Louis Baum University of Michigan, REU Student Summer 2011 UM Nuclear Sciences Group
Overview Background Why Detectors Time of Flight Road Blocks Conclusion Main Side Project
Californium 252 Fission Source Cf-252 α γ n SF H.I. 2β- Cf-248 Cm-248 α Pu-244 High spontaneous fission branch 1 3.09% 3.87 neutrons per fission 4 50 uci 5/15/05 - ~42,000 neutrons/sec (July 2011)
Background Neutron was detected in 1932 by James Chadwick with proton recoil methods Neutrons are harder to detect then gammas counts Gamma Neutron spectrum Energy http://www.oken.co.jp/o/eng_s/toku2.html
Why Neutrons are important constituents of matter Neutrons are vital for analysis of many nuclear reactions Nuclear security- radioactive material Nuclear reactors- clean energy Develop an advanced lab for physics majors
Detectors Plastic scintillator Optical couple (grease) Mu metal shield Photomultiplier tube High Voltage Base
Photomultiplier tubes Converts photons to a fast current pulse Anode Pulse Out Photoelectric effect or Compton scatter http://commons.wikimedia.org/wiki/file:photomultipliertube.png
Time of Flight Method n/γ Stop Detector Neutrons γ Start detector Cf-252 γ-rays Scintillator Scintillator
Time-of-Flight Spectrum using Cf-252 Path length 30 cm 1000 900 Prompt Gamma Peak 800 700 Counts 600 500 400 300 200 Full Width at Half Maximum ~ 4 ns SF Neutron Continuum 100 0-15 - 10-5 0 5 10 15 20 25 30 35 Time of Flight (ns)
Triggers and Thresholds Single Channel Analyzer (SCA) Constant Fraction Discriminator (CFD) 2
CFD Circuit CFD - Constant Fraction Discriminator TAC - Time to Pulse Height Converter MCA - Multi Channel Analyzer 0-5 V
No CFD Circuit TAC - MCA - Time to Pulse Height Converter Multi Channel Analyzer 0-5 V
The Apparatus Stop detector Start detector
Road Blocks Low Count Rate vs Separation Time Jitter Efficiency Correction
Time- of- Flight Spectrum 3000 path length 60 cm 2500 counts 2000 Over night Run 1500 1000 500 0-20 - 10 0 10 20 30 40 50 60 Time (ns)
MCNP- Polimi Detector SimulaFon Code Timing Resolution of 4 ns Time Jitter Dependent on the detector geometry Trigger dependent
DetecFon Efficiency CalculaFons Absolute Efficiency 0.008 0.007 0.006 0.005 0.004 0.003 0.002 0.001 Threshold Value 100 KeVee 250 KeVee 350 KeVee 600 KeVee 0 0 1 2 3 4 5 6 7 8 9 10 Neutron Energy (MeV)
Neutron Energy Spectrum Deduced Cf-252 corrected counts 14 hour run Neutron Energy (MeV) 4,5,6
Neutron Energy Spectrum Deduced Cf-252 Non-CFD CFD 5 2 corrected counts Neutron Energy (MeV) 4,5,6
Side Project Digitizer PSD work at the University of Notre Dame Accelerator Lab
Pulse Shape Discrimination Arbitrary Voltage Short gate Long gate Slower Decay N- Pulse γ- Pulse 0 5 10 15 20 25 30 Time (ns) M. Ojaruega
PSD of Cf-252 using liquid scintillator Neutrons Short Gate Long Gate Gammas
Uncalibrated Neutron Energy Spectrum counts Channel Number
Digitizer Pros: Can use 6-detector array Post processing flexibility High sample rate Cons Large data files
Acknowledgements NSF REU program U of M Fred Becchetti Mitaire Ojaruega Michael Febbraro Ramón Torres-Isea
Works Cited 1. Wolfram Alpha knowledgebase, 2011. 2. www.ortec-online.com/download/sca-introduction.pdf 3. O. I. Batenkov et al., INDC(NDS)-146, 1983. 4. E. Hjalmar, H. Slatis, and S. G. Thompson. Energy Spectrum of Neutrons from Spontaneous Fission of Californium-252, Phys. Rev. 100, 1542 1543 (1955). 5. A. S. Smith, P. R. Frelos, and J. H. Roberts. Spontaneous Fission Neutron Spectrum of Cf-252, Physical Review Vol. 108, # 2 1957. 6. N. V. Eremin, S. S. Markochev, A. A. Paskhalov, and E. A. Tsvetkov. Measuring High Energy γ Ray Emission Accompanying the Spontaneous Fission of Cf-252, Bulletin of the Russian Academy of Sciences. Physics, 2011, Vol. 75, No. 4, pp. 544 546. 7. I. A. PaweAlczak, J. T oke, E. Henry, M. Quinlan, H. Singh, W. U. Schroder. NSTAR - A Capture Gated Plastic Neutron Detector,. 8. Cf Nuclei S. A. Pozzi,, J. A. Mullens and J.T. Mihalczo. Analysis of neutron and photon detection position for the calibration of plastic (BC-420) and liquid (BC-501) scintillators, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. Volume 524, Issues 1-3, 21 May 2004, Pages 92-101.