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A hexagonal uranium calorimeter for measuring electromagnetic showers at the CERN ISR
Nuclear Instruments and Methods in Physics Research 219 (1984) 311-321 North-Holland, Amsterdam
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A HEXAGONAL URANIUM CALORIMETER FOR MEASURING ELECTROMAGNETIC SHOWERS AT THE CERN ISR R. C A R O S I 3, A . D I C I A C C I O 1, C.W. F A B J A N 2, H. G O R D O N 1, M. H A R R I S 2, G. K E S S E L E R 2, J.v.d. L A N S 2, I. M A N N E L L I 2, L. O L S E N 2, G. P I E R A Z Z I N I 3, W.J. W I L L I S 2, W. W I T Z E L I N G 2 a n d C. W O O D Y 1 1 Brookhaoen National Laboratory, Upton, NY, USA e CERN, Geneva, Switzerland 3 I N F N and lnstituto di Fisica dell'Universita, Pisa, Italy
Received 15 July 1983
A uranium-scintillator sandwich electromagneticcalorimeter has been constructed for the detection of photons produced at small angles at the CERN ISR. The calorimeter was designed to have good energy and spatial resolution to resolve the two photons from ~t° decays up to a maximum energy of - 25 GeV (P-r - 4.5 GeV/c). The longitudinal division into a front and back section provided information for discriminating between electromagnetic and hadronic showers. The mechanical design is described along with test beam results and the operation of the calorimeter in the ISR.
1. Introduction
2. Mechanical design
We present a description of a uranium-scintillator calorimeter used to study photon and meson production in hadron collisions at the CERN Intersecting Storage Rings (ISR). The calorimeter was situated in the forward direction inside the magnet cone of the Axial Field Spectrometer (AFS) (1), shown in fig. 1, with the front face at a distance of 1.76 m from the intersection of the colliding beams. At this position, the separation of the two photons from meson decays of interest was small and good spatial resolution was necessary to resolve the photons as separate showers at high PT. On the other hand, the fact that the photons were close together permitted a design with small solid angle coverage. The general requirements of the design were: 1) good spatial resolution for resolving electromagnetic showers; 2) good energy resolution; 3) the ability to distinguish between electromagnetic and hadron showers; 4) the ability to provide a fast trigger; 5) compact overall design, as the space available in the AFS magnet cone was limited. The main physics goal of the detector was to study the production of direct single photons relative to ~r°s at small center of mass angles (0cm - 11°). In addition, the invariant cross section for ~r° production and the ratio ~//~r° has been measured in this region. We report here on the design, construction, testing and operation of the calorimeter at the ISR.
Uranium is a particularly suitable material for an absorber in terms of the requirements for this detector. It has a short radiation length ( - 3.2 mm) which results in a short overall length. Also, the ratio of interaction length to radiation length is larger than for most other absorber materials (19.4 compared with 6.8 for iron and 17.5 for lead) which is beneficial for hadron rejection. Uranium can be easily handled in the form of thin plates, and its natural radioactivity can be used for calibration when used in conjunction scintillator. The disadvantages of using uranium are that it may not be as readily available as other materials and that machining must be done under special controlled conditions. The design of the calorimeter centered around a supply of existing hexagonal uranium plates [2]. The plates were 30 cm wide, 1.6 ± 0.1 mm thick depleted U 238 and had three 14 mm diameter holes located near the edges in three comers. These holes were used to support the stack of plates with three 13 mm stainless steel support rods passed through the holes and fastened to hexagonal end plates. The front plate was 9 mm stainless steel and the back plate was 10 mm aluminum with feed-through holes for the photomultiplier tubes used in the readout. The stack was formed by planes of scintillator strips interleaved between the uranium plates and was divided longitudinally into a front and back section. The hexagonal geometry allowed the scintillators to be arranged in three stereo readout views, denoted Y, U and V. Each
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gap contained two 2.5 mm scintillator readout planes rotated 120 ° with respect to each other which repeated in the sequence YU, VY, UV as shown in fig. 2. The advantages of having two sampling layers per absorbing layer were that there was better correspondence between the energy in the different views and there were more photoelectrons detected per unit of energy deposited in the absorber, which improved the energy resolution. The gap spacing was maintained by three 6.3 mm spacers located on each support rod between the uranium plates and insured that no mechanical stress was placed on the scintillators when the stack was assembled. The front section consisted of 12 gaps containing 8 planes of scintillator for each view and totaled 6.5 radiation lengths (0.26 absorption lengths). The back section consisted of 24 gaps with 16 planes for each view and was 13.0 radiation lengths (0.52 absorption lengths). The choice of 6.5 r.1. for the division between the front and back sections was made to maximize the separation of
two nearby photons. The e/~r rejection of the calorimeter could have been improved by reducing the depth of the front section. A summary of the properties of the calorimeter is given in table 1. The scintillator planes were comprised of twenty four 1 cm wide strips in the front section and ten 3 cm wide strips in the back. The back section was slightly larger to allow for the larger transverse size of showers after the first 6 radiation lengths. This minimized the effect of side leakage for showers near the edge of the front section. The fiducial region in the front was surrounded by a "veto" region consisting of six 35 mm wide scintillator strips 8 layers deep which overlapped the outside edge of the back section. This was used in the analysis of single photon events to detect the presence of additional photons in the calorimeter when one shower was found in the fiducial region. The scintillator material was Nuclear Enterprises NE 104B chosen for its high light output and compatability
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R. Carosi et aL / Hexagonal uranium calorimeter PHOTOMULTIPLIER
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with BBQ wavelength shifter readout. The strips were placed in trays made of 0.05 mm thick annealed aluminum which provided optical isolation without the need for individual wrapping of each strip. Details concerning the construction of these trays can be found in ref. 3. The strips were left unpolished at the readout end and were polished at the opposite end. An aluminum
Table 1 Properties of the calorimeter sections
Number of uranium plates (1.6 mm) Number of scintillator planes (2.5 mm) Y U V Total Width of scintillator strips (cm) Scintillator strips per view Number of gaps Gap width (mm) Radiation length Nuclear collision length Absorption length
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spacer was fixed to the tray at the non-readout end for mechanical support and alignment with the support rods. Fig. 3 shows the front and back tray assemblies. The strips were read out on one end with 4 mm thick wavelength shifter bars arranged in 6 sectors around the perimeter of the hexagon. The bars were made of acrylic plastic doped with BBQ wavelength shifter with a concentration of 80 mg/l. Each bar integrated the light longitudinally from the stack of scintillator strips into a single readout channel. Each sector consisted of an outer layer with 12 readout channels for the front section plus one for the veto, and an inner layer consisting of 6 channels for the back section. The back strips near the support rods were read out with two BBQ bars to avoid dead regions near the corners of the calorimeter. The BBQ bars were placed in machined aluminum trays which provided precision mechanical alignment with the scintillator strips as well as optical separation of each readout channel. The scintillators were separated from the BBQ bars by an air gap of - 0.5 mm. Each bar was glued to an acrylic light guide which transported the light to a photomultiplier tube located at the rear of the calorimeter. The light guides were doped with UV absorbing material to reduce the sensi-
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tivity to Cherenkov light. This minimized the problem of spurious hits in the fight guides from charged particles not associated with electromagnetic showers which were present during normal operating conditions at the ISR. The light guides were coupled to the photomultiplier tubes with small silicone plastic discs which also contained UV absorber. The photomultipliers were 10 stage Hammamatsu R647-04 phototubes which have an active photocathode 9 mm in diameter. The standard resistor divider chain bases were modified for high rate by adding capacitors on the last 4 stages and included a separate output from the last dynode for triggering. Each tube was mounted in a hole in the back plate containing an O-ring which held the tube in place and provided a light tight seal. Additional mechanical support was provided by a 1 cm thick foam rubber mask placed over the tubes behind the back plate. Cooling was provided using forced air distributed over the bases from a nozzle located at the back of the calorimeter. The Axial Field Magnet produced a field of up to 300 G in the region of the calorimeter and required that the pliototubes be magnetically shielded. Each tube was located inside a 20 mm o.d., 16 mm i.d., 62 mm long iron cylinder glued to the inside of the back plate
extending 10 mm beyond the photocathode. In addition, two 5 mm thick iron plates were added to the end of the stack and a 3 mm thick iron shield was placed around the exterior of the calorimeter in the back to reduce the field to an acceptable level. A scintillation hodoscope was mounted to the front of the calorimeter in order to provide information on charged particles. It consisted of eight 3 cm wide scintillator strips, identical to those used in the back section. The strips were glued directly to acrylic light guides and were read out with the same type of phototubes as were used in the calorimeter. The strips were mounted parallel to the U strips and covered the entire fiducial region of the front section. The total number of phototubes in the system was 122 (19 from each sector of the calorimeter plus 8 from the hodoscope). High voltage was provided by a stabilized programmable high voltage system (Danfysik Model 150) and was under the control of the online computer of the AFS. The anode signals were read out with a standard CAMAC system using LeCroy 2282A analog to digital converters (ADCs). These ADCs had a sensitivity of 0.25 pC/channel and were linear to + 0.1% over their full dynamic range. An internal calibration feature allowed measuring the conversion factor from channel number to charge for each channel separately. Pedestals were also recorded for each channel. The phototube anode outputs were adjusted such that the ADCs were operating over approximately half of their full dynamic range during normal operation. The dynode outputs were used for triggering, as described in a later section.
3. Calibration system The energy calibration of each strip as welt as the calorimeter as a whole was established with the calorimeter in a test beam. Two independent calibration systems were used to maintain the energy calibration during actual running conditions. The first system used the natural radioactivity of the uranium as a source of energy deposition in the scintillator. This provided a measurable DC current at the anode output of each phototube. The output currents depended on the size and number of strips read out by each phototube, and varied from 47 nA for the smallest strips in the front section to 705 rLA for the largest strips in the back. These currents were large compared to the phototube dark currents (typically ~<0.5 nA) but were too small for the weakest channels to produce signals in the ADCs which could be used for calibration, even with a gate length as long as 10 ffs. The currents gave rise to a minimum charge of 4.7 x 10 -5 p C / n s in the front strips and a maximum charge of 7.0 x 1 0 - 4 p C / n s in the back. During normal running conditions at the
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ISR, the DC current produced by the flux of particles into the calorimeter was comparable to the current produced by the uranium noise• Hence the calibration of the strips using the DC currents could only be performed during periods with no beams in the ISR. The second calibration system used was a light flasher system comprised of a xenon flash lamp (EGG FX-280), a vacuum photodiode (VPD) (ITT F W l l 4 , 2-1/4" diameter) used to monitor the light source, and a system of optical fibers which carried the light from a diffusing box to the phototubes in the calorimeter. The system is shown in fig. 4. The light from the flash lamp was reflected off the diffusing mirror and viewed by the photodiode. This provided a pulse to pulse measure of the light'output from the flash lamp which served as a reference for comparison with the light seen by the phototubes. Six optical fiber bundles with ends facing the diffusing mirror carried the light 5 meters to the calorimeter• The fibers came in close proximity to the ISR beams and were exposed to a high dosage of radiation while the ISR was running (up to 2700 fads/month). Initially, standard glass communication fibers were used and were severely radiation damaged after only a few months• Subsequently, the fibers were changed to radiation resistant plastic fibers (FORT Fibre Optique TIS FP01). These fibers were 1 mm diameter and had an attenuation length of - 5 0 0 d b / k m . Seven such fibers were grouped together for each of the six fiber bundles going to the calorimeter. The fiber bundles connected to the back of the calorimeter with a screw type connector for easy dismounting. Inside the calorimeter, the light from each bundle was distributed to the 19 PMs of each sector by individual quartz fibers 0.4 mm in diameter and 20-30 cm in length. They were coupled to the light guides approximately 1 cm from the phototubes. The quartz fibers were less sensitive to radiation damage than the plastic fibers• No detectable effect of radiation damage to either the quartz or plastic fibers has been observed over a period of one year in the ISR.
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The light flasher system could be pulsed manually or under the control of the online computer at the AFS. The flash lamp was pulsed and the VPD and phototube signals were read out with ADCs. The output of the VPD was directly proportional to the number of photons impinging on the photocathode and was extremely stable with respect to voltage variations on the VPD. The ratio of the pulse height produced by the flasher in each phototube to the pulse height obtained from the VPD could be used to measure the relative gain of each phototube. This had the advantage that the gains could be measured and adjusted while beams were present in the ISR. The flasher system was also used to measure the linearity of each phototube in the calorimeter• This was carried out by varying the voltage applied to the flash lamp to change the amount of light observed by the phototubes over a dynamic range similar to that encountered with real electromagnetic showers from the ISR. The linearity of the VPD was measured separately in laboratory tests to be better than 0.6% over the dynamic range used in the measurements. The linearity of the phototubes were then measured relative to the VPD. A correction of the form P~or~ect*d= Praw + A • P r a2w , where Praw was the measured pulse height in ADC channels, was deduced from the measurements. The average value of A was 3.58 × 10 -3. This correction compensated for a slight saturation effect in the phototubes for large pulse heights and amounted to a correction of ~< 1% to the measured energy for 20 GeV showers.
4. Beam tests
The performance of the calorimeter was investigated in a test beam of electrons and hadrons of known momenta at the CERN Proton Synchrotron. The tests were carried out to determine the relative energy calibration of each channel and to study the energy resolution, position resolution, linearity and hadron rejection of the calorimeter under controlled conditions. The test beam results were also compared with the predictions of the EGS [4] Monte Carlo program. The relative energy calibration of each channel was determined by scanning across the strips with a ~'beam with a momentum of 7 GeV/c. The phototube gains were adjusted to give the same value for the minimum ionizing peak in the pulse height distribution for non-interacting beam particles in the center of each strip. The DC current produced by the uranium noise was measured for each channel with equalized phototube gains to establish a calibration current for each strip• The flasher pulse height distribution was also measured for each channel with all phototubes set to their desired gains. This established the relationship
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between phototube gain, uranium current and flasher pulse height. The uranium currents served as reference values for all later calibrations, whereas the flasher pulse heights could be reestablished at any time by setting the currents to their reference values and remeasuring the flasher pulse height distributions for each channel. The single ended readout of the strips resulted in a significant variation in pulse height along the length of each strip due to the light attenuation in the scintillator. This feature was used to give a ftatter PT response for the trigger used at the ISR as described later. However, a correction had to be applied to the raw pulse heights before they could be used to determine the shower energy using the three views. The pulse height variation as a function of the distance from the phototube readout end was measured in the laboratory using a radioactive source. The front strips were scanned along the center of the strip, while the back strips were scanned at three positions corresponding to the top, middle and bottom third of the strip. The measurements were fit to a smooth function to give an attenuation curve for each type of scintillator strip used in the calorimeter. Fig. 5 shows the attenuation curve for a typical front strip, and for the middle position of one of the back strips. The attenuation curves were used to correct the pulse heights in the offline analysis to obtain a uniform energy response over the entire calorimeter. Fig. 6 shows the measured energy for 7 GeV electrons for a scan along one of the strips after correcting for the attenuation. The variation in the reconstructed energy was ~< 2% across the strip. The energy resolution was measured by exposing the calorimeter to beams of electrons with momenta of 4, 7
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and 10 GeV. Fig. 7 shows the pulse height distributions for the three incident beam energies. The energy resolution was parameterized by the expression o ( E ) / E a / i f f , where E is given in GeV. Using a single view, we measured a value of a - 0.17, while averaging the energy for all three views, we found a - 0A1. These agreed well with the values predicted by the EGS Monte Carlo program. Fig. 8 shows the electron energy as measured by the calorimeter versus the incident beam energy. The deviation from linearity measured over this range of electron energies was ~< 2%. The precision in determining the true linearity was limited by the relative energy calibration between runs taken at different momenta and the
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d e t e r m i n a t i o n of the absolute energy of the beam. Fig. 8 also shows the energy response predicted by EGS. A small deviation from linearity was predicted by E G S d u e to energy leakage. T h e leakage varied from 2.1% at 4 G e V to 3.1% at 10 GeV. The conversion factor between A D C pulse height a n d total energy m e a s u r e d in the calorimeter could be d e t e r m i n e d from the m e a s u r e m e n t s with electrons a n d c o m p a r e d with the conversion factor b a s e d o n the d E / d x energy loss for m i n i m u m ionizing particles passing t h r o u g h the calorimeter. T h e energy scale det e r m i n e d with electrons was - 6 M e V / A D C channel, while the factor o b t a i n e d using m i n i m u m ionizing particles was - 7 . 5 M e V / A D C channel. This difference has b e e n observed in other sampling calorimeters * [5]. This stresses the need to determine the energy scale with electromagnetic showers w h e n the calorimeter is to be used to measure electromagnetic energy. In the analysis of data taken with this calorimeter, the absolute energy scale was m o n i t o r e d t h r o u g h o u t the experiment using p h o t o n showers f r o m ¢r° decays. The procedure required t h a t the reconstructed ~r° mass for real d a t a be the same as that o b t a i n e d with M o n t e Carlo generated events where the ~r° energies were known. A detailed description of the procedure used is given in ref. 6.
* The difference is mainly due to the fact that the ratio of energy deposited in the absorber to the energy deposited in the scintillator depends on the ratio of the densities for d E / d x energy loss and on the ratio of the radiation lengths for electromagnetic showers.
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for the electron b e a m spread uniformily over one 3 m m bin. A G a u s s i a n fit to the shower position, shown by the curve, gives a resolution of o = 2.3 m m after unfolding the spread due to the beam. The ability of the calorimeter to resolve two showers in the fiducial region d e p e n d e d on the energy of the two showers a n d the a m o u n t of overlap in each of the 3 views. A measure of this resolving power is the ability to distinguish two p h o t o n s from 7r° decays. This was studied by p r o d u c i n g ~r°s in the test b e a m via the charge exchange reaction ~r-p ~ ~r°n using a b e a m of 16 G e V ~r- incident on a polyethylene target. Fig. 10 gives the y y mass distribution for events with two reconstructed showers. It shows a clean ~r° peak, with a width of o / m e a n - 0 . 1 2 , a n d also indicates a slight b u m p in the region of the ,/. Fig. 11 shows the distance between the two showers across the face of the calorimeter determined from the reconstructed shower positions for events in the ~.o peak. The m i n i m u m y y separation was 2.7 cm, consistent with the separation expected for 7r°s decaying with the m i n i m u m opening angle for the calorimeter situated at a distance of 1.65 m from the target. In ISR data, ~r°s were observed with a distance of ~< 2 cm between the two showers. The discrimination against h a d r o n showers was based o n measuring the difference between the longitudinal a n d transverse energy distributions for electromagnetic a n d h a d r o n i c showers. The ratio of the energy in the front section to the total energy in the calorimeter was used as a measure of the longitudinal shower development. Fig. 12 shows the ratio Efront/Etotal for electrons
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a n d ~r- showers with a m o m e n t u m of 7 GeV. Requiring Efront/Etota I > 0.2 eliminated 62% of the h a d r o n showers a n d retained 99% of the electron showers at this m o m e n t u m . T h e effectiveness of this cut was nearly i n d e p e n d e n t of energy between 4 a n d 10 GeV. Additional h a d r o n rejection was provided by requiring the transverse size of the shower to be small. Fig. 13 gives the p r o b a b i l i t y of r e c o n s t r u c t i n g a h a d r o n shower with a given fraction of the initial b e a m m o m e n t u m (7 G e V / c ) after making the Efront/Etotal cut a n d requiring the shower radius * to be less t h a n 1 cm. The misidentification probability is plotted as a function of the reconstructed h a d r o n shower energy divided by the incident b e a m energy. T h e curve flattens out near E r e c o n s t r u c t o d / E b e a m ~ 0.75 due to a c o m p o n e n t from ~rcharge exchange, a n d a small electron c o n t a m i n a t i o n in the h a d r o n b e a m which was not eliminated d u e to C h e r e n k o v counter inefficiencies. W e estimate the true p r o b a b i l i t y for a h a d r o n to be misidentified as an electromagnetic shower with >/80% of the initial particle m o m e n t u m to be < 1%. This rejection was achieved with an efficiency for real electromagnetic showers of 96%. * We define the shower radius as R = ere,(x,
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where E i and x~ are the energy and position of the ith strip, £the mean shower position, and the sum running over 4-3 strips from the shower maximum.
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cutoff energies recommended in EGS (1.5 MeV for electrons and 100 keV for photons). We found for example that the shower radius increased 10% by lowering the photon cutoff energy to 10 keV but worsened the agreement with the longitudinal development of the shower. Since we have used EGS in the analysis of ~r°, 71and single photon events produced in the ISR, we found it necessary to modify the transverse energy distribution produced by EGS in order to accurately reproduce the overlap of multiple showers in the calorimeter. This was done by selecting a sample of real photon showers from the ISR data which were used to define a collection of transverse shower profiles. These profiles were used to redistribute the energy given by EGS on an event by event basis in the Monte Carlo keeping the total energy and Etront//Etotal ratioconstant. This resulted in excellent agreement in the shower radius for real and Monte Carlo data and preserved the agreement for the other shower parameters.
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Fig. 13. Probability of a hadron shower (7 GeV ~r-) to be misidentified as an electron shower as a function of the ratio of the reconstructed shower energy to the initial hadron energy.
We found that the EGS Monte Carlo produced satisfactory agreement with the test beam measurements of energy resolution, linearity, position resolution and the front-to-back ratio. However, the average shower radius for electrons predicted by EGS was 6.3 mm compared with a measured value of 8.3 mm. This was believed to be due to several effects. First, the Monte Carlo did not include the effects of cross talk between channels which was possible due to a small amount of optical coupling between the scintillator strips, BBQ bars and light guides. This coupling increased the shower radius by - 10% based on test beam measurements with muons. Secondly, the Monte Carlo did not accurately represent the detailed geometry of the calorimeter in terms of spaces and material between the strips and plates. We found however that increasing the effective thickness of the scintillator by 12%, thereby reducing the density, resulted in an increase in the shower radius of only 2%. The remaining difference ( - 20%) we attributed to the fact that EGS did not exactly reproduce the true energy deposition far away from the central core of the shower. This was in part due to the treatment of low energy photons and electrons below the
In this section we briefly describe the operation of the calorimeter at the ISR and the trigger used for data taking. Further details on the performance can be found in refs. 6 and 7. The trigger used the dynode signals from the phototubes to require a minimum energy deposition in each of the three views. For each view the signals from the 24 strips in the front were added together in a linear adder circuit to provide a signal for fast triggering. Each signal was added to the sum with a preselected weight by adjusting the value of a weighting resistor in the adder circuit for each channel. This feature was used to equalize the contribution of each channel to the sum, as described below. A similar circuit was used for the back strips, veto strips and hodoscope strips. The standard trigger configuration consisted of adding the trigger sums for the front and back of each view separately and requiring a minimum energy deposition in each view. The trigger logic is shown in fig. 14. The U view was oriented such that the scintillator strips were approximately parallel to the direction defining PT in the laboratory, as shown in fig. 2, and were read out along the edge farthest away from the outgoing beam. The observed pulse height for a given energy deposition varied along the length of the strips due to the light attenuation in the scintillator. Since the light output varied roughly as the inverse of the distance from the readout end, and since PT = E . sin0 increases approximately linearly with x, triggering on a threshold for the observed pulse resulted in a threshold of roughly constant PT over the region covered by the calorimeter. The threshold in the U view was set to determine the
R. (_'arosi et al. / Hexagonal uranium calorimeter
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energy and consequently the PT of the trigger. Since the Y and V views were rotated with respect to the U view, they were not related to PT in a simply way. The thresholds in these views were typically set at a factor of 3 lower than the threshold in the U view to allow for the pulse height variation due to the attentuation in the scintillator. Normally, the veto and hodoscope trigger sums were not used for normal data taking, but were used for special purpose triggers. We found that the ratio of the anode to dynode output for each channel varied signifiantly (up to a factor of 2) due to differences in the performance of the individual phototubes and bases. This implied that, since the phototube gains were set on the basis of the anode current produced by the uranium noise, a given energy deposition would contribute differently to the trigger sum for different strips. The difference in the effective trigger threshold for each strip produced a trigger rate which varied from strip to strip due to the sharply falling energy spectrum. This non-uniformity was corrected for by adjusting the weighting resistors
for each channel in the sum to compensate for the unequal anode to dynode ratios. The ratios in the U view were adjusted to +5% which was sufficient to provide good uniformity in tiggering over the entire calorimeter. Calibration of the calorimeter at the ISR was done using the anode currents and the light flasher system. The anode currents were set to their calibration values usually at the beginning of each ISR running period and the flasher pulse height distributions remeasured. The flasher pulse heights could then be used to recalibrate the tubes with beams circulating in the ISR. With no re,calibration using the flasher, the average phototube gain drift over a 48 h ISR run was typically less than 5%. The calorimeter trigger was put in coincidence with a "minimum bias" trigger, generated by scintillation counters in the AFS, in order to define the trigger used for data taking. The minimum bias trigger required an inelastic event occurring in the intersection region to trigger either the "beam-beam" on "inner hodoscope"
R. Carosi et al. / Hexagonal uranium calorimeter
counters, shown in fig. 1. The final trigger rate depended on the thresholds set for the three views, the ISR luminosity and the beam conditions. For normal luminosity the trigger rate at low threshold was high ( - 300 Hz at PT -- 1 G e V / c ) . Data at low threshold was collected in a dedicated mode of running where the calorimeter was the only trigger in use at the AFS. The trigger rate for a higher threshold (PT > 3 G e V / c ) was ~< 0.5 Hz and permitted calorimeter triggered data to be taken in parallel with other triggers from the AFS. The calorimeter was in use at the ISR for 16 months, approximately 9 months in a data taking mode. Data was collected for an equivalent integrated luminosity of 7.1 × 10 36 cm -2. The main items requiring repair during that period were phototubes and summing amplifier cards, both at the rate of about one repair every 3 months. N o sign of deterioration due to radiation damage to the scintillator, wavelength shifters or the quartz or plastic light fibres has been observed during the entire period of operation. We wish to thank the C E R N ISR division and the members of experiment R807 for their dedicated efforts in the construction of this calorimeter and bringing it into operation. We wish to extend special thanks to J.
321
Renand for his help throughout this project, to G. M a y m a n for his help during the early construction, and to F. Meyer for his work on the linearity measurements. We also acknowledge the support of t h e collaborating h o m e institutions and Research Councils and the US Department of Energy.
References [1] H. Gordon et al., Nucl. Instr. and Meth. 196 (1982) 303; O. Bother et al., Nucl. Instr. and Meth. 196 (1982) 315; O. Botner et al., Nucl. Instr. and Meth. 179 (1981) 45; O. Botner et al., IEEE Trans. Nucl. Sci. NS-28 (1981) 510. [2] C.W. Fabjan et al., Nucl. Instr. and Meth. 141 (1977) 61. [3] M. Harris and C. Woody, CERN Technical Note GE/81-7. [4] R.L. Ford and W.R. Nelson, SLAC 210, June (1978). [5] A. Babaev et al., Nucl. Instr. and Meth. 122 (1974) 313; J.H. Cobb et al., Nucl. Instr. and Meth. 158 (1979) 93; O. Botner et al., Nucl. Instr. and Meth. 179 (1981) 45. [6] T. Akesson et al., (Production of 9 ° and ,/o at 11 ° in pp collisions at ¢i = 63 GeV) Z. Phys C. 18 (1983) 5. [7] T. Akesson et al., (High PT Direct Photon Production at 11 ° in pp collisions at ¢i = 63 GeV) Phys. Lett. 123B (1983) 367.
311
A HEXAGONAL URANIUM CALORIMETER FOR MEASURING ELECTROMAGNETIC SHOWERS AT THE CERN ISR R. C A R O S I 3, A . D I C I A C C I O 1, C.W. F A B J A N 2, H. G O R D O N 1, M. H A R R I S 2, G. K E S S E L E R 2, J.v.d. L A N S 2, I. M A N N E L L I 2, L. O L S E N 2, G. P I E R A Z Z I N I 3, W.J. W I L L I S 2, W. W I T Z E L I N G 2 a n d C. W O O D Y 1 1 Brookhaoen National Laboratory, Upton, NY, USA e CERN, Geneva, Switzerland 3 I N F N and lnstituto di Fisica dell'Universita, Pisa, Italy
Received 15 July 1983
A uranium-scintillator sandwich electromagneticcalorimeter has been constructed for the detection of photons produced at small angles at the CERN ISR. The calorimeter was designed to have good energy and spatial resolution to resolve the two photons from ~t° decays up to a maximum energy of - 25 GeV (P-r - 4.5 GeV/c). The longitudinal division into a front and back section provided information for discriminating between electromagnetic and hadronic showers. The mechanical design is described along with test beam results and the operation of the calorimeter in the ISR.
1. Introduction
2. Mechanical design
We present a description of a uranium-scintillator calorimeter used to study photon and meson production in hadron collisions at the CERN Intersecting Storage Rings (ISR). The calorimeter was situated in the forward direction inside the magnet cone of the Axial Field Spectrometer (AFS) (1), shown in fig. 1, with the front face at a distance of 1.76 m from the intersection of the colliding beams. At this position, the separation of the two photons from meson decays of interest was small and good spatial resolution was necessary to resolve the photons as separate showers at high PT. On the other hand, the fact that the photons were close together permitted a design with small solid angle coverage. The general requirements of the design were: 1) good spatial resolution for resolving electromagnetic showers; 2) good energy resolution; 3) the ability to distinguish between electromagnetic and hadron showers; 4) the ability to provide a fast trigger; 5) compact overall design, as the space available in the AFS magnet cone was limited. The main physics goal of the detector was to study the production of direct single photons relative to ~r°s at small center of mass angles (0cm - 11°). In addition, the invariant cross section for ~r° production and the ratio ~//~r° has been measured in this region. We report here on the design, construction, testing and operation of the calorimeter at the ISR.
Uranium is a particularly suitable material for an absorber in terms of the requirements for this detector. It has a short radiation length ( - 3.2 mm) which results in a short overall length. Also, the ratio of interaction length to radiation length is larger than for most other absorber materials (19.4 compared with 6.8 for iron and 17.5 for lead) which is beneficial for hadron rejection. Uranium can be easily handled in the form of thin plates, and its natural radioactivity can be used for calibration when used in conjunction scintillator. The disadvantages of using uranium are that it may not be as readily available as other materials and that machining must be done under special controlled conditions. The design of the calorimeter centered around a supply of existing hexagonal uranium plates [2]. The plates were 30 cm wide, 1.6 ± 0.1 mm thick depleted U 238 and had three 14 mm diameter holes located near the edges in three comers. These holes were used to support the stack of plates with three 13 mm stainless steel support rods passed through the holes and fastened to hexagonal end plates. The front plate was 9 mm stainless steel and the back plate was 10 mm aluminum with feed-through holes for the photomultiplier tubes used in the readout. The stack was formed by planes of scintillator strips interleaved between the uranium plates and was divided longitudinally into a front and back section. The hexagonal geometry allowed the scintillators to be arranged in three stereo readout views, denoted Y, U and V. Each
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gap contained two 2.5 mm scintillator readout planes rotated 120 ° with respect to each other which repeated in the sequence YU, VY, UV as shown in fig. 2. The advantages of having two sampling layers per absorbing layer were that there was better correspondence between the energy in the different views and there were more photoelectrons detected per unit of energy deposited in the absorber, which improved the energy resolution. The gap spacing was maintained by three 6.3 mm spacers located on each support rod between the uranium plates and insured that no mechanical stress was placed on the scintillators when the stack was assembled. The front section consisted of 12 gaps containing 8 planes of scintillator for each view and totaled 6.5 radiation lengths (0.26 absorption lengths). The back section consisted of 24 gaps with 16 planes for each view and was 13.0 radiation lengths (0.52 absorption lengths). The choice of 6.5 r.1. for the division between the front and back sections was made to maximize the separation of
two nearby photons. The e/~r rejection of the calorimeter could have been improved by reducing the depth of the front section. A summary of the properties of the calorimeter is given in table 1. The scintillator planes were comprised of twenty four 1 cm wide strips in the front section and ten 3 cm wide strips in the back. The back section was slightly larger to allow for the larger transverse size of showers after the first 6 radiation lengths. This minimized the effect of side leakage for showers near the edge of the front section. The fiducial region in the front was surrounded by a "veto" region consisting of six 35 mm wide scintillator strips 8 layers deep which overlapped the outside edge of the back section. This was used in the analysis of single photon events to detect the presence of additional photons in the calorimeter when one shower was found in the fiducial region. The scintillator material was Nuclear Enterprises NE 104B chosen for its high light output and compatability
313
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with BBQ wavelength shifter readout. The strips were placed in trays made of 0.05 mm thick annealed aluminum which provided optical isolation without the need for individual wrapping of each strip. Details concerning the construction of these trays can be found in ref. 3. The strips were left unpolished at the readout end and were polished at the opposite end. An aluminum
Table 1 Properties of the calorimeter sections
Number of uranium plates (1.6 mm) Number of scintillator planes (2.5 mm) Y U V Total Width of scintillator strips (cm) Scintillator strips per view Number of gaps Gap width (mm) Radiation length Nuclear collision length Absorption length
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spacer was fixed to the tray at the non-readout end for mechanical support and alignment with the support rods. Fig. 3 shows the front and back tray assemblies. The strips were read out on one end with 4 mm thick wavelength shifter bars arranged in 6 sectors around the perimeter of the hexagon. The bars were made of acrylic plastic doped with BBQ wavelength shifter with a concentration of 80 mg/l. Each bar integrated the light longitudinally from the stack of scintillator strips into a single readout channel. Each sector consisted of an outer layer with 12 readout channels for the front section plus one for the veto, and an inner layer consisting of 6 channels for the back section. The back strips near the support rods were read out with two BBQ bars to avoid dead regions near the corners of the calorimeter. The BBQ bars were placed in machined aluminum trays which provided precision mechanical alignment with the scintillator strips as well as optical separation of each readout channel. The scintillators were separated from the BBQ bars by an air gap of - 0.5 mm. Each bar was glued to an acrylic light guide which transported the light to a photomultiplier tube located at the rear of the calorimeter. The light guides were doped with UV absorbing material to reduce the sensi-
R. Carosi et al, / Hexagonal uranium calorimeter
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tivity to Cherenkov light. This minimized the problem of spurious hits in the fight guides from charged particles not associated with electromagnetic showers which were present during normal operating conditions at the ISR. The light guides were coupled to the photomultiplier tubes with small silicone plastic discs which also contained UV absorber. The photomultipliers were 10 stage Hammamatsu R647-04 phototubes which have an active photocathode 9 mm in diameter. The standard resistor divider chain bases were modified for high rate by adding capacitors on the last 4 stages and included a separate output from the last dynode for triggering. Each tube was mounted in a hole in the back plate containing an O-ring which held the tube in place and provided a light tight seal. Additional mechanical support was provided by a 1 cm thick foam rubber mask placed over the tubes behind the back plate. Cooling was provided using forced air distributed over the bases from a nozzle located at the back of the calorimeter. The Axial Field Magnet produced a field of up to 300 G in the region of the calorimeter and required that the pliototubes be magnetically shielded. Each tube was located inside a 20 mm o.d., 16 mm i.d., 62 mm long iron cylinder glued to the inside of the back plate
extending 10 mm beyond the photocathode. In addition, two 5 mm thick iron plates were added to the end of the stack and a 3 mm thick iron shield was placed around the exterior of the calorimeter in the back to reduce the field to an acceptable level. A scintillation hodoscope was mounted to the front of the calorimeter in order to provide information on charged particles. It consisted of eight 3 cm wide scintillator strips, identical to those used in the back section. The strips were glued directly to acrylic light guides and were read out with the same type of phototubes as were used in the calorimeter. The strips were mounted parallel to the U strips and covered the entire fiducial region of the front section. The total number of phototubes in the system was 122 (19 from each sector of the calorimeter plus 8 from the hodoscope). High voltage was provided by a stabilized programmable high voltage system (Danfysik Model 150) and was under the control of the online computer of the AFS. The anode signals were read out with a standard CAMAC system using LeCroy 2282A analog to digital converters (ADCs). These ADCs had a sensitivity of 0.25 pC/channel and were linear to + 0.1% over their full dynamic range. An internal calibration feature allowed measuring the conversion factor from channel number to charge for each channel separately. Pedestals were also recorded for each channel. The phototube anode outputs were adjusted such that the ADCs were operating over approximately half of their full dynamic range during normal operation. The dynode outputs were used for triggering, as described in a later section.
3. Calibration system The energy calibration of each strip as welt as the calorimeter as a whole was established with the calorimeter in a test beam. Two independent calibration systems were used to maintain the energy calibration during actual running conditions. The first system used the natural radioactivity of the uranium as a source of energy deposition in the scintillator. This provided a measurable DC current at the anode output of each phototube. The output currents depended on the size and number of strips read out by each phototube, and varied from 47 nA for the smallest strips in the front section to 705 rLA for the largest strips in the back. These currents were large compared to the phototube dark currents (typically ~<0.5 nA) but were too small for the weakest channels to produce signals in the ADCs which could be used for calibration, even with a gate length as long as 10 ffs. The currents gave rise to a minimum charge of 4.7 x 10 -5 p C / n s in the front strips and a maximum charge of 7.0 x 1 0 - 4 p C / n s in the back. During normal running conditions at the
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ISR, the DC current produced by the flux of particles into the calorimeter was comparable to the current produced by the uranium noise• Hence the calibration of the strips using the DC currents could only be performed during periods with no beams in the ISR. The second calibration system used was a light flasher system comprised of a xenon flash lamp (EGG FX-280), a vacuum photodiode (VPD) (ITT F W l l 4 , 2-1/4" diameter) used to monitor the light source, and a system of optical fibers which carried the light from a diffusing box to the phototubes in the calorimeter. The system is shown in fig. 4. The light from the flash lamp was reflected off the diffusing mirror and viewed by the photodiode. This provided a pulse to pulse measure of the light'output from the flash lamp which served as a reference for comparison with the light seen by the phototubes. Six optical fiber bundles with ends facing the diffusing mirror carried the light 5 meters to the calorimeter• The fibers came in close proximity to the ISR beams and were exposed to a high dosage of radiation while the ISR was running (up to 2700 fads/month). Initially, standard glass communication fibers were used and were severely radiation damaged after only a few months• Subsequently, the fibers were changed to radiation resistant plastic fibers (FORT Fibre Optique TIS FP01). These fibers were 1 mm diameter and had an attenuation length of - 5 0 0 d b / k m . Seven such fibers were grouped together for each of the six fiber bundles going to the calorimeter. The fiber bundles connected to the back of the calorimeter with a screw type connector for easy dismounting. Inside the calorimeter, the light from each bundle was distributed to the 19 PMs of each sector by individual quartz fibers 0.4 mm in diameter and 20-30 cm in length. They were coupled to the light guides approximately 1 cm from the phototubes. The quartz fibers were less sensitive to radiation damage than the plastic fibers• No detectable effect of radiation damage to either the quartz or plastic fibers has been observed over a period of one year in the ISR.
315
The light flasher system could be pulsed manually or under the control of the online computer at the AFS. The flash lamp was pulsed and the VPD and phototube signals were read out with ADCs. The output of the VPD was directly proportional to the number of photons impinging on the photocathode and was extremely stable with respect to voltage variations on the VPD. The ratio of the pulse height produced by the flasher in each phototube to the pulse height obtained from the VPD could be used to measure the relative gain of each phototube. This had the advantage that the gains could be measured and adjusted while beams were present in the ISR. The flasher system was also used to measure the linearity of each phototube in the calorimeter• This was carried out by varying the voltage applied to the flash lamp to change the amount of light observed by the phototubes over a dynamic range similar to that encountered with real electromagnetic showers from the ISR. The linearity of the VPD was measured separately in laboratory tests to be better than 0.6% over the dynamic range used in the measurements. The linearity of the phototubes were then measured relative to the VPD. A correction of the form P~or~ect*d= Praw + A • P r a2w , where Praw was the measured pulse height in ADC channels, was deduced from the measurements. The average value of A was 3.58 × 10 -3. This correction compensated for a slight saturation effect in the phototubes for large pulse heights and amounted to a correction of ~< 1% to the measured energy for 20 GeV showers.
4. Beam tests
The performance of the calorimeter was investigated in a test beam of electrons and hadrons of known momenta at the CERN Proton Synchrotron. The tests were carried out to determine the relative energy calibration of each channel and to study the energy resolution, position resolution, linearity and hadron rejection of the calorimeter under controlled conditions. The test beam results were also compared with the predictions of the EGS [4] Monte Carlo program. The relative energy calibration of each channel was determined by scanning across the strips with a ~'beam with a momentum of 7 GeV/c. The phototube gains were adjusted to give the same value for the minimum ionizing peak in the pulse height distribution for non-interacting beam particles in the center of each strip. The DC current produced by the uranium noise was measured for each channel with equalized phototube gains to establish a calibration current for each strip• The flasher pulse height distribution was also measured for each channel with all phototubes set to their desired gains. This established the relationship
R. Carosi et al. / Hexagonal uranium calorimeter
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between phototube gain, uranium current and flasher pulse height. The uranium currents served as reference values for all later calibrations, whereas the flasher pulse heights could be reestablished at any time by setting the currents to their reference values and remeasuring the flasher pulse height distributions for each channel. The single ended readout of the strips resulted in a significant variation in pulse height along the length of each strip due to the light attenuation in the scintillator. This feature was used to give a ftatter PT response for the trigger used at the ISR as described later. However, a correction had to be applied to the raw pulse heights before they could be used to determine the shower energy using the three views. The pulse height variation as a function of the distance from the phototube readout end was measured in the laboratory using a radioactive source. The front strips were scanned along the center of the strip, while the back strips were scanned at three positions corresponding to the top, middle and bottom third of the strip. The measurements were fit to a smooth function to give an attenuation curve for each type of scintillator strip used in the calorimeter. Fig. 5 shows the attenuation curve for a typical front strip, and for the middle position of one of the back strips. The attenuation curves were used to correct the pulse heights in the offline analysis to obtain a uniform energy response over the entire calorimeter. Fig. 6 shows the measured energy for 7 GeV electrons for a scan along one of the strips after correcting for the attenuation. The variation in the reconstructed energy was ~< 2% across the strip. The energy resolution was measured by exposing the calorimeter to beams of electrons with momenta of 4, 7
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and 10 GeV. Fig. 7 shows the pulse height distributions for the three incident beam energies. The energy resolution was parameterized by the expression o ( E ) / E a / i f f , where E is given in GeV. Using a single view, we measured a value of a - 0.17, while averaging the energy for all three views, we found a - 0A1. These agreed well with the values predicted by the EGS Monte Carlo program. Fig. 8 shows the electron energy as measured by the calorimeter versus the incident beam energy. The deviation from linearity measured over this range of electron energies was ~< 2%. The precision in determining the true linearity was limited by the relative energy calibration between runs taken at different momenta and the
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d e t e r m i n a t i o n of the absolute energy of the beam. Fig. 8 also shows the energy response predicted by EGS. A small deviation from linearity was predicted by E G S d u e to energy leakage. T h e leakage varied from 2.1% at 4 G e V to 3.1% at 10 GeV. The conversion factor between A D C pulse height a n d total energy m e a s u r e d in the calorimeter could be d e t e r m i n e d from the m e a s u r e m e n t s with electrons a n d c o m p a r e d with the conversion factor b a s e d o n the d E / d x energy loss for m i n i m u m ionizing particles passing t h r o u g h the calorimeter. T h e energy scale det e r m i n e d with electrons was - 6 M e V / A D C channel, while the factor o b t a i n e d using m i n i m u m ionizing particles was - 7 . 5 M e V / A D C channel. This difference has b e e n observed in other sampling calorimeters * [5]. This stresses the need to determine the energy scale with electromagnetic showers w h e n the calorimeter is to be used to measure electromagnetic energy. In the analysis of data taken with this calorimeter, the absolute energy scale was m o n i t o r e d t h r o u g h o u t the experiment using p h o t o n showers f r o m ¢r° decays. The procedure required t h a t the reconstructed ~r° mass for real d a t a be the same as that o b t a i n e d with M o n t e Carlo generated events where the ~r° energies were known. A detailed description of the procedure used is given in ref. 6.
* The difference is mainly due to the fact that the ratio of energy deposited in the absorber to the energy deposited in the scintillator depends on the ratio of the densities for d E / d x energy loss and on the ratio of the radiation lengths for electromagnetic showers.
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for the electron b e a m spread uniformily over one 3 m m bin. A G a u s s i a n fit to the shower position, shown by the curve, gives a resolution of o = 2.3 m m after unfolding the spread due to the beam. The ability of the calorimeter to resolve two showers in the fiducial region d e p e n d e d on the energy of the two showers a n d the a m o u n t of overlap in each of the 3 views. A measure of this resolving power is the ability to distinguish two p h o t o n s from 7r° decays. This was studied by p r o d u c i n g ~r°s in the test b e a m via the charge exchange reaction ~r-p ~ ~r°n using a b e a m of 16 G e V ~r- incident on a polyethylene target. Fig. 10 gives the y y mass distribution for events with two reconstructed showers. It shows a clean ~r° peak, with a width of o / m e a n - 0 . 1 2 , a n d also indicates a slight b u m p in the region of the ,/. Fig. 11 shows the distance between the two showers across the face of the calorimeter determined from the reconstructed shower positions for events in the ~.o peak. The m i n i m u m y y separation was 2.7 cm, consistent with the separation expected for 7r°s decaying with the m i n i m u m opening angle for the calorimeter situated at a distance of 1.65 m from the target. In ISR data, ~r°s were observed with a distance of ~< 2 cm between the two showers. The discrimination against h a d r o n showers was based o n measuring the difference between the longitudinal a n d transverse energy distributions for electromagnetic a n d h a d r o n i c showers. The ratio of the energy in the front section to the total energy in the calorimeter was used as a measure of the longitudinal shower development. Fig. 12 shows the ratio Efront/Etotal for electrons
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a n d ~r- showers with a m o m e n t u m of 7 GeV. Requiring Efront/Etota I > 0.2 eliminated 62% of the h a d r o n showers a n d retained 99% of the electron showers at this m o m e n t u m . T h e effectiveness of this cut was nearly i n d e p e n d e n t of energy between 4 a n d 10 GeV. Additional h a d r o n rejection was provided by requiring the transverse size of the shower to be small. Fig. 13 gives the p r o b a b i l i t y of r e c o n s t r u c t i n g a h a d r o n shower with a given fraction of the initial b e a m m o m e n t u m (7 G e V / c ) after making the Efront/Etotal cut a n d requiring the shower radius * to be less t h a n 1 cm. The misidentification probability is plotted as a function of the reconstructed h a d r o n shower energy divided by the incident b e a m energy. T h e curve flattens out near E r e c o n s t r u c t o d / E b e a m ~ 0.75 due to a c o m p o n e n t from ~rcharge exchange, a n d a small electron c o n t a m i n a t i o n in the h a d r o n b e a m which was not eliminated d u e to C h e r e n k o v counter inefficiencies. W e estimate the true p r o b a b i l i t y for a h a d r o n to be misidentified as an electromagnetic shower with >/80% of the initial particle m o m e n t u m to be < 1%. This rejection was achieved with an efficiency for real electromagnetic showers of 96%. * We define the shower radius as R = ere,(x,
- x)2/~e,,
where E i and x~ are the energy and position of the ith strip, £the mean shower position, and the sum running over 4-3 strips from the shower maximum.
R. Carosi et al. / Hexagonal uranium calorimeter I
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319
cutoff energies recommended in EGS (1.5 MeV for electrons and 100 keV for photons). We found for example that the shower radius increased 10% by lowering the photon cutoff energy to 10 keV but worsened the agreement with the longitudinal development of the shower. Since we have used EGS in the analysis of ~r°, 71and single photon events produced in the ISR, we found it necessary to modify the transverse energy distribution produced by EGS in order to accurately reproduce the overlap of multiple showers in the calorimeter. This was done by selecting a sample of real photon showers from the ISR data which were used to define a collection of transverse shower profiles. These profiles were used to redistribute the energy given by EGS on an event by event basis in the Monte Carlo keeping the total energy and Etront//Etotal ratioconstant. This resulted in excellent agreement in the shower radius for real and Monte Carlo data and preserved the agreement for the other shower parameters.
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Fig. 13. Probability of a hadron shower (7 GeV ~r-) to be misidentified as an electron shower as a function of the ratio of the reconstructed shower energy to the initial hadron energy.
We found that the EGS Monte Carlo produced satisfactory agreement with the test beam measurements of energy resolution, linearity, position resolution and the front-to-back ratio. However, the average shower radius for electrons predicted by EGS was 6.3 mm compared with a measured value of 8.3 mm. This was believed to be due to several effects. First, the Monte Carlo did not include the effects of cross talk between channels which was possible due to a small amount of optical coupling between the scintillator strips, BBQ bars and light guides. This coupling increased the shower radius by - 10% based on test beam measurements with muons. Secondly, the Monte Carlo did not accurately represent the detailed geometry of the calorimeter in terms of spaces and material between the strips and plates. We found however that increasing the effective thickness of the scintillator by 12%, thereby reducing the density, resulted in an increase in the shower radius of only 2%. The remaining difference ( - 20%) we attributed to the fact that EGS did not exactly reproduce the true energy deposition far away from the central core of the shower. This was in part due to the treatment of low energy photons and electrons below the
In this section we briefly describe the operation of the calorimeter at the ISR and the trigger used for data taking. Further details on the performance can be found in refs. 6 and 7. The trigger used the dynode signals from the phototubes to require a minimum energy deposition in each of the three views. For each view the signals from the 24 strips in the front were added together in a linear adder circuit to provide a signal for fast triggering. Each signal was added to the sum with a preselected weight by adjusting the value of a weighting resistor in the adder circuit for each channel. This feature was used to equalize the contribution of each channel to the sum, as described below. A similar circuit was used for the back strips, veto strips and hodoscope strips. The standard trigger configuration consisted of adding the trigger sums for the front and back of each view separately and requiring a minimum energy deposition in each view. The trigger logic is shown in fig. 14. The U view was oriented such that the scintillator strips were approximately parallel to the direction defining PT in the laboratory, as shown in fig. 2, and were read out along the edge farthest away from the outgoing beam. The observed pulse height for a given energy deposition varied along the length of the strips due to the light attenuation in the scintillator. Since the light output varied roughly as the inverse of the distance from the readout end, and since PT = E . sin0 increases approximately linearly with x, triggering on a threshold for the observed pulse resulted in a threshold of roughly constant PT over the region covered by the calorimeter. The threshold in the U view was set to determine the
R. (_'arosi et al. / Hexagonal uranium calorimeter
320
TRIGGER DISCRIMINATORS
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energy and consequently the PT of the trigger. Since the Y and V views were rotated with respect to the U view, they were not related to PT in a simply way. The thresholds in these views were typically set at a factor of 3 lower than the threshold in the U view to allow for the pulse height variation due to the attentuation in the scintillator. Normally, the veto and hodoscope trigger sums were not used for normal data taking, but were used for special purpose triggers. We found that the ratio of the anode to dynode output for each channel varied signifiantly (up to a factor of 2) due to differences in the performance of the individual phototubes and bases. This implied that, since the phototube gains were set on the basis of the anode current produced by the uranium noise, a given energy deposition would contribute differently to the trigger sum for different strips. The difference in the effective trigger threshold for each strip produced a trigger rate which varied from strip to strip due to the sharply falling energy spectrum. This non-uniformity was corrected for by adjusting the weighting resistors
for each channel in the sum to compensate for the unequal anode to dynode ratios. The ratios in the U view were adjusted to +5% which was sufficient to provide good uniformity in tiggering over the entire calorimeter. Calibration of the calorimeter at the ISR was done using the anode currents and the light flasher system. The anode currents were set to their calibration values usually at the beginning of each ISR running period and the flasher pulse height distributions remeasured. The flasher pulse heights could then be used to recalibrate the tubes with beams circulating in the ISR. With no re,calibration using the flasher, the average phototube gain drift over a 48 h ISR run was typically less than 5%. The calorimeter trigger was put in coincidence with a "minimum bias" trigger, generated by scintillation counters in the AFS, in order to define the trigger used for data taking. The minimum bias trigger required an inelastic event occurring in the intersection region to trigger either the "beam-beam" on "inner hodoscope"
R. Carosi et al. / Hexagonal uranium calorimeter
counters, shown in fig. 1. The final trigger rate depended on the thresholds set for the three views, the ISR luminosity and the beam conditions. For normal luminosity the trigger rate at low threshold was high ( - 300 Hz at PT -- 1 G e V / c ) . Data at low threshold was collected in a dedicated mode of running where the calorimeter was the only trigger in use at the AFS. The trigger rate for a higher threshold (PT > 3 G e V / c ) was ~< 0.5 Hz and permitted calorimeter triggered data to be taken in parallel with other triggers from the AFS. The calorimeter was in use at the ISR for 16 months, approximately 9 months in a data taking mode. Data was collected for an equivalent integrated luminosity of 7.1 × 10 36 cm -2. The main items requiring repair during that period were phototubes and summing amplifier cards, both at the rate of about one repair every 3 months. N o sign of deterioration due to radiation damage to the scintillator, wavelength shifters or the quartz or plastic light fibres has been observed during the entire period of operation. We wish to thank the C E R N ISR division and the members of experiment R807 for their dedicated efforts in the construction of this calorimeter and bringing it into operation. We wish to extend special thanks to J.
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Renand for his help throughout this project, to G. M a y m a n for his help during the early construction, and to F. Meyer for his work on the linearity measurements. We also acknowledge the support of t h e collaborating h o m e institutions and Research Councils and the US Department of Energy.
References [1] H. Gordon et al., Nucl. Instr. and Meth. 196 (1982) 303; O. Bother et al., Nucl. Instr. and Meth. 196 (1982) 315; O. Botner et al., Nucl. Instr. and Meth. 179 (1981) 45; O. Botner et al., IEEE Trans. Nucl. Sci. NS-28 (1981) 510. [2] C.W. Fabjan et al., Nucl. Instr. and Meth. 141 (1977) 61. [3] M. Harris and C. Woody, CERN Technical Note GE/81-7. [4] R.L. Ford and W.R. Nelson, SLAC 210, June (1978). [5] A. Babaev et al., Nucl. Instr. and Meth. 122 (1974) 313; J.H. Cobb et al., Nucl. Instr. and Meth. 158 (1979) 93; O. Botner et al., Nucl. Instr. and Meth. 179 (1981) 45. [6] T. Akesson et al., (Production of 9 ° and ,/o at 11 ° in pp collisions at ¢i = 63 GeV) Z. Phys C. 18 (1983) 5. [7] T. Akesson et al., (High PT Direct Photon Production at 11 ° in pp collisions at ¢i = 63 GeV) Phys. Lett. 123B (1983) 367.