adjacent detectors used to investigate fluctuations in time and density measurements. The recording electronics is kept at room temperature in a hut at the centre of the array. The signal, EHT and fibre optic cable from each detector to the recording hut is buried under about 0.3 m of snow to keep any temperature variations to a minimum,
N.J. T. Smith et al. / Search for ultra high energy "f -ray sources
ALUMINIZEDMYLAR SHEET •
Im ,-
~l
BLACK ~ , ~ INTERIOR ~U-METAL FIBRE IELD, , OPTIC
7 r-,
....0o.,° EGS
'
Fig. 2. The detector sits on four extendable legs so that as the snow level rises, due to snow accumulation, the detectors can be raised. The 3 in. PMT views the scintillator from below to remove the possibility of occasional prepulses due to Cherenkov emission in the glass face of the PMT. The inside of the detector is painted black, with the exception of a sheet of aluminized Mylar above the scintillator, so that only prompt light reaches the PMT and the delayed light due to multiple reflections is absorbed.
lator made in the early 1960s by Prof. J. Linsley using the method described by Clark et al. [3]: the scintillators were loaned to us by Linsley for this experiment. The scintillator is viewed, from below, by a 3 in. EMI 9281B photomultiplier (PMT) with a bialkali photocathode surrounded by a mu-metal shield (EMI type PS7B). The PMTs, which have a rise time of 2.5 ns, where evaluated for their performance down to - 8 0 ° C [4]. The mean monthly temperature ranges from - 20 o C to - 65 o C. The boxes, housing the scintillator and PMT, are mounted on four extendable tubular legs so that they can be raised when show accumulation ( - 0 . 3 m per year) requires it. At the start of the 1988 Austral winter the scintillator in each detector was approximately 2 m above the show level. After the first year of operation each detector was be covered with 6 m m of lead to take advantage of the multiplication of energetic electrons in the shower and of the conversion of photons above 1 MeV to improve the angular resolution of the telescope [5,61.
623
3. System electronics The electronics system used for SPASE is based on the design that had been successfully implemented at Haverah Park of the G R E X "t-ray telescope [7] and will only be described in outline here. All signal processing is done in a building held at room temperature at the array centre. At each detector there is a headunit with a dynode chain and PMT. E H T is distributed to the detectors from the centre along E H T cable (Suhner GX3272) and signals are transmitted to the processing electronics via high bandwidth cable (Delta Enfield TR233). All cables are buried under about 0.3 m of show to keep temperature variations to a minimum. Signals received from the detectors are split three ways. One is input to a low level discriminator, D1, set at a third of a particle: a particle is defined as the signal detected from a fully traversing relativistic particle. The output from the D1 discriminator is input into LeCroy 4208 TDCs. These have 1 ns timing resolution with the capability to measure signals positive and negative with respect to a c o m m o n signal. The second signal is input to a high level discriminator, D2, set at 1 particle. If five D2 signals occur in 1 I~s then a shower trigger is produced. The relative arrival times at each detector are measured at the D1 level. The third split signal is input into an l l - b i t charge integrating LeCroy 4300 A D C giving pulse height information. The density recording system saturates at - 150 m -2. When a shower trigger is produced the event time is latched. The system clock has a period of 100 ~s and is synchronised every hour by a signal from a rubidium clock housed in the main South Pole building complex 300 m away. These data along with the pulse height and time information are transmitted to the control computer ( U M A N 1000 32-bit computer running under O S / 9 ) , via an inhouse built fast transfer (FT) module [8] in 1 ms. The FT module controls the sequencing of data and commands to the LeCroy modules via a C A M A C data bus without the direct intervention of the computer. It dumps data into an inhouse designed G P I B deep fifo memory which has the capacity to store nearly 90 events. The computer can then interrogate this information while the FT module waits for the next event. In this way events that are separated by a millisecond can be recorded even though it takes the computer about 0.5 s to process an event. These data and a simple on-line analysis, arrival direction using a plane shower front fit and a simple centre of mass core location, are transferred to magnetic tape (Thorn E M I 0.5 in. 9900 streamer tape deck) in a compressed format. The SPASE system records about 0.6 million events, just over 70 Mbytes of data, a week. In addition to the event data the D1 and D2 rates for all channels along with timing calibration data, described below, are dumped to tape every 100 min.
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N.J.T. Smith et al. / Search for ultra high energy 7 -r~(v source.s
4. System calibrations Before data from the array can be analysed three i m p o r t a n t m e a s u r e m e n t s / c a l i b r a t i o n s must be made. These are: the coordinates of the detectors with the orientation of the axes with respect to the G r e e n w i c h meridian, the relative system delays between detector channels and the value of the single particle used in the analysis to calculate the density at a detector. The coordinates of each detector were d e t e r m i n e d using a theodolite and a distance meter (Hewl e t t - P a c k a r d 3808A). The relative distances between detectors were measured to better t h a n 1 cm. The orientation of the array was d e t e r m i n e d with respect to the Greenwich meridian using a sun-dial technique in the early s u m m e r a n d using the transit of the Sun with lines of detectors as it set towards the beginning of winter. The o r i e n t a t i o n is accurate to within 0.2 o. The Antarctic Plateau is k n o w n to drift by a b o u t 10 m per year so it will be necessary for the array to be resurveyed annually to look for any changes in detector separation or changes of orientation with respect to the Greenwich meridian. The most critical part of the shower m e a s u r e m e n t with this i n s t r u m e n t is the d e t e r m i n a t i o n of the relative arrival times of the shower front at each detector. It is therefore necessary to determine the total relative system delay for each channel. The total delay has four c o m p o n e n t s : the electronic cable lengths, tube transit time and p r o p a g a t i o n delays through the T D C modules a n d discriminator electronics, respectively. The cable lengths were measured to 0.03 ns by measuring the frequency of standing waves w h e n the signal cable was shorted at one end [9]. The tube transit times were measured with respect to a s t a n d a r d tube using one of the scintillator boxes which was fitted with two tubes looking at the same scintillator blocks. These times were measured using the SPASE recording system. The variation in transit time as a function of P M T E H T was found to be - 1 . 1 8 n s / 1 0 0 V: this figure was used to adjust the time delays whenever a n E H T adjustment was required. The T D C and discriminator delays were similarly measured. As a further check on the stability of the timing channels the array was equipped with a 0.5 m W green He Ne laser ( M e l l e s - G r i o t ) as first i m p l e m e n t e d in the Haverah Park G R E X telescope [10]. The o u t p u t from the laser is m o d u l a t e d by a POckel Cell triggered by a 5 kV step generator (Optronics SPG5000). The resulting light pulse has a risetime of 1.5 ns a n d a width of 10 ns. This is transmitted by 5 0 / 1 2 5 graded index fibre optic cable to each detector. The relative arrival times of this light pulse at each detector can be used to look for drifts in overall inherent delays. With a similar system
at H a v e r a h Park drifts as small as 0.25 n s / m o m h wouM have been detected. A single particle peak was not clearly visible in the ungated spectrum from the detectors so d e t e r m i n i n g the b a c k g r o u n d rate above a specific density a n d setting thresholds for discriminators was not straight forward. The b a c k g r o u n d rate was determined from measurem e n t s with a 10 × 10 cm 2 block of scintillator in close proximity to a PMT. The single particle peak stood away from the noise in this setup. The D2 discriminators were then adjusted to give the same rate per unit area as the small block. This rate was approximately three times that at sea level for a similar setup. The D1 discriminators were then set to 1 / 3 the signal size of the D2 threshold. All D1 a n d D2 discriminators were set to the same voltage level for each detector channel. The D1 a n d D2 rates were 724 Hz _+8% a n d 271 Hz + 8 % respectively, The rates have remained stable throughout the first eight m o n t h s of operation to within this precision. Using two small scintilation detectors (100 cm 2 scintillator) a " M u o n Telescope" was constructed and used to gate the signals from a detector so that the single particle peak could be measured at various positions across the scintillator. The detector response to fully traversing particles was f o u n d to be uniform to better t h a n 20%. The weighted single particle, as would be observed if a good ungated spectrum was observed, agreed with that deduced from the b a c k g r o u n d rate of the small block.
5. Prelimina~ results The array started recorded on 21st D e c e m b e r 1987 with approximately 40% on time until late J a n u a r y while modifications a n d calibrations were being made to the array. The South Pole station was closed for the winter on the 16th F e b r u a r y 1988 and from then ran with > 95% o n t i m e t h r o u g h o u t the winter. Approximately 2 million events, which were recorded before station closing, having been analysed [11]. These d a t a have been used to check various properties of the array, including its angular resolution, time a n d density error functions a n d the sensitivity to different source regions. Checks can be m a d e t h r o u g h o u t the year as a b o u t 1-2% of the data are t r a n s m i t t e d via the c o m p u t e r and satellite link every week. In this way the winterover scientist gets input from the rest of the SPASE collaboration a b o u t the p e r f o r m a n c e of the array a n d can correct any problems that arise with a 2 3 week turn around. In practice the level of p e r f o r m ance has been so high that this facility has so far been little used.
N.J.T. Smith et al. / Search for ultra high energy "y-ray sources
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The zenith angle, 0, distribution of analysed events is shown in fig, 3. At the South Pole the declination of a star is 0 - 90 o and so on the same diagram we indicate the position of some interesting sources. SN1987A is in an ideal position for observation. When two adjacent detectors, e.g. 7 and 8, are compared from the viewpoint of time differences, the observed spreads reflect the angular distribution of the showers triggering the detectors and the thickness of the shower disc (which is a function of core distance and zenith angle) as well as the instrumental performance. The error in time measurement, as a function of density, deduced from detectors 7 and 8 is shown in fig. 4. The arrival time error along with the density error deduced from 7 and 8 is then used to weight detector times and densities in the full analysis.
We have used two independent fitting programs to assign shower directions, In both cases the basic strategy is to locate the shower core and to find a best fit of the times recorded by the detectors to a curved shower front by an iterative procedure, with the shower direction (zenith and azimuth angle) as the free parameters. Only showers with cores inside the array are included in the analysis because their directions can be more reliably determined. The Leeds analysis uses shower curvature determined from a full Monte Carlo simulation of photon showers. The Bartol analysis uses a curvature determined experimentally by studying the residuals of fits to plane shower fronts. The two programs also use different algorithms to assign weights to the times of the various detectors depending on their pulse heights, Given an algorithm for assigning shower direction, it is possible to explore the angular resolution experimentally by dividing the array into two independent subarrays and then constructing the distribution of differences in space angle, ,P. Fig. 5 shows this distribution for the subarrays 4, 7, 9, 14 and 5, 8, 11, 12. The value of q'~s measured from this distribution is 2.6 °. It is generally supposed that, to find the resolution of the whole array, one should divide the width, 'Prms, of the difference distribution by 2. One factor of ~ - is due to the increased number of times available and another factor of v~- to decreased Poissonian fluctuations. This would make the angular resolution 1.3 ° for showers satisfying the selection criterion given in fig 5. Simulations give useful insight into this question. With simulated showers, where the true direction is known, one can compare the result of the subarray analysis (as applied to simulated showers) with the angular resolution of the array determined by using the same direc-
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The array operated satisfactorily during its first nine months and for half that time the air temperature was less than - 6 0 o C, close to the minimum temperature which normally occurs towards the end of the Austral winter. We therefore feel confident that the experiment will run through the next few years without any major difficulties and that the problems of running an air shower experiment in the Antarctic have been solved. The majority of the data arrived at Leeds and Bartol at the end of 1988 when the main analysis and a serious search for cosmic ray point sources was started. Initial results show that the angular resolution is of the order of a degree and that the array is behaving in a stable manner despite the harsh Antarctic conditions.
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tion-finding algorithm on the whole array. Fig. 6 shows the results of such a comparison made with the Leeds analysis. Two points are apparent. First, the subarray analysis indeed give a difference approximately equal to twice the angular resolution of the whole array. Second, the resolution steadily deteriorates as the primary energy decreases: the calculated value of ~rm~ is approximately proportional to $30°5, where $30 is the density recorded at 30 m from the shower core. Analysis of simulated showers with the Leeds scheme, coupled with checks between the consistency of subarrays using both real and simulated shows, indicates that the rms error in space angle averaged over the expected size distribution of photon showers falling within 45 m of the center is expected to be 1.1 ° for photon showers. Internal consistency checks of the Bartol analysis using subarrays indicates an rms error in space angle of 1.3 ° for ordinary hadronic showers, which is quite consistent with 1.1 ° for photon showers. Although the conclusion of both the Bartol and the Leeds analysis schemes is that the overall angular resolution of the array for photon showers is = 1.1 °, there are still differences of comparable magnitude between the directions assigned by the two schemes to individual showers. It is expected that the differences can be greatly reduced when the structure of the shower front has been more fully investigated, as this determines the weight given to each timing measurement. The value of having two analyses starting from different assumptions is clear to us.
Acknowledgements Many people have helped in the setup phase of the experiment. The SPASE collaboration are indebted to the following: The N S F support under grant DPP8613231; I T T / A n t a r c t i c Services for constructing the detectors and providing invaluable help and assistance in setting up the experiment at the South Pole and especially R. Burdie for his help in making the detector modules light tight and E. Siefka for help in setting up our computer networks and transmission of data of Leeds and Bartol; Staff at Leeds University with special thanks to S.D. Bloomer acting as intermediary between the South Pole scientists and our Computer Manufacturer, J. MacMillan for doing environmental cold tests on our equipment. D. Pearce, A. Price and G. Wright for construction of components of the experiment. Staff at the BarIol Research Institute and in particular R. Pfeiffer for design of the detector and L. Shulman for help and advice on suitability of components of the experiment. Finally, we express our deep appreciation to John Linsley for making available to us the large number of scintillator blocks that the experiment requires. This was, indeed, crucial for implementing SPASE. We also thank Derek Swinson for carefully checking the scintillators and preparing them for shipment to Antarctica.
References [1] A.M. Hillas, Proc. Vulcano Workship on H E - U H E Behaviour of Accreting X-Ray Sources (1986) p. 331. [2] T. Gaisser, A. Harding and T. Stanev, Nature 329 (1987) 314. [3] Clark et al., Rev. Sci. Instr. 28 (1959) 43. [4] J.E. MacMillan and R.J.O. Reid, submitted to J. Phys. E.
N.J. T. Smith et al. / Search for ultra high energy T -ray sources
[5] J. Linsley, private communication (1986); and Proc, 20th Int. Cosmic Ray Conf., Moscow, Vol. 2 (1987) p. 442. [6] S.D. Bloomer, J. Linsley and A.A. Watson, J. Phys. G14 (1988) 645. [7] Eames et al., in preparation; Brooke et al., Proc. 19th Int. Cosmic Ray Conf., La Jolla vol. 3 (1985) p. 426.
[8] [9] [10] [11] [12]
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Patel et al,, in preparation. G. Fidercaro, Suppl. Nuovo Cim. 2 (1960) 254. Patel et al,, in preparation. Gaisser et al., submitted to Phys. Rev. Lett, Bloomer et al., in preparation; Eames et al. Proc. 20th Int. Cosmic Ray Conf., Moscow, vol. 2 (1987) p. 449.