1. MoNA detector physics
How to detect neutrons.
Thomas Baumann NSCL

2. What is a neutron?
Together with protons, they form the nucleus of an atom.
They are about as massive as protons.
They have no charge.
Neutrons only interact with the nucleus.
Atom
Nucleus
10–10 m
10–14 m
Outline:
what’s a neutron
neutron detection principles
MoNA detector module parts
scintillator
PMT
electronics
spectra
trigger logic

3. An atom is mostly empty space (for a neutron)
Outline:
what’s a neutron
neutron detection principles
MoNA detector module parts
scintillator
PMT
electronics
spectra
trigger logic
Neutrons don’t interact much with matter!

4. 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’)

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

6. MoNA detector module
voltage divider
PMT
magnetic shield
light guide

7. How a MoNA module works n p
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).

8. 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).

9. Photo-multiplier tube
photocathode
dynodes
anode e–
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!

10. 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.

11. tright= dright/vlight
MoNA electronics
tleft= dleft/vlight
tright= dright/vlight
CFD
TDC
TDC
CFD
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.

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

13. Cosmic rays Cosmic rays are energetic particles from outer space.
90% are protons.
They hit the outer atmosphere of our Earth (thank you!) and cause nuclear reactions.

14. Proton shower over Chicago

15. Cosmic rays in MoNA
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.

16. Cosmic rays in MoNA
TDC spectra:
self stop delay

17. Cosmic rays in MoNA
detector width
self stop delay

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

19. 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

20. 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).

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

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

23. 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.

24. 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.

25. 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.

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

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

28. MoNA-LISA

29. Acknowledgements