Next Generation Scintillation Detectors: Development of Quantum Dot Doped Scintillator. Lindley Winslow University of California Los Angeles

Transcription

1 Next Generation Scintillation Detectors: Development of Quantum Dot Doped Scintillator Lindley Winslow University of California Los Angeles

2 I am particularly interested in applications to... e - e - νi νi Nucleus Z Nucleus Z+2 Nuclear Process Neutrinoless Double Beta Decay

3 Neutrinoless Double Beta Decay The sum of the electron energies gives a spike at the endpoint of the neutrino-full double beta decay.

4 An explosion of technology!...and even KamLAND and SNO are getting in on the action!

5 An analytical form for comparing experiments:

6 How many sigma you would like to be able to measure.

7 Detector Efficiency Isotopic abundance Molecular Weight

8 Exposure time Background rate

9 Total Mass Being big is what kiloton-scale scintillators are good at!

10 Energy resolution is what they are not so good at... Energy resolution

11 Wrapped up in the background rate, is some method to convince yourself that you saw neutrinoless double beta decay. Best way would be to tag the daughter, but tracking the electrons would be nice too!

12 The angular correlation between outgoing electrons is fairly nucleus independent... Kotila and Iachello Angular Correlation And new physics could show up in this distribution! Phys.Rev.D76:093009,2007 Ali, Borisov, Zhuridov One electron energy

13 Can we do something better with Liquid Scintillator detectors?

14 Basic Principle of Neutrino Detectors Physics Light PMTs νe e - Z νe e -

15 Typical PMT Detection Efficiency: Peak Efficiency nm

16 Tune Scintillator Emmission: Nuclear Instruments and Methods in Physics Research A 440 (2000) 360 } 371 Typically, 200 photons detected per MeV with ~3ns timing resolution. Example is Borexino Scintillator.

17 Cerenkov Light Scintillation Light

18 Directionality Energy Resolution

19 The Cerenkov light is still there... Number of Cerenkov Photons for a 1MeV e- For KamLAND scintillator, this is 60 (10) photons per MeV above 400nm below 400nm the light is absorbed and reemitted as scintillation light.

20 What are the handles in a scintillator detector? Polarization? Number Timing Wavelength

21 Geant4 simulation NEW! Simplified R=6.5m spherical geometry. Simulating single 5MeV electrons. Current KamLAND scintillator and PMTs. Can we pick out the Cerenkov signal? From: Christoph Aberle

22 Results for 100 e- events: Cerenkov light more important at longer wavelengths.

23 As expected Cerenkov light is directed forward...

24 and the Cerenkov light arrives earlier... Note: 3ns transit time spread of KamLAND PMTs is not great.

25 Now with a 35ns cut we can pull out a directional signal... Event by event is going to be difficult, unless...

26 With perfect timing...

27 Much better directional distribution... and even event by event looks possible.

28 So the timing and photocathode coverage requirements point to something like the LAPPD (higher quantum efficiency would be nice too).

29 So new photodetectors can be used to tune all 3. Number Timing Wavelength

30 But can we do anything to the step before? Physics Light PMTs νe e - Z νe e -

31 Quantum Dot Doped Scintillator

32 What are quantum dots?

33 What are Quantum Dots? Quantum Dots are semiconducting nanocrystals. A shell of organic molecules is used to suspend them in an organic solvent (toluene) or water. Common materials are CdS, CdSe, CdTe...

34 Quantum Dot Materials Overlap with Candidate Isotopes! Isotope Endpoint Abundance 48 Ca MeV % 150 Nd MeV 5.6% 96 Zr MeV 2.8% 100 Mo MeV 9.6% 82 Se MeV 9.2% 116 Cd MeV 7.5% 130 Te MeV 34.5% 136 Xe MeV 8.9% 76 Ge MeV 7.8% 128 Te MeV 31.7%

35 Palo Verde Chooz The Previous Generation of Short Baseline Reactor Experiments

36 Aging of the Palo Verde Scintillator: Making stable metal doped scintillator is tricky.

37 Chooz s rising threshold: Instability affect quality of data and duration of data taking.

38 An older Double Chooz plot: Attenuation Length Wavelength

39 Quantum dots provide the chemistry for suspending isotope in scintillator.

40 Why are they so popular? Because of their small size, their electrical and optical properties are more similar to atoms than bulk semiconductors. In fact, the optical properties of quantum dots with diameter <10nm is completely determined by their size. Their size is easily regulated during their synthesis. bigger smaller

41 Example CdS Quantum Dot Spectra: They absorb all light shorter than 400nm and re-emit it in a narrow resonance around this wavelength. Very Useful for Biology, Solar Cells, and LEDs! surface states which can be eliminated with a second shell.

42 My scintillator is toluene with PPO Counts [Arbtrary Units] Toluene + 5 g/l PPO Wavelength [nm] Adding quantum dots will hopefully tune and narrow the peak of this curve.

43 First Results from ν. Because ν s are worth it.

44 Available at: JINST 7 (2012) P07010 arxiv:

45 Let s start with some basic measurements!

46 First spectrometer data with excitation with 280nm LED. Samples are: 20mL toluene + 5 g/l PPO g/l quantum dots.

47 How much light? Excite the scintillator with a 280nm LED. PMT Peak Sensitivity Counts [Arbtrary Units] Toluene + 5 g/l PPO NN-Labs 360nm Dots NN-Labs 380nm Dots Wavelength [nm] These dot have a 20% quantum efficiency, state of the art is > 80%.

48 How much light? Excite the scintillator with a 280nm LED. Counts [Arbtrary Units] PMT Peak Sensitivity Toluene + 5 g/l PPO Sigma-Aldrich 380nm Dots Sigma-Aldrich 400nm Dots Sigma-Aldrich 420nm Dots Wavelength [nm]

49 Do Quantum Dots Age? One of the NSF reviewers asked if this was an issue. Counts [Arbtrary Units] NN-Labs 380 nm Dots December Batch 1 December Batch 2 June 2011 June 2010 Toluene + 5 g/l PPO Wavelength [nm] No evidence for aging. The bigger issue for us seems to be batch to batch variations.

50 Sample 20mL 90 Sr β=1mev To 1GS/s waveform digitizer. Dark Box Simple Two PMT Setup

51 Does the scintillator still scintillate? Study the scintillator with a 90 Sr beta source. Rate per 20.0 ADC Units [Hz] Toluene + 5 g/l PPO Sigma-Aldrich 380nm Dots NN-Labs 360nm Dots NN-Labs 380nm Dots Sigma-Aldrich 400nm Dots Charge [ADC Units] The light yield is reduced compared to the standard scintillator

52 Do quantum dots change the timing characteristics of the scintillator? 4 10 Toluene + 5 g/l PPO Sigma-Aldrich 380 nm Dots 3 10 NN-Labs 360 nm Dots Photon Arrival Time [ns] The answer is no, though the quantum dot scintillator seems to have a slightly larger late light component.

53 Fitting to a three exponential model + PMT response: 4 10 Toluene + 5 g/l PPO Sigma-Aldrich 380 nm Dots 3 10 NN-Labs 360 nm Dots Photon Arrival Time [ns]

54 Quantum dots allow you unprecedented control over the wavelength response of your metal-doped scintillator.

55 So this is the idea... Better Scintillator Better Photo-Detectors = Better

56 Next Steps: ν. Last Spring 1L Detector - Now More quality control of the dots before using. Nitrogen purging for better light yield Larger quantum quantities Attenuation length measurements

57 The 1 L detector can be a neutron detector! Cadmium is a good alternative to Gadolinium.

58 Next Steps: ν. 1m 3 Detector Make use of knowledge from 1L detector Hopefully, experiment with new photodetectors. Make measurement of two neutrino double beta decay in 116 Cd.

59 Recall you can have Two Neutrino Double Beta Decay: ν. e - νe e - νe Nucleus Z Nucleus Z+2 Nuclear Process With 10g of 116 Cd, I expect 1000 events in 6 months.

60 Next Steps: ν. We are here Staged refurbishment of KamLAND between

61 Next Steps: ν. We are here Exciting work ahead!

62 The End