Vanderbilt Neutron Detector Array for the FMA

Short description of the array

The Vanderbilt neutron array consists of 16 liquid scintillator (NE213) detectors situated downstream with respect to the target chamber. The scintillators are coupled to PM tubes. The PM tubes require HV between 1500 and 2000 V which is supplied by the LeCroy main frame which can be controlled remotely. The electronics related to the neutron detectors, including the HV power suppliers, is located in the rack just behind the AYEBALL.

Each PM tube produces dynode (positive) and anode (negative) signals. The anode signal is integrated using two time gates. One gate covers the whole signal giving the total energy (E) and the second one is delayed by about 35 ns and integrates the slow component of the anode pulse (SE). The dynode pulse is used to obtain time with respect to the Rf signal from ATLAS (RF).

High Voltage control

HV for the neutron detectors can be controlled from the DAQ ALPHA. Tha main frame has number 16 and channels from 0 through 15 are used. The above values are stored in a file: U19:[FMA.NEUTRON]NELOW.HV. To apply them say: restore_hv U19:[FMA.NEUTRON]NELOW.HV. If you want to switch HV off say: restore_hv U19:[FMA.NEUTRON]NEZERO.HV. Note that the main frame has to be in a remote mode! Details of HV control can be found here.

The following voltages are applied to the individual neutron detectors:

          ch#0        1850 V
          ch#1        1650 V
          ch#2        1650 V
          ch#3        1750 V
          ch#4        1750 V
          ch#5        1800 V
          ch#6        1700 V
          ch#7        1600 V
          ch#8        1625 V
          ch#9        1600 V
          ch#10       1550 V
          ch#11       1500 V
          ch#12       1800 V
          ch#13       1700 V
          ch#14       1500 V
          ch#15       1650 V

Daphne histograms

The following important histograms related to the neutron detectors are sorted by DAPHNE at the moment:

• ESE0:ESE15 (2d) - total energy vs slow component for each neutron detector
• RF0:RF15 (1d) - neutron TOF, i.e. neutron detector vs RF

A combination of 1d gates put on each RFn spectrum and 2d gates put on each ESEn matrix provides discrimination between neurons and gamma rays.

• NE (2d) - total energy vs detector number
• NSE (2d) - slow component vs detector number
• NRF (1d ) - neutron detector TOF vs detector number

The above spectra give access to spectra corresponding to individual neutron detectors.

• NHIT (1d) - neutron detector hit pattern (neutrons + gammas)
• NHITN (1d) - neutron detector hit pattern (only neutrons)
• NHITE (1d) - total energy QDC hit pattern (neutrons + gammas)
• NHITSE (1d) - slow component QDC hit pattern (neutrons + gammas)
• NHITRF (1d) - neutron TOF TDC hit pattern (neutrons + gammas)

The above hit pattern spectra are being regularly displayed on the secondary screen and then deleted to provide a way to monitor the neutron detectors.