Research

Brief outline of some of the research:
1. Topic: Fission:
1.i. Fragment spectrometer for improved yield data
- Collaboration with LANL
- Gathering mass data
- Current detector development focus on Z determination through fragment range in ionization chamber
1.iii. Fission simulations with focus on fission product distributions and decay, including n-fission and photo-fission
- Application in active interrogation and nonproliferation

2. Topic: Development of novel types of detectors/detection modes:
2.i. Room temperature semiconductor detector development - AlSb (III-V material)
- Predicted high electron and hole mobility and lifetime, high resistivity
- Have grown thin film, high quality material
2.ii. Optical effects in crystals for dosimetry
- Neutrons displace lattice atoms, change index of refraction
- New approach to N dosimetry, possibly gamma blind
- Unpowered materials used with laser-based examination, for integrating and live-time dosimetry
2.iii. Radiolytic ozone production from radiation fields, better low level characterization, many applications
2.iv. Spatial imaging of special nuclear materials. Applications to passive interrogation and nonproliferation


There are many research topics important to the nuclear engineering community within radiation detection but a few strong motivating themes dominate: nuclear safeguards and nonproliferation, and improved data, especially fission data. I built a multidisciplinary research program on radiation detection and detector development at the University of New Mexico with an emphasis on addressing these needs. My goal is to continue to conduct research in areas of high importance to the nuclear engineering community while supporting the overall mission of UNM Nuclear Engineering.

Below I highlight my research focus areas, beginning with addressing needs in fission data, making these data useful through simulations, and then new detector development work that can find applications in materials control and accountability for safeguards, and for detection out in the field.

1. Fission Fragment Data:

Model of fissioning system, fission preformation may be inferred from the fission products.

Fission fragment data is incredibly important for theory supporting advanced reactor concepts and for active interrogation in support of nonproliferation and safeguards. In terms of fundamental data, it is incredible what is still not known about fission. The report of the “Nuclear Physics and Related Computational Science R&D for Advanced Fuel Cycles (AFC) Workshop” [1] identifies in its section on “Nuclear Measurements” the need for new fission cross section measurements. The report singles out specifically the area of fission fragment yields and their kinetic energies as an area where data are needed to achieve a better understanding of the fission process.

i. Light ion experiments:
We performed beam experiments at Lawrence Berkeley National Laboratory in collaboration with colleagues at PNNL to examine fission fragments following 45 MeV 6Li on 232Th. Using coincidence gamma rays, both symmetric and antisymmetric fission products were observed, notably in the A=95, 122, and 132 mass regions. Many isomeric states were observed in the study, and previously unknown isomer half lives and gamma rays from neutron rich nuclei were determined [2].

ii. Neutron beam work

UNM Fission Fragment Spectrometer and preliminary 252Cf mass data (uncorrected)

Another approach to fission fragment data is high resolution data on fission mass distributions. We are in collaboration with researchers at the Los Alamos Neutron Science CEnter (LANSCE) at LANL on the SPIDER project (SPectrometer for Ion DEtermination in fission Research), to measure fission fragments, particle by particle, over a range of incident neutron energies over a range of fission targets. We were originally funded through a contract with Los Alamos and then changed over to a DOE-NEUP award.
See summary at: http://news.unm.edu/news/unm-professor-leads-team-exploring-nuclear-fission

We are building spectrometers and detectors at both LANL and UNM. The spectrometers are based on time-of-flight (TOF) fragment velocity measurements and ionization chamber (IC) energy measurements, with the data correlated particle-by-particle, which itself is very involved. From E and v for each particle, following E=1/2 mv2, we extract fragment mass. A mass distribution plot from the UNM spectrometer from correlating v and E is presented in the figure (before E loss corrections). Some of our TOF work is presented in reference [3] and more papers are in preparation.

The full collaboration with LANL is focused on building high resolution, good geometric efficiency spectrometers and gathering fission fragment data. LANL has considerable resources and personnel for their system development, and so the UNM effort is focused on prototyping detectors and methods to increase capabilities for both UNM and LANL, and in the process we make our own full spectrometer system and gather independent data sets. For example, we designed and tested the UNM ionization chamber that LANL based their IC design on. We built a full UNM-SPIDER detector and tested it with both alpha and fission sources, and with neutron beams on 235U at LANSCE.

By examining depth of penetration in the gas in the ionization chamber, using the IC as a time projection chamber, we are able to extract information on the charge state of the fragment. My group at UNM is currently focused on using an active cathode method in the ionization chamber to extract proton number (Z) information, beyond what the LANL group is currently doing with their focus on mass (A). Our capabilities thus will give us Z in addition to A (and thus we measure N). Moving forward to measure both fragments simultaneously in binary fission we will measure fission TKE and, almost directly, neutron multiplicity, both extremely important for theory. We are in the process of producing strong and novel data sets to deliver to theorists and the nuclear community.

iii. Fission simulations
A significant part of fission data is being able to use the fission data, especially important for applications such as active interrogation. To find clandestine special nuclear materials (SNM), such as may be smuggled in cargo to set a scenario, neutron or photon beams may be used to penetrate the cargo and induce fission. The delayed signal following fission, especially high-energy photons, is much stronger and easier to detect than the natural decay radiation from U or Pu.

To understand both the possibilities and limitations we have written Monte Carlo simulations, in Geant4 and in MCNP, to compare fission fragment distributions and thus the delayed radiation, to enable photofission using data from surrogate entrance channels to examine both photon and neutron beam interrogation, and tracking burnup following fission fragment production [4-7]. The collaboration in simulations with Prof. de Oliveira (NE) has been mutually beneficial as we bring complementary expertise, his on MCNP simulation coding and mine on the Geant4 coding, physics processes, and realistic detection considerations.

2 Novel detector development

Not only are we developing the fission fragment spectrometers and fission data, but we are working on novel detectors and materials that can serve as enabling technologies.

i. Room temperature semiconductor detectors

Probe station for radiation characterization with AlSb semiconductor samples and TEM profile image of different GaSb diode structures.

In nonproliferation, interdiction of SNM smuggling, or identification of radioactive threats - all important applications of nuclear science - gamma ray spectroscopy is an important mode of identification and quantification. Unfortunately, typical detectors such as NaI or other scintillators have low resolution, while high-resolution semiconductor detectors such as Ge are difficult to field.

There has been a lot of research on room temperature semiconductor detectors, and CdZnTe (CZT) has been of great interest. Due to poor hole mobility in CZT the induced signal pulse varies greatly depending on position of radiation interaction relative to the electrodes. This is one reason usable CZT crystals are on the mm scale, greatly limiting efficiency. Though tricks have been implemented using multiple electrodes, this is still a materials deficiency. AlSb had been proposed as a very strong candidate for room temperature detectors, with electron and hole properties predicted to be similar and near that for electrons in CZT [8], which would give a more symmetric detector response allowing larger theoretical volumes. As a dual carrier detector, this also improves statistics and therefore resolution over CZT. At issue is how to grow pure and defect free materials to test the theorized properties. LBNL was researching the material properties, improving methods in growing bulk crystals of AlSb, but Al oxides rapidly and Sb reacts even with the growth crucibles and high enough quality material was never obtained.

The expected characteristics of AlSb are too tantalizing, and there are other techniques to achieve high quality growth. Teaming with Prof. G. Balakrishnan in Electrical Engineering, an expert in epitaxial growth of antimonides, we have been pursuing a campaign to grow high purity, low defect AlSb for radiation detection an atom layer at a time using molecular beam epitaxy (MBE). We have ideas to scale up to thicker detectors. MBE growth allows much more control over the crystal than bulk techniques, and we are already seeing success with thin films. To distinguish roles in this, Prof. Balakrishnan's students grow the materials, and my student does materials electronic testing (Hall measurement, mobility, resistivity, carrier concentration, and soon carrier lifetime) and characterizing the radiation response, which feeds in to the next iteration of growth and material doping. Our research strengths are very complementary in this collaboration to make room temperature detectors.

We have grown and tested thin film GaSb [9], which has much more ideal purity and low defect properties. We have grown and tested several samples of AlSb, manuscript in preparation, and already used these for alpha spectroscopy, and results are guiding specifics of current material growth. If we prove the predicted properties then this opens a new material for room temperature detection and spectroscopy.

ii. Optical interrogation of crystals for dosimetry

Setup for measuring crystal refractive index change due to radiation, and time response of system without/with crystal.

We are also using standard materials in novel ways, such as crystals, which can survive extremely harsh conditions for use in neutron dosimetry. This may find use for materials control and accountability for safeguarding electrochemical fuels reprocessing, for example. This can be used for integrating dosimetry or, in the right conditions, live time dosimetry.

Radiation causes material damage, which can be assessed nondestructively in optical materials. Neutrons have a much higher efficiency than photons for causing lattice displacements and, with that change in the crystal structure and thus density, a change in the index of refraction. A method for extremely high resolution measurement of the index of refraction in crystals was invented by my collaborator, Prof. Jean-Claude Diels in physics, using interactions of optical combs. Mode locked lasers have a pulsing with the repetition rate based on the resonance and gain conditions in the cavity. Resonances for an inserted Fabry Perot etalon, our crystal sample, have a higher repetition rate. The light must meet the resonance conditions of both the cavity and the etalon simultaneously, and so as the index of refraction of the etalon changes the ratio of the frequencies change. We performed proof of concept experiments with a crystal irradiated by the UNM PuBe neutron howitzer, and our paper was recently accepted by the journal Nuclear Instruments and Methods A [10]. This work is based on Prof. Diels' expertise in optical frequency combs, and my group's experimental work on irradiation and interpretation of radiation interactions and damage. My group is working on applying this to neutron dosimetry, including matrial exposure to radiation and measurement of index changes, calibration of index change, and developing methods for stand off measurements for application to live time in situ measurements within reactors, which will be a big step forward in reactor diagnostics.

Novel applications of detectors
iii. Radiolytic Ozone
Radiolysis from ionizing radiation may produce ozone, and so ozone is investigated as a marker for possible radioactive sources for detection use in the field. Low-level production studies, through SBIR funding, are presented in [11]. My group at UNM is doing all the radiation work and radiolytic ozone characterization work, with project collaboration with Tanner Research Inc.

iv. Spatial imaging of SNM
It may be important to perform spatial imaging on radioactive materials, for example with mystery cargo. Several techniques exist for spatial imaging of radioactive materials such as Compton camera and coded aperture. Compton cameras require high enough energy photons for Compton scattering, which is not typical for special nuclear materials. Coded apertures are difficult for small angular distributions. We are examining, simulating, and performing proof of concept measurements on other spatial imaging techniques appropriate for SNM in cargo.

v. Muon imaging
Another research example is the use of muons for imaging, with proof of principle experiments such as we ran imaging the UNM reactor [12,13].

vi. Environmental work
Finally and of great importance in New Mexico, with Prof. Kenya da Cunha of civil engineering we are looking at U decay series in soils and water near mines in the four corners area [14] and examining uranium transport in the environment, manuscript in preparation. Prof. da Cunha does the chemical preparation of the samples and I perform the radiation measurements in my lab to identify and quantify isotopes, and we work together on the analysis. While this is not a novel detection technique, it is significant for understanding the front end of the nuclear fuel cycle.

References
g.s. denotes my graduate students
1. http://www.mcs.anl.gov/events/workshops/nprcsafc/Report_FINAL.pdf
2. Fission fragment isomers populated via 6Li + 232Th, J.J. Ressler, J.A. Caggiano, C.J. Francy, P.N. Peplowski, J.M. Allmond, C.W. Beausang, L.A. Bernstein, D.L. Bleuel, J.T. Burke, P. Fallon, A.A. Hecht, D.V. Jordan, S.R. Lesher, M.A. McMahan, T.S. Palmer, L. Phair, N.D. Scielzo, P.G. Swearingen, G.A. Warren, M. Wiedeking, Phys. Rev. C, 014301 (2010).
3. Development of position-sensitive time-of-flight spectrometer for fission fragment research, C.W. Arnold, F. Tovesson, K. Meierbachtol, T. Bredeweg, M. Jandel, H.J. Jorgenson, A. Laptev, G. Rusev, D.W. Shields, M. White, R.E. Blakeley g.s., D.M. Mader g.s., A.A. Hecht, Nuc. Instr. Meth. A 764, 53 (2014); doi:10.1016/j.nima.2014.07.001
4. Comparison of Geant4 and MCNP6/CINDER for use in Delayed Fission Radiation Simulation, A.A. Hecht, R.E. Blakeley g.s., W.J. Marting.s., E. Leonardg.s., Annals of Nuclear Energy 69, 134 (2014); doi:10.1016/j.anucene.2014.02.004
5. Dual neutral particle induced transmutation in CINDER2008, W.J. Martin g.s., C.R.E. de Oliveira, A.A. Hecht, Nucl. Instr. Meth. A 767, 163 (2014); doi:10.1016/j.nima.2014.08.048
6. Reactor fuel depletion benchmark of TINDER, W.J. Marting.s., C.R.E. de Oliveira, A.A. Hecht, Annals of Nuclear Energy 73, 547 (2014); doi:10.1016/j.anucene.2014.04.036
7. Dual neutral particle beam and density analysis for enhanced SNM detection, Rodney L Keith g.s., William J Martin g.s., Cassiano de Oliveira, Adam Hecht* Submitted to Nucl. Instr. Meth. B
8. Room-Temperature Replacement for Ge Detectors-Are We There Yet? P.N. Luke, M. Amman, IEEE Trans. Nuc. Sci. 54, 834 (2007); doi:10.1109/TNS.2007.903184
9. Thin Film Gallium Antimonide for Room Temperature Radiation Detection, Erin I. Vaughan g.s., Nassim Rahimi, Ganesh Balakrishnan, Adam Hecht*, Submitted to Journal of Electronic Materials
10. Novel Techniques for High Precision Refractive Index Measurements, and Application to Assessing Neutron Damage and Dose in Crystals, K. Masuda, E.I. Vaughan g.s., L. Arissian, J.P Hendrie, J. Cole g.s. g.s., J.-C. Diels, A.A Hecht*, Nuc. Instr. Meth. A (2014), accepted 11/11/14. http://dx.doi.org/10.1016/j.nima.2014.11.031
11. Radiolytic yield of ozone in air for low dose neutron and x-ray/gamma-ray radiation, J. Cole g.s., S. Su g.s., R.E. Blakeley g.s., P. Koonath, A.A. Hecht, Radiation Physics and Chemistry 106, 95 (2015); doi: 10.1016/j.radphyschem.2014.06.008
12. Imaging a nuclear reactor using cosmic ray muons, J. Perry g.s., M. Azzouz, J. Bacon, K. Borozdin, E. Chen, J. Fabritius II, E. Milner, H. Miyadera, C. Morris, J. Roybal, Z. Wang, B. Busch, K. Carpenter, A.A. Hecht, K. Masuda, C. Spore, N. Toleman, D. Aberle, Z. Lukić, J. Appl. Phys. 113, 184909 (2013); doi: 10.1063/1.4804660
13. Identifying Nuclear Materials Using Tagged Muons, C. L. Morris, J. D. Bacon, K. Borodzin, J. M. Durham, J. M. Fabritius II, E. Guardincerri, A. Hecht, E. C. Milner, H. Miyadera, J.O. Perry g.s., D. Poulson g.s. , Submitted to Nucl. Instr. Meth. A
14. Ground water contamination with 238U, 234U, 235U, 226Ra and 210Pb from past uranium mining: Cove Wash, Arizona, K. da Cunha, H. Henderson, B.M. Thomson, A.A. Hecht, Environmental Geochemistry and Health (2013); doi:10.1007/s10653-013-9575-2