The RHIC Zero Degree Calorimeters


PHENIX ZDC Shower Maximum Detector

    The Zero Degree calorimeters are small transverse area hadron calorimeters located downstream of the DX dipole magnets in each of the Heavy Ion Experiments at RHIC. The detectors measure neutral energy within a 2 mrad cone about the beam direction (since charged particles are swept away by the DX magnet). The actual detector location is ~18m from the interaction point and the horizontal acceptance is to +/- 5cms.
    The design is based on the requirements for Au-Au runs but the dettectors are also used in p-p and d-Au runs.
    The detectors serve two roles: The energy measurement basically counts the number of free "spectator" neutrons. This is used for event-by-event characterization(usually in conjuction with the BBC). Also coincidence signals from detectors on either side of the interaction region are used for luminosity monitoring. During 200  GeV Au-Au running (ie in Run IV) the ZDC coincidence rate has an effective cross section of
10.4 barns (with an uncertainty of about 5%).
    The energy scale of the ZDC is determined from the inclusive spectrum obtained with RHIC data. You can usually make out the first few peaks (ie 1n and 2n, etc) in this spectrum which gives you the best possible Energy calibration. However we try to "dead-reckon" the relative gains of the 3 PMT's- each measuring successive depth segments in the calorimeter- using cosmic ray data. Typically this has been succesful and the width of the 1-n peak is about 20%,  consistent with what we expected for optimal balancing.
      The ZDC timing resolution is ~ 100 psec depending on the electronics setup, empirical slewing corrections.. (in other words depending on which experiment you're in). Since you measure time in 2 ZDC's, on either side of the IR, you can derive 2 quantities from them- the vertex position and the event time.

The PHENIX Shower Max Detectors(SMD) are layers of position sensitive hodoscopes sandwiched between the first and second module of the ZDC's. Since a ZDC module is 2 hadronic interaction lengths deep, the actual amplitude measured in the Shower Max detector will fluctuate from one shower to another (but less so in a typical event with many neutrons).
    The purpose of the SMD is to measure the centroid of showers in the ZDC in 2 coordinates-x and y. The first SMD layer has 21 strips of 0.5 cm *0.5 cm scintillators each with wavelength shifter fiber readout. Groups of 3 fibers are read out by a single channel of a multi-anode PMT. So the total width of the SMD, 10.5 cms,  is subdivided into 7 samples. In d-Au runs (Run-3) the horizontal measurement accuracy of the centroid was about 0.1 to 0.2 mm and correlated with accelerator instrumentation beam position. Similarly, the vertical coordinate has 8 elements of 4 scintillator strips.

The ZDC's are described in the following NIM article  (). Also relevant is the following article in Phys. Rev. Letters which analyzes the basic cross sections contributing to the ZDC data. The measurements relate directly to the NIM article discussing the calculation of cross sections.

C. Adler et al., Nucl. Instr. And Meth. A470, 488 (01)  or in archives at nucl-ex/0008005.
M. Chiu et al. Phys. Rev. Lett. 89 (02) 012302
A. J. Baltz C. Chasman and S. White, Nucl. Instr. And Meth. A417, 1 (98) or at  nucl-ex/9801002

    Some of the design considerations are based on the NA49 measurement in which we tried to understand the role of free neutron measurement as opposed the more traditional fixed target implementation.

H.~Appelshauser et al.[NA49 Collaboration],``Spectator nucleons in Pb + Pb collisions at 158-A-GeV"
Eur. Phys. J. A2, 383 (1998)

Info for PHENIX shifts on ZDC/SMD operation
    ZDC/SMD keeps an electronic logbook location

The High Voltage of ZDC/SMD shares a crate with BBC. Also the FEM's for ZDC/SMD are the same as BBC at the "back end".
   HV is accessed through the corrsponding line in

The ZDC (2* 3 PMT's) and SMD (2 MAPT's) are monitored with led flasher data which you can track with the on-line plots.

Also see Denisov's zdc tutorial at

The first ZDC module was installed in the RHIC tunnel on March 19th '99.




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(last updated: Nov. 2003)