IEEE Transactions on Nuclear Science, Vol. NS-30, No. 1, February 1983
NEUTRON MULTIPLICATION MEASUREMENT INSTRUMENT
K. V. Nixon, E. J. Dowdy, S. W. France, D. R. Millegan, and A. A. RobbaLos Alamos National Laboratory
Mail Stop J562Los Alamos, New Mexico 87545
Summary
The Advanced Nuclear Technology Group of theLos Alamos National Laboratory is now using intelli-gent data-acquisition and analysis instrumentation fordetermining the multiplication of nuclear material. - _Earlier instrumentation, such as the large NIM-cratesystems, depended on house power and required addi-tional computation to determine multiplication or toestimate error. The portable, battery-powered multi-plication measurement unit, with advanced computa-tional power, acquires data, calculates multiplication,and completes error analysis automaticaUy. Thus, themultiplication is determined easily and an availableerror estimate enables the user to judge the signifi-cance of results.
Introduction
The technique used, to measure multiplication,analyzes the time correlation of neutrons emitted bythe fission process. If neutron detection events areuncorrelated (that is, random in time), the distributionof counts during an interval of fixed size is a Poissondistribution. However, because neutrons detectedfrom a fissioning system exhibit time correlationstemming from the nature of the fission process, weexpect a broader distribution than that of Poisson.We measure this deviation from a Poisson distributionusing the first two moments of the distribution andrelate this deviation to the neutron multiplication ofthe fissioning system. This theory is discussed inanother paper.'
Operation
The instrument is controlled by switches and push-buttons on the front panel. START, STOP, and RESETpushbuttons control the acquisition of data (Fig. 1). yAnalysis is automatic when acquisition is complete,and results are stored in the external memory. A16-position thumbwheel switch selects which valuewill be displayed on the 16-character liquid crystaldisplay (LCD). In conjunction with the ENTER push-button, another thumbwheel switch provides manualentry of constants such as deadtime or livetime. If theappropriate values are not entered manually, the micro-processor assigns default values for the constants.
The operator begins a typical measurement sequence byconnecting the instrument to a neutron detector pod with arandom neutron source, such as 241AmB, placed near thepod. He places the function switch at 0, tau, and pushes theSTART button. This measurement determines the value oftau, deadtime. After 106 intervals are counted, tau iscalculated and stored in the external memory. The neutronsource is replaced with a spontaneous fission source, 252Cf,and the operator begins the acquisition with the functionswitch either in position 1 or 2. At the completion of 10'intervals, Ys is calculated and stored in the externalmemory. If these values are known from earlier measure-
Fig. 1. Multiplication measurement instrument in operation.
ment, the user can omit this sequence and manually entertau and Ys from the front panel. The operator turns on theSTANDBY switch to maintain the values of tau and Ys andturns off the instrument.
After the detector pod and instrument are taken to themeasurement location, the user positions the detector nextto the nuclear material. The operator turns on the powerand pushes the START button with the function switch inposition 4. After 106 intervals are acquired, the value ofthe multiplication and the associated error estimates arecalculated and stored in the memory. If the printer isconnected, the microcomputer prints the original data andthe calculated results at the end of each measurement. Ifonly LCD display is preferred, the user moves the functionswitch to position 3.
Figure 2 shows the functional block diagram of the multi-plication measurement instrument. The NS80P50, a memberof the 8048 family of single-component microcomputers, isa central processor that controls the operation of the entireinstrument. This microcomputer allows the program memoryin the form of a 2732 erasable, programmable, read-onlymemory (EPROM) to be attached piggyback style. Thus, 4096bytes of program memory are provided in the small andconvenient package.
The microcomputer provides for 256 bytes of internalrandom access memory indirectly addressable as workingregisters. Two 6514 microcomputers provide an external1024-by-8-bit random access memory and supply the neededstorage of the large histogram (256 channels by 24 bits). Thefirst three stored pages constitute the histogram memory; thelast page, calculated values, such as tau, multiplication, andmean. This memory has a 3.6-V back-up voltage when thestand-by switch is on and the power switch is off. Theback-up system enables retention of the calculated valueswhile draining a maximum of 2 mA from the battery. Thus,the measured values of tau and Ys can be stored wheneverthe instrument is turned off and relocated at a target object.
Interfaced with the central processor by low-power,small-scale, integrated circuits, a number-oriented micro-processor, MM57109, provides calculational power.2Commands from the central processor are latched forcontinuous input into the number-oriented microprocessor.The calculator output is multiplexed onto the data bus, andcircuitry provides a handshake style of data exchange.
Input from the START, STOP, RESET, and ENTER DATAcontrol buttons causes an interrupt to the central processorand sets an associated bit in a latch. The microcomputerpolls the latch to determine the appropriate subroutine.
These interrupts and all other switch signals from the frontpanel are input to the microcomputer through Intel 8243input/output expanders. These devices provide the I/Oexpansion that consists of four 4-bit bidirectional static I/Oports. Thus, a single 4-bit master control port provides themany I/O control lines for the microcomputer.
The 5- by 7-in. dot-matrix LCD, EPSON MA-1955B,comes with the low-power circuitry needed to furnish thedrivers and multiplexers for the dot matrix. The charactergenerator, two 6654 CMOS EPROMs, furnishes a custom setof characters that includes the standard ASCII upper- andlower-case alphabet, some of the Greek alphabet, exponen-tials, and other characters used to display scientific data.
The serial port provides the data to be produced on theprinter or transmitted over the RS232 link to remotelocations. The serial port consists of a 6402 universalasynchronous receiver/transmitter and some discretecomponents to shift the logic levels to those compatible withRS232. The 6402 is now an industry standard and interfacesdirectly with the single-chip microcomputer.
The input signal from the neutron pod is provided at thefront panel through a standard high-voltage (SHV) connector.A single printed circuit card furnishes the preamplification,amplification, and discrimination. This unit also sumns thehigh-voltage output with the signal input, allowing a singlecable to the detector pod. This arrangement eliminates theneed for external preamplifiers and multiple cables that aredifficult to deploy for field measurement. The discriminatoroutput is a standard digital logic signal connected to an INTEL8253 counter/timer. This integrated circuit furnishes three16-bit timer/counter registers that are used for the datascaler and the interval timer. The 8253 interfaces directlywith the central processor through the multiplexed data busand allows efficient and convenient program control.
PREAMP-AMP HI STOGRAMtD SCR IM NATOR | 1 7 MEMORY
HIGH VOLTAGE SCALER 16-CHARACTERPOWER SUPPLY_L C D D ISPLAY l
r PARAI Fl _T| NUMBER- ll
T~~~O R232 MICROPROCESSOR
EXTERNALPROGRAMMEMORY +5V +1 2V -1 2V
FAST CHARGER -W 12-V BATTERY LOW VOLTAGE
7.6 AMPERE HOUR POWER SUPPLY
Fig. 2. Functional block diagram.
Software
The program was written with ASM-48 assemblylanguage, the only language available with the 8048 familymicrocomputers. A 4096-byte program memory was designedto accommodate extensive analysis. Over three-fourths ofthe program memory is used, which allows some expansionand indicates the size of the programming effort. Themodular software is efficient for development and testingand enables the transfer of useful subroutines to otherinstruments under development.3
The flow charts (Figs. 3-5) show the program structure.After the POWER ON RESET is complete, the micro-computer control lines are initialized and READY is printedon the display (Fig. 3). The program waits for an interrupt;when interrupt recognition occurs, the program identifieswhich of the four possible subroutines to service. If theDATA ENTER button is pushed, the control computer readsthe manual data thumbwheel switches. If the FUNCTIONswitch is at 0, the value of the data switches is read as 0 to9.99 ws and is stored ip the tau location in the memory. Ifthe FUNCTION switch is at 1, the data switches are read asX.XX for the mantissa of Ys and stored in that location. Theexponential of Ys is entered when the FUNCTION switch isat 2. The lifetime (L) of the pod is manually entered asXXX Pis when the FUNCTION switch is at 4. When theIVIANUAL DATA button is pushed, all other values for theFUNCTION switch trigger the LCD signal FC SWITCHWRONG.
A RESET interrupt clears the interval counter and thehistogram in the same manner as does a POWER ONRESET. The START interrupt clears the histogram andbegins the data acquisition. Acquiring data continues until1 06 intervals are counted and stored or until the STOPinterrupt is activated (Fig. 4). These actions cause theprogram to transfer to the COUNT END subroutine, whichcalculates tau, Ys, or multiplication (Fig. 5). The calculatedvalues are stored in the memory for LCD retrieval or serialport printout.
The CALCULATE subroutine requires only that theprogrammer point to a command table, a list of calculatorkeystroke entries necessary for solving the desired equation,before calling the calculator subroutine. The microprocessoroperation is similar to that of hand-held calculators, such asthe Hewlett-Packard HP-21 or the National SemiconductorNovus 4520, the Scientist. The user develops calculationsequences without detailed knowledge of the design or ofmicrocomputers. Once a table of keystrokes to solve theequation has been developed and tested on the calculator, itcan be entered directly into the program memory. Figure 6shows the structure of the CALCULATE subroutine. Noticethat besides the 64 calculator commands, memory-to-calculator inputs or calculator-to-memory outputs arepossible. The hexidecimal byte FF indicates the end of thetable and signifies that control will be passed back to thecalling routine. Fig. 3. Interrupt software flow chart.
The multiplication unit, tested and proven to be a
versatile and easily used instrument that saves the operatoranalysis time, contrasts sharply with earlier systems.'Furthermore, because error analysis is available at the scene,
the user can quickly judge the accuracy of the answers.
We are indebted to M. M. Stephens of the SafeguardsAssay Group for the design and development of the neutronamplifier-discriminator.
References
1. A. A. Robba, E. J. Dowdy, and H. F. Atwater, "NeutronMultiplication Measurements Using Moments of the NeutronCounting Distribution," submitted to Nuclear Instrumentsand Methods, October 1982.
2. S. N. Kim, "Number Cruncher (MM57109) Interface toMicroprocessor," Application Note AN-186, IM-B2077,National Semiconductor Corporation, July 1977.
3. K. V. Nixon and C. Garcia, "Hand-Held Pulse-Train-Analysis Instrument," IEEE Transactions on Nuclear Science,February 1983.
4. C. D. Ethridge, E. J. Dowdy, C. N. Henry, andD. R. Millegan, "A Microprocessor-Based Neutron CountMoments Logic Module for Special Nuclear MVaterial Assayby the Neutron Fluctuation Method," First Symposium onSafeguards and Nuclear Material Management, April 1979,Brussels, Belgium, LA-UR-78-31 74.
