The Center for Chemical Characterization and Analysis (CCCA) provides a state-of-the-art capability in chemical analysis by neutron activation analysis (NAA). This powerful technique requires facilities and expertise which cannot be made available in every TAMU laboratory which might benefit from its use. Therefore, the laboratory is operated to provide research support to campus users, while also participating in original research in method development and service analysis for industrial customers.

    This guide has been prepared in an effort to provide information to potential users as to the general capabilities of the technique as well as to suggest possible applications. However, full utilization of the methodology can only be attained if users realize its likely benefits and incorporate NAA into their programs.

   Our staff stands ready to help in this endeavor in any way we can. Users are encouraged to contact Dr. Dennis James at (409)845-7630, FAX (409)845-1655, Email wd-james@tamu.edu or check our Internet site at http://www.chem.tamu.edu/services/naa/index.html for additional information or with any questions.
 
 
 

    Table of Contents

                    Introduction

                    Instrumentation
                            Neutron Sources
                            Gamma Ray Detectors
                            Pulse Height Analysis
                            Data Reduction
                            Compton Suppression y-SpectroscopyApplications

                    Applications
                            INAA (multielement)
                            INAA (single element)
                            FNAA

                    Submission of Samples

                    Sample Preparation
                            Laboratory Procedures
                            Cleaned Vials
                            Solids
                            Liquids
                            Others
                            Sealing Vials

                    Irradiation Scheduling

                    Spectroscopy (Counting)

                    P-Tube Operation

                    Appendices (not published on internet)
                            A-Weighing Procedures
                            B- Counting System Procedures
                            C- Pneumatic Tube Procedures
                            D- Charge Schedule
 

Introduction:

    The Neutron Activation Analysis Laboratory of the Center for Chemical Characterization and Analysis continues a long history of involvement in the development of nuclear analytical techniques at Texas A&M University. Much of the early development of the NAA method, especially the first applications of automation, were accomplished here by the forerunners of our center.  The first two meetings of the Modem Trends in Activation Analysis symposia, the premier scholarly international conference for practitioners of activation analysis methods, was organized by TAMU personnel and held on our campus in 1961 and 1965. The George Hevesy Medal, an award presented in memory of the "Father of NAA" to individuals who have demonstrated exceptional leadership and innovation in this field, has been awarded twice to TAMU researchers. No other institution can boast this record. This long history of participation in method research and development and application of the technique serves to define the commitment for our modern effort. The laboratory seeks to make activation analysis methodology available to TAMU researchers in whatever form it may be required. We have an active program which embraces academic research in many departments, focusing on the transfer of expertise in the procedures to students, who in turn, enter the scientific community with a better understanding of nuclear analytical techniques. The laboratory carries out its function by engaging in three major activities: method development research - necessary for us to retain our position of international leadership in the field and to ensure the capabilities we provide are always state-of-the-art; research support - provision of the necessary expertise and facilities to TAMU experimenters to support a wide variety of research; and service - extension of our support to the private sector m Texas and throughout the United States. The laboratory is also unique in that it provides NAA services not only from reactor produced thermal neutrons, but also accelerator based fast neutrons. This expands our capabilities to include those elements best determined using particle emitting reactions. Fast neutron activation analysis (FNAA) for oxygen determinations has become the definitive method over the last two decades. Determinations of several minor elements, including nitrogen, silicon and aluminum, are also routinely performed by FNAA. Our laboratory has a very active FNAA program which services requests from university and industrial customers.
 
 

Instrumentation:
 
        Neutron Sources - The laboratory uses two primary sources of neutrons for its measurements. TAMU's Nuclear Science Center operates a 1 Megawatt TRIGA research reactor approximately 46 hours per week. This facility provides neutron fluxes as high as 2 X 10¹³ n cm^-2s^-1. Pneumatic
irradiation positions are available for production of short-lived species and the return of samples to laboratory facilities. Transfer times are on the order of two seconds. The CCCA operates a laboratory at the NSC, housing high resolution germanium gamma spectroscopy and pulse height analysis instrumentation. For longer lived nuclides, rotisserie irradiation positions are available. Typical long irradiation times are from a few hours to fourteen hours (full operating day), however longer times can be achieved as required by irradiating over a period of days or even weeks. Samples irradiated in rotisserie positions are generally returned to the CCCA laboratories on main campus for processing. The laboratory operates two sealed tube neutron generators. These are located in the basement of the Teague building on campus. Both are manufactured by Kaman Sciences; one being the A710 model, the second the A711. The A710, about 25 years old, was refurbished in 1982 to upgrade to A711 electronics. It has a total neutron yield of about 10¹¹ neutrons per second. The newer version, the A711, has a live times higher output and is physically somewhat larger, but otherwise is very similar. These systems are very simple, straightforward to operate and easily automated. Associated with the A71 1 system is an automated sample transfer system which allows repetitive analysis of sequential samples without technician intervention. The transfer system is controlled by a Toshiba programmable logic controller (PLC). The PLC is readily programmed in "ladder logic" and has proven to be very reliable.

        Gamma Ray Detectors - The center operates several high purity germanium detector (HPGE) systems. In general, our detectors have been chosen for superior resolution. There is always a trade-off with HPGE detectors between resolution (the degree with which the detector can distinguish between gammas of different energies) and efficiency (the proportion of the emitted gammas which are actually detected). The larger high efficiency detectors usually have a somewhat degraded resolution due to the longer charge collection times required for the higher volumes of crystal Detectors which boast both high efficiency (>40-50%) and good resolution (<1.75 keV) are still very uncommon and therefore often prohibitively expensive. Poor resolution cannot be overcome by altering experimental conditions, therefore we buy the detectors with the best resolution we can find. Most of our detectors have relative efficiencies on the order of 25% (relative to a 3"X3" NaI(Tl) scintillator at 25 cm from the source). We have recently purchased two new high efficiency (~60%) detectors as well.  All of our currently used detectors were purchased from EG&G Ortec.

        Pulse Height Analysis - Signals produced by the gamma spectrometers are processed by a series of Nuclear Instrument Module (NIM) electronics to amplify and shape the analog pulses for input to a pulse height analysis (PHA) system These modules are of the highest quality, generally purchased from either EG&G Ortec or Canberra Industries. The multichannel analyzer (MCA) system installed in our laboratory is purchased from Canberra Industries. It is based on the new DEC (Digital Equipment Company) Alpha 64 bit processing technology. The CPU is a Digital Workstation 600au computer which communicates with acquisition front end modules via Ethernet. The analog to digital converters we use are very fast (5 ~s fixed conversion time), which reduces analyzer dead time and allows an increased rate of input.   We have also recently purchased a new digital signal processor which is the wave of the future for nuclear pulse processing.  Fast neutron activation analysis for oxygen presents a separate type of data acquisition and analysis. Our setup uses a PC based multichannel scaling (MCS) card purchased from Canberra Industries to store data from both the neutron monitor (BF3 neutron detector) and from the gamma ray of interest (Na~) detector). Each analytical run (generator warm up, irradiation, delay for transfer, and count) is documented in a single MCS sweep. MCS spectra of unknown materials are compared to those of standard materials for computation of oxygen concentrations. All calculations are performed on the data after transfer to the AXP system described above.

        Data Reduction - Spectra recorded during gamma spectroscopic acquisitions are reduced using commercial software purchased from Canberra Industries. The Peak program which determines peak area for gamma rays of interest and the NAA program are used to produce elemental concentration results for samples analyzed. The algorithms used in these programs are well documented and represent best current doctrine on spectral data resolution. Final results are routinely output into standard spreadsheet format for further manipulation by our users and for electronic transfer to remote computing facilities, either on campus or to other locations via Internet. Additional sophisticated statistical treatment of final data for certain programs is available within our laboratory.

        Compton Suppression y-Spectroscopy - The laboratory is in the process of constructing a Compton suppression spectroscopy system based on a large NaI~) annulus shield detector. The annulus is operated in anti-coincidence mode, allowing for the discrimination of any energy deposits in the germanium detector it surrounds, coincident with a signal in the shield itself The result is a great reduction in the Compton continuum of the gamma spectrum and therefore, a corresponding increase in the sensitivity of measuring small peaks which fall on this continuum. This will help in the detection of elements which exhibit gammas of relatively low energy (few hundred keV).
 
 

Applications:

        INAA (multielement) - By far, the most common use of the activation analysis methods is for instrumental multielement neutron activation analysis (INAA). The scope of applications on the TAMU campus is far too great to discuss in any detail in this document. The laboratory cooperates with perhaps a dozen academic departments in the use of INAA for the production of data of value to their programs. In most instances, the graduate students, postdocs or the professors do the majority of the hands-on work themselves. We provide a training environment to teach them the necessary procedures for them to come into our laboratories and use our existing equipment to perform their own analyses. This arrangement allows for a very large data production with only a minimal support personnel requirement within the center. An average of some 30,000+ analytical determinations are performed each year in this way. We realize that user performed analysis is not always the best choice, depending on the needs of the research program. In those cases, we offer fee for service support as well. Some of our larger users have included the departments of chemistry, geology, oceanography, animal science and anthropology.

        INAA (single element) - Elemental determinations of specific elements of interest are done routinely in our laboratory. In addition to an active program supporting campus users, extensive use is made of this capability by private industry. Determination of the halogens in plastics and production chemicals and transition metals in catalysts make up a large portion of this work.

        FNAA - Most of the FNAA work, as previously mentioned, is for the determination of oxygen. We average some 1000 to 1500 samples submitted per year, primarily from our industrial customers.
 
 

Submission of Samples:

        The details of sample submissions for NAA cannot be described without knowledge of the sample matrix and elements of interest. It is true that due to the high sensitivity the technique enjoys, the sample portions required for analyses often can be quite small We have analyzed individual hair samples weighing only a fraction of a milligram for arsenic, for example. However, that is far from an optimum case. hi general, quantities desirable for INAA are on the order of milligrams up to perhaps 0.2 g for solids and a few milliliters for liquids. FNAA generally requires more sample with some 10 to 20 g optimum for most materials. FNAA for oxygen, depending on the oxygen content, is performed on samples of a few milliliters up to 25 mL. For the best sensitivity in oxygen determinations of organic liquids, it is important that the 25 mL is available. We strongly urge that anyone interested in submitting samples for NAA work would contact the laboratory to discuss particulars of the project. Earl involvement of the laboratory in the project (prior to sample collection, if possible) is the best way to ensure the integrity of the samples for trace analysis.
 
 

Sample Preparation:

        The laboratory is equipped with sample preparation facilities which can be made available to NAA users. It is particularly advisable that our facilities be used, in fact, because the automatic balances we use store the sample weights in a data file which is directly compatible with the run description file used by the NAA software. This avoids tedious input of tabulated data by hand and the possible errors in doing so. The step by step details of the sample weighing procedures are presented in Appendix A to this document.

        Laboratory Procedures - The procedures we have implemented in our sample preparation laboratory are designed to assure accurate work without contamination of the sample or the lab.  Obviously, good standard laboratory procedures must be practiced. CCCA personnel will work with those without the necessary training to insure they are aware of those procedures they need to know.

        Cleaned Vials - The 2/5 dram and 2.5 mL polyethylene vials have been precleaned. It is important that we do not contaminate them after cleaning. The bags of vials should not be opened without gloved hands. Nothing goes inside the bag except a gloved hand or a cleaned instrument (tweezers, etc.). In the sample preparation lab, we use only polyfilm gloves. Latex gloves with talc cannot be used. Gloves must be changed regularly and without fail after touching anything else which might contaminate the sample. Identification is placed on the vials using an organic Sharpie permanent marker only. All instruments which come in contact with the sample of the irradiation vial should be cleaned. We usually rinse spatulas and tweezers with deionized/distilled water and dry them on Kimwipes between samples Post sample contact cleaning will depend heavily on the sample type. Disposable spatulas may be required with samples which cannot be completely removed from transfer instruments.

        Solids - Solids are generally weighed directly into preweighed, precleaned polyethylene vials for irradiation. They may be analyzed either after drying or 11as is11, depending on the design of the experiment. Drying of most materials can be accomplished in an oven at 1100 C. An oven is available in the laboratory if needed. Drying time is dependent on sample mass; perhaps 2 hours for milligram quantities or overnight for larger quantities. Precleaned weighing bottles (we often substitute liquid scintillation vials) should be used for drying and storing dried material. Lyophilization is available for samples which 'night tend to loose volatile elements of interest upon heating (such as organic tissue, etc.)

        Liquids - liquid samples often are handled in exactly the same manner as the solids described above. However, there are a few other things to consider. The total volume of liquid to be sealed in the container should not exceed 60% of the container volume. This will avoid over-pressuring the vial during irradiation in many cases. For pneumatic tube irradiations, we generally transfer no more than 1.2 mL into the 2/5 dram irradiation vials. These can be safely irradiated up to 10 minutes, even if the liquid is fairly volatile, if the vial is adequately heat sealed. All liquid samples (and standards) are weighed into the irradiation vials; we do not depend on volumetric transfers for accuracy. In some cases liquids are transferred after irradiation into an un-irradiated vial to avoid the blank. In this case accurate weighing of the sample is performed after transfer.It is sometimes advisable to evaporate the liquids to dryness~before irradiation. This is especially true for longer irradiations, or ptube irradiations of very small quantities of liquids which would become dispersed over a wide geometry during transfer. A heat lamp is generally used for this purpose. Samples should not be left unattended without proper control of the drying area (let someone know, leave a note) and should be removed from under the lamp immediately after dryness occurs (do not let the residue bake!).  Certain liquids, such as oils, are especially difficult because they cannot be evaporated and upon heating tend to seep through the polyethylene container. They also can cause sealing the vial to be very difficult if any is allowed to touch the rim of the vial. We often double encapsulate oils, first in the 2/5 dram vial, then a 2 dram vial for longer irradiations.

        Others - The variation of sample types which may be submitted for NAA is so great all possible procedures for handling them cannot be described here. CCCA personnel are available to discuss these procedures as needed.

        Sealing Vials - All vials irradiated in the reactor must first be heat sealed. We use a soldering iron arrangement located in the fume hood in the main sample preparation laboratory. The sealing process is critical. Poor seals will cause samples to open during irradiation, spilling sample material into the rotisserie can. This will contaminate all the other vials in the can. Even if cleanup is possible, it is extremely tedious and produces liquid radioactive waste. All users should get detailed directions from CCCA technicians before attempting their own seals! Failure to do so may result in the loss of all the samples in the can.
 
 

Irradiation Scheduling:

        Scheduling of neutron irradiations at the Nuclear Science Center can be done through our laboratory or independently. Depending on the source of payment for the irradiations, it is generally easier to work the schedules through our laboratory to avoid conflicts between irradiation and counting schedules. Scheduling is generally discussed in detail before the onset of major projects. Samples submitted on a fee for service basis, of course, will be scheduled by our technicians. Please contact the laboratory for clarification of the irradiation schedules, charges and potential neutron charge support available to TAMU users.
 
 

Spectroscopy (Counting):

        Laboratory users are welcome to make use of our high resolution gamma spectroscopy system after an appropriate training session. Appendix B of this document is intended as a step by step procedure to be used as a refresher for that training by our users. Scheduling of counter time should be done as soon as possible to avoid conflicts. Scheduled times should be adhered to. Situations have arisen in the past in which missed count periods resulted in lost data.
 
 

P-Tube Operation: