Neutron Beam characterization of the Breazeale Nuclear Reactor at the Pennsylvania State University, Radiation Science and Engineering Center

Participants: J. S. Brenizer, Professor of Mechanical and Nuclear Engineering
  K. Ünlü , Professor of Mechanical and Nuclear Engineering
   
  C. Trivelpiece, Graduate Student
   
Services Provided: Neutron Beam Laboratory
   
Sponsor: Department of Energy, under Nuclear Energy Engineering Research (NEER) grant

 

Neutron Beam Applications

Neutron beams are used in a variety of experimental methods at research reactors all across the United States. Such applications include: neutron radiography and tomography, neutron depth profiling, neutron diffraction, etc. The aforementioned techniques employ neutron interactions, which are characterized by energy dependent cross sections. Accordingly, information characterizing the energy spectrum of these neutron beams is useful.

The current method being used for neutron spectroscopy at the Breazeale Nuclear Reactor is a time-of-flight (TOF) method using a slow neutron chopper that was brought from Cornell University, Ward Center for Nuclear Sciences.

Figure 1: Slow neutron chopper device (chopper slit and disk)

The TOF technique is based on measuring the time it takes neutrons to travel a predetermined distance. For this experiment, we measure the amount of time it takes for neutrons to travel from the aperture of the slow neutron chopper itself (Figure 1), to a lithium loaded glass detector. When the reactor is at power, a collimated neutron beam is incident on the chopper slit that can be seen in Figure 1. Inside the chopper device, a disk rotates and essentially fragments the incident neutron beam. Every rotation of the disk triggers an optical sensor that signals the beginning of the TOF measurement. The TOF measurement is completed for given neutrons when neutrons enter the detector and produce pulses. A computer program is used that provides the ability to measure multiple channels (corresponding to multiple neutron flight times) for each rotation of the chopper disk. The neutron spectrum is obtained from a multi-channel scaler (MCS) and each channel in the collected spectrum corresponds to specific neutron energy. Figure 2 is a typical energy spectrum collected using the current experimental setup. The neutron energy spectrum detectable with this particular system ranges from 0 ~ 0.3 eV.

Figure 2: Neutron energy spectrum collected using TOF slow neutron chopper method.

 

Currently, work is being done to make the slow neutron chopper system portable so that the system can be used at other research reactors. The original chopper setup consisted of the chopper, a collimating tube, detector and detector shielding. Also, multiple nuclear instrumentation modules (NIM) and bins needed for pulse processing and data analysis.

Recently, equipment was purchased to accomplish the goal of making the analysis portion of the chopper system portable. The new equipment includes:

Figure 3: Neutron Collinator Tube and Detector Shielding

 

Two ORTEC instrumentation modules (Spectroscopy Amplifier and an SCA) are used in the signal processing; however, these modules were also used in the original setup. The original MCS was located on a PCI card and required a computer with an open PCI slot for operation. The Multiport II MCA/MCS device uses an internal analog-to-digital converter (ADC) and a USB connection to connect to the Dell notebook, eliminating the need for a PCI card based MCS as well as a desktop computer with an open PCI slot. The Multiport II works like any other nuclear instrumentation module in that it is compatible with any NIM bin. This compatibility, coupled with the USB connection capability, makes the Multiport II one of the key pieces in making the slow neutron chopper system portable.

The design of the experimental setup (slow neutron chopper, collimator tube, detector and shielding) must also be redesigned in order to make the slow neutron chopper system portable. To accomplish the goal of being able to ship the system to other research reactors, system components must disassemble and reassemble with ease, and still meet their experimental needs. Currently, the slow neutron chopper is mounted on a stand which keeps the chopper slit at a constant height corresponding to the height of the neutron beam.

Figure 4: Proposed Polyethylene / Borated Aluminum Shielding Annuli

The collimator and the detector (w/ shielding) are aligned so that the slit and the detector are at the same height. The original system will be redesigned to give the components adjustable heights, so that the system can be used in different nuclear research reactors. The distance from the front of the collimator tube to the back of the detector shielding is approximately 2.5 meters (Figure 3). This length poses a problem when considering shipping methods. The collimator tube will be redesigned into sections, probably three, that can be easily attached together while keeping the strict alignment tolerances needed for this experiment. The detector shielding must also be redesigned for shipping purposes. One idea being discussed involves machining polyethylene and borated aluminum annuli, in which the detector will be placed (Figure 4). The polyethylene will be wrapped with boroflex to provide additional neutron shielding. Shipping cases and packing material will be purchased to complete this stage of the project.

To date, two experimental runs have been made at a reactor power of 850 kW. The first run was made with the original system (desktop PC, PCI based MCS card) to ensure that the system was properly aligned in the beam. Upon completion of this run, the detector was immediately connected to the new equipment (Multiport II, Dell notebook), and a second neutron energy spectrum was collected. The two spectra were compared and found to be similar, implying that the new equipment is suitable for use in this experiment. Experimental runs at different power levels will be made in the near future.

As mentioned earlier, the detectable neutron energy range for the slow neutron chopper system is 0 ~ 0.3 eV. It is desirable to measure the higher end of the neutron beam energy spectrum. An ORTEC Model 525-780 Helium-3 Neutron Spectrometer has been ordered which will allow neutron energy spectrum measurements of both thermal and fast neutrons. This spectrometer utilizes the 3He(n,p) 3H reaction. The incident neutron energy is determined by measuring the energies of the proton and the triton which are produced during the reaction. Shipping cases and materials will also be purchased so the helium-3 spectrometer can be shipped along with the slow neutron chopper system to characterize the energy spectrum of other neutron beam facilities.

Progress is being made to make the neutron beam characterization equipment portable, and extend the detectable energy range of the current setup. When completed, the slow neutron chopper system, combined with the helium-3 spectrometer, will make a useful tool for characterizing the energy spectrum of neutron beams at many research reactors across the United States.