MoNA detector physics

1. MoNAdetector physics How to detect neutrons. Thomas BaumannNSCL

2. What is a neutron? 10–10 m 10–14 m • Together with protons, they form the nucleus of an atom. • They are about as massive as protons. • They have no charge. • neutrons don’t interact much with matter

3. Neutron detection • Neutrons can’t be detected directly. • Nuclear reactions are needed that create an energetic charged particle: • Neutron capture (e.g. 10B + n  7Li* + ) • Neutron scattering (e.g. H + n  H’ + n’)

4. MoNA detector module plastic scintillator light guide photo-multiplier tube voltage divider

5. MoNA detector module

6. How a MoNA module works n • Plastic scintillator is made up of H and C. • The neutron scatters off one of these. • The moving charged particle excites the scintillation material and causes light emission (scintillation). p

7. How a MoNA module works • The scintillation light travels along the detector bar (total internal reflection). • At each end, the light is detected by photo-multiplier tubes (PMTs).

8. Photo-Multiplier Tube photocathode dynodes anode • The photons release photo-electrons at the photo cathode of the PMT. • MoNA uses 12-stage PMTs, that multiply the initial photo-electron by 3107 at 1800 V. • Now we have an electric pulse! e–

9. MoNA electronics t • The anode signal is used for timing. • A constant fraction discriminator (CFD) triggers on a specific fraction of the pulse. • Output of the CFD is a logic pulse. • This provides amplitude-independent timing.

10. MoNA electronics • The CFD logic output provides the start signal for the time-to-digital converter (TDC). • A common stop signal stops all TDCs at the same time. tleft= dleft/vlight tright= dright/vlight CFD TDC TDC CFD 0 0

11. MoNA electronics delay • In coincidence experiments, the common stop is provided by a timing detector in front of the Sweeper. CFD common stop CFD TDC CFD TDC 0 0

12. Cosmic rays • Cosmic ray muons are used to test and calibrate MoNA. • MoNA is also used as a sky survey instrument for cosmic rays. • For cosmic ray runs, the timing is done differently: • There is no charged particle to give a common stop. • MoNA runs in self-stop mode. • The first MoNA hit creates the common stop.

13. Cosmic rays µ TDC spectra: self stopdelay 0 0

14. Cosmic rays detectorwidth µ 0 0 0 self stopdelay

15. TDC spectra • The time-difference spectrum of cosmic rays shows the edges of the detector bar. W.A. Peters, PhD thesis

16. QDC spectra • Timing information is not all that is recorded. • The signal integral is recorded using charge-to-digital converters (QDCs). • The charge-integral is a measure of the produced light in the scintillator bar. QDC gate t

17. QDC spectra • The light output is related to the deposited energy in the scintillator. • So the QDC signal tells us how much energy was deposited (we have to assume which particle it scattered off, however).

18. Cosmic–Ray QDC spectrum • Shows peak from cosmic muons. • Pedestal peak and CFD threshold are also visible. W.A. Peters, PhD thesis

19. MoNA trigger logic • MoNA trigger logic is based on field-programmable gate arrays (FPGAs).

20. MoNA trigger logic • The level-1 trigger logic has 32 inputs (one detector wall). • It provides the trigger for the level-2 logic, scalers, and QDC gates.

21. MoNA trigger logic • The level-2 trigger logic has trigger inputs from the level-1 logic and the charged particle detectors. • It provides coincidence logic, readout control, veto, fast clear, multiplicity, and bit-pattern readout.

22. FPGA basics • The FPGA is made up of configurable logic blocks (CLBs). • Shown here is one half of a CLB (one slice), containing 2 logic cells.

23. FPGA basics • The CLBs are surrounded by programmable input/output blocks (IOBs), which in turn are interconnected.

24. FPGA basics • The FPGA contains a matrix of 32 by 48 CLBs, totaling 6912 logic cells or 322970 system gates!