1. Neutron Detection with MoNA LISA
Anthony Kuchera
2016 PAN

2. How neutrons interact with matter
No Charge – does not interact by the coulomb force.
This means it is possible for neutrons to pass through significant volumes of material without any interaction (difficult for detection!).
Neutrons can scatter or be absorbed and converted:
Neutron energy and angle are significantly changed compared to before interaction
Replaced by one or more secondary radiations, typically heavy charged particles
Most neutron detectors measure the secondary radiation events which can be measured directly.

3. Neutron scattering
Elastic neutron scattering becomes useful because energy can be transferred to the scattering nucleus which can recoil with sufficient energy to generate a signal in some medium.
Hydrogen targets (protons) are common because the neutron elastic scattering cross section is large and a neutron can transfer up to its entire energy with a hydrogen nucleus.
Therefore recoiling protons are commonly used as the signature for neutron detection. 8

4. Scintillation Detectors
Scintillators convert kinetic energy of particles into detectable light with high efficiency.
High-Z materials are better for gamma detection (NaI, CsI).
Organic scintillators are better for neutron detection (hydro-carbons).

5. Scintillation Detectors
Scintillators convert kinetic energy of particles into detectable light with high efficiency.
High-Z materials are better for gamma detection (NaI, CsI).
Organic scintillators are better for neutron detection (hydro-carbons).
Kinetic energy from charged particle passing through molecules is absorbed causing them to excite.
De-excitation from the first excited singlet state emits scintillation light to lower states.

6. Zoom to nuclear scale (10-15 m)H n

7. Zoom to nuclear scale (10-15 m)H n

8. Zoom to nuclear scale (10-15 m)H n

9. Charged particle recoils
It is the secondary event of a neutron interacting with hydrogen or carbon nucleus that induces the scintillation process.
A fast moving neutron can cause a proton to recoil in the energy range from zero to the initial total energy of the neutron.
Proton recoils have a higher scintillation efficiency than carbon, but the interaction with carbon also plays a role.

10. How to “catch” the scintillation light
Scintillation light is emitted in all directions.
In practice, we want to detect as much of this light as possible.
What can we do?
Wrap in reflective material
Wrap with black material
Convert light to signal pulse
Figure from Thomas Baumann
J. K. Smith, Ph.D. Dissertation, NSCL 2014

11. Photomultiplier tubes (PMTs)
Scintillating detectors need light output to be converted into an electrical signal for counting.
The most common tool for this is the PMT.
Amplification can turn a few hundred photons into 1010 electrons!
The charge is then collected at the anode of the PMT
Figure from:

12. Any questions so far?

13. Neutron detection at NSCL
Neutrons at NSCL are commonly measured using their “time of flight”:
Intermediate energy ( MeV) neutrons detected with MoNA-LISA.
Low energy neutrons (down to 100 keV) detected with LENDA.

14. MoNA-LISA
Modular Neutron Array and Large multi-Institutional Scintillator Array
288 scintillators (200x10x10 cm3), each with 2 PMTs.
Horizontal position determined from time difference between PMTs.
Neutron kinetic energy measured from time of flight from target to bar.

15. Study of very neutron-rich nuclei
As nuclei become more neutron rich, the neutron separation energy becomes lower and eventually there are no more bound states.
Resonant states can exist for very small times before decaying.
12O
13O
14O
15O
16O
17O
18O
19O
20O
21O
22O
23O
24O
25O
26O Z
10N
11N
12N
13N
14N
15N
16N
17N
18N
19N
20N
21N
22N
23N 8C 9C
10C
11C
12C
13C
14C
15C
16C
17C
18C
19C
20C
21C
22C 7B 8B 9B
10B
11B
12B
13B
14B
15B
16B
17B
18B
19B
6Be
7Be
8Be
9Be
10Be
11Be
12Be
13Be
14Be
15Be
16Be
4Li
5Li
6Li
7Li
8Li
9Li
10Li
11Li
12Li
13Li
Neutron-rich nuclides with Z<9
All nuclei outlined in red have been measured with MoNA
2He
3He
4He
5He
6He
7He
8He
9He
10He 1H 3H 4H 5H 6H 7H
Stable
Bound
Unbound 2H n N

16. Spectroscopy of neutron unbound nuclei
Gamma Spectroscopy: Bound states
Invariant Mass Spectroscopy: Neutron-unbound states
S. R. Stroberg et al. Phys. Rev. C (2014)
C. R. Hoffman et al. Phys. Lett. B 672 (2009)

17. MoNA-LISA-Sweeper Experiments
fragments
neutrons
beam
To determine the decay energy from parent to daughter, we need to measure:
𝐸 𝑑𝑒𝑐𝑎𝑦 = 𝑚 𝑓 2 + 𝑚 𝑛 2 +2( 𝐸 𝑓 𝐸 𝑛 − 𝑝 𝑓 𝑝 𝑛 cos 𝜃) − 𝑚 𝑓 −𝑚 𝑛

18. MoNA-LISA-Sweeper Experiments
fragments
We need the x, y, z positions and the energy/momentum
neutrons
beam
To determine the decay energy from parent to daughter, we need to measure:
𝐸 𝑑𝑒𝑐𝑎𝑦 = 𝑚 𝑓 2 + 𝑚 𝑛 2 +2( 𝐸 𝑓 𝐸 𝑛 − 𝑝 𝑓 𝑝 𝑛 cos 𝜃) − 𝑚 𝑓 −𝑚 𝑛

19. Time differences in a single bar
Lets look at interactions in three different locations of a detector… t1
t1 = t2 → ∆t = t1-t2 = 0 t2
t1 < t2 → ∆t < 0 t1 t2 t1
t1 > t2 → ∆t > 0 t2

20. Horizontal position determination
Taking the time difference between two PMTs for one bar gives a relative position within the bar which can be calibrated to units of distance.
J. K. Smith, Ph.D. Dissertation, NSCL 2014

21. Time-of-flight spectra
In addition to the positions, we need to obtain the time it takes for the neutrons to travel from (very near) the target to the detector array.
J. K. Smith, Ph.D. Dissertation, NSCL 2014

22. Kinetic energy determination
We can get the velocity of the neutrons by measuring their times of flight and knowing the distance traveled. Together with the known neutron mass we can get their energy and momentum.
𝑣= 𝑥 2 + 𝑦 2 + 𝑧 2 𝑡
𝐾𝐸= 𝑚 𝑛 𝑐 − 𝑣 2 𝑐 2 − 𝑚 𝑛 𝑐 2

23. 𝐸 𝑑𝑒𝑐𝑎𝑦 = 𝑚 𝑓 2 + 𝑚 𝑛 2 +2( 𝐸 𝑓 𝐸 𝑛 − 𝑝 𝑓 𝑝 𝑛 cos 𝜃) − 𝑚 𝑓 −𝑚 𝑛
Decay energy
By measuring the positions and energies of the neutron(s) and fragment we can reconstruct the decay energy.
𝐸 𝑑𝑒𝑐𝑎𝑦 = 𝑚 𝑓 2 + 𝑚 𝑛 2 +2( 𝐸 𝑓 𝐸 𝑛 − 𝑝 𝑓 𝑝 𝑛 cos 𝜃) − 𝑚 𝑓 −𝑚 𝑛
Neutron
0.85 MeV
Fragment
A-1X+n AX

24. Energy calibration and gain-matching
Detectors can have small variations that require operating voltages to be adjusted to match output signals in energy.
For detecting low energy particles (>10 MeV), sources with well-known decay energies can be used for calibration.
To gain-match and calibrate MoNA-LISA, we need higher energy signals.
Thankfully in nature we have a reliable source…

25. Cosmic ray muons
Radiation from the sun and other galactic sources are mainly made of very energetic protons with some heavier charged particles.
From the interaction with the earth’s atmosphere, several types of secondary radiation are produced with energies up to 100s of MeV.
Upper atmosphere
Earth
Primary cosmic rays
Protons, heavy charged particles
109 – 1020 eV
Secondary radiation
Muons, pions, electrons
100s of MeV

26. Cosmic ray muons
At sea level, 80% of the remaining secondary radiation is made of muons with an intensity of about 1 muon/cm2/minute.
The energy deposited for muons is about 2 MeV/cm.
Upper atmosphere
Earth
Primary cosmic rays
Protons, heavy charged particles
109 – 1020 eV
Secondary radiation
Muons, pions, electrons
100s of MeV

27. Gain-matching MoNA-LISA
We can adjust the voltage applied to the PMTs to make sure the outputs from the different PMTs give an equal signal for the same energy signal.
J. K. Smith, Ph.D. Dissertation, NSCL 2014

28. Conclusions
One of the big challenges in detecting neutrons is the ability for them to scatter multiple times in detector without depositing its full energy.
Neutrons are typically measured “indirectly” by secondary radiation which produces signals that can be more easily measured.
Organic scintillator detectors are commonly used which produces a signal of light when a neutron transfers energy to a charged particle (typically proton) and that charged particle is absorbed into an molecule which is then excited and de-excites.
For fast neutrons, time-of-flight techniques can be used to determine the energy of a neutron.

29. Any questions?

30. Hands-on activity: what you will do
Observe cosmic rays interacting with detectors using oscilloscope.
Use a source of gammas/neutrons to determine detector response as a function of position.
Create a function to calibrate time difference (ns) and peak height (mV) into position (cm).
Determine source location at unknown position using signals on oscilloscope and your calibration function.