LA-7108 UC•15 e 4 l1111ued February 1978 Neutron Detector Suitcase for the Nuclear Emergency Search Team E J Dowdy C N Henrv R D Hastings s W • Franca clenllllc laboratory of the Univenity of California LOS ALAMOS NEW MEXICO 17545 I An Afflrmotiva Adlcn Ec aual Opportunily En p loyer UHITl D f TAT•• D•PAIITMICN'I' Of' SNWRGY CONTRACT W -t•o••rtQ II This work was partly supported by the US Depmment of Bnergy Safeguards and S curity Mlrtnfkt f OOM1 l J'lal t GSI llll-U15 1 76-100 IOJ•llJ S J 1111 ti -un Ufl fff lffO 101-ll $ 1%4-250 '· ' 6 00 UMU - · '· l2¼• 2'o Ul•l1S 0 •llS HM TJ 10 n U M u u 1100 IJ 30 J1MDO 4 01 1' • u 'l'-'JO ffi •n ' ' w ••'11•«4 • 111'11 •I do rpo-fH If 1lw t1Altf'4 S Atn 4 untnL Ntll htr di• Un kd lhodH U IJ td PfJ1111Vl l' l1 ol fl rU IUll' lln' r u 1 fffllllO ffl n lf COI Jbliil ln w lff'a•l•ula n l J th tr l'lft O 'tff ffllkH AMI WlntlllY UPN• tit lfllplitd n u tU'lt nc ' ' -mn Ult ta I bl h1f or 1'f11Q bil U I toe th in'Uh ' ' ll' °fit '- 11 u ful u ut nn mr»fffi t fl•tt ••• 111 _ 1 1 P 4 tl ff j J lt•fl •lt11rh t tit U l'P_ U lli•I tl l UM 1 h nJ p nkl't IIYM'4 llfht aMH jffi--'SU S S 1• 1' SU -600 60lsVp NEUTRON DETECTOR SUITCASE FOR THE NUCLEAR EMERGENCY SEARCH TEAM by E J Dowdy C N Henry R D Hastings S W France - - ·i ABSTRACT A portable high-efficiency neutron detection system has heen constructed for tbe l' uclear Emergency Search Team It includes an alarm system based on tbne Interval meaeuremente of the incoming neutron detection pulses The Y•tem la de llgned for transportation by vehicle in searob lng for neutron-emitting radioactive materials INTRODUCTION Elements of the Loa Alamos Scientiffo Laboratoey LASL act both operationally and as a research and develoPlllent agency for the Nuclear Emergency Search Team NEST NEST is responsible for responding in a timely fashion to acts of extortion that invclve threatened use of special nuclear materials SNM or other radioactive materials One possible phase of the response to credible threats is to 11earch for materialll claimed to be held by the extortionist A rll' t move ie to dispatch a few trained penionnel with hand-held or otherwise portable instrumente that can detect the latent radiations expected The only radiations from radioactive materials detectable at a distance are neutrons and gamma rays and all current search instruments are based on detection of these penetratiDg radiations Thia report describes a high-efficiency portable neutron detection ystem developed for the NEST The syetem--detectors amplifiers discriminators power supplies batteries battery charger and deto ction logic circuitry-is packaged in a single 46 by 66 by 30 cm custom-made aluminum •suitcase• that weighs approximately 32 kg The three-section suitcase is shown in Fig 1 and details of the various components are described in the following sections DETECTORS In each outer section of the suitcase there are eight 'He-filled neutron sensitive proportional counters 4 lt 10' Pa fill prlll su re fl cm in diameter with a 63 cm active length They are in a planer array in each Bl ction with the outer shells of adjacent detectors in contact The detectors are hare on the outer side of the closed suitcase and are in contact with a 0 64-cm-thick sheet of polyethylene on the inner aide The bate side of the detector array is intended to be exposed in the expecto d direction of the radiation souzce The rather small amount of moderatiDg material near the detectors Will chosen Ill a reault of earlier neutron detector improvement studies 1 as well ae to save weight The neutron suitcase was designed to be carried in a vehicle in normal traffic pattemo end the radiation source is expected to be remote and enclosed Our studies indicate that the resultant neutron spectrum at the 1 Fig l The neutron detector suitcase detector location is soft and too much moderating material would decrease the detection efficiency ELECTRONICS All the electronic circuits the batteries and battery charger the signal conditioners and the detection logic are contained in the center section of the suitcase A Power Supplleo The low-voltage 12 6-V power supply eonaista of 10 D-size rechargeable NiCd batteries with an approximate 65-h lifetime between charges The highvoltage 1410-V supply consists of 47 B-sl re nonrechargeable 30-V carbon dry cells The drain on the high-voltage supply is email so the supply lifetime is es entlally the shelf life of the batteries Figure 2 ahowa the charger for the low-voltage battery pack partly removed l'rom its storage compartment 2 Fig 2 Charger for low-voltage battery pock lifted from its storage compartment B Signal Conditioning All 16 proportional count ere are ganged together Into a single a1I1plifier and discriminator With 1410V biaa on the countel'1 the amplitude of pul llls from the counteta due to the 'He n • p T reaction is approximately 4 mV The amplifier gain is appro imately 101 resulting in an approximately 1-V pulse from the amplifier for the 4-mV input pulse The discriminator threshold is variable but it is normally set to provide a logic output pulse for analog signals ebove approximately 1 V C Detection Logic The detection logic is hased on meMurement of the time intervals between pulsee as implemented by a-fold analysis • This type of analy is was chosen for detector systems whose normal count rate ie low Successive short counting intervals should yield small numbera that would p1'1 Clude OaU oian statistical analyses and cha nee overlap of a counting Interval and the optimum time window for the source is highly bnprobable The s-fold techni- que based on the Poist10n distribution is a preferred method for analyzing source-ielated counts The a-fold circuit diagramed in Fig 3 produces an lllarm output when a compa1it10n scaler ac• cumulates a clock count number smaller than the number that repreaentll the average background count ar-rival rate diminished by a certain amount to provide the proper trip level The clock pulse scaler content is measured when the neutron pulse input scaler accumulates the number of pulses that the operator hes preselected In a nonnal mobile aeerch the backf ToUnd is expected to fluctuate the logic circuitry automatically accounts for this longterm• luctuetion The prinm-y clock frequency is adjusted digitally in direct proportion to the incoming neutron pulse rate The •hort-terrn count rate is thus compared with a longer term• but non stationary background count rate The false alarm probability per comparison Is constant but cause the number of comparison• between the incoming pulae rate and the CUffi l l baekf Tound rate i proportional to the average count rate the false alarm rate per unit time is directly proportional lo the average count rate S-FOLD WlrH KEYBOARD ENrRY-OIAGRAM LOAO RIM SERI IL AOl ER I ii El@@ IIJ fil@ -UR ER IUDISTl R 'ii'1 c J r t l J fii l l---- L _ _ _ _ __ _ _ _ _ _--1 C t - Fil c _ c _ · · a The a-fold circuit 3 The logic circuitry Fig 8 operates 8ll follows The primary clock frequency Fo is approximately 1 MHz This frequency is divided by 2 before being scaled ln binary counter D The contents of binary counter D ls loaded into the buffer when the con tents of binaxy counter N equals 32 and binary counter Dis reset As Fig 3 indicates the number in binary counter N is M- 1 times the number of input pulses from the discriminator The load and reset pulses are generated whenever the content of binary counter N equals 32 which requi res a certain •· lapsed time At Thia At corresponds to M x 32 input pulses The count accumulated in binary counter D in At ls Binary counter A ia presented with a pulse frequency CONTROL PANEL The suitcase control panel is contained in a sep111ate comparlment in the center section which is removable from the center but is connected to the suitcase by 2 4-m-long cables Figure 4 showa the control panel removed Figure 6 is a closeup of the panel The functions and indicators on this panel are as Collows A Power A three-position rotary switch selects the power source for the suitcase The OFF CHG position is the normal OFF which is also used to charge the lowvoltage batteries There are two ON positions one provides power from the internal batteries the other selects the external power source that can be any 12-V de supply of adequate power rating B Internal Battery Test that is independent of the primary clock frequency Fo but is adjusted as At changes This change in 6t provides for adjustments appropriate to long•term background fluctuations Binary counter A receives a load and reset pulse on th average of every MIR seconds and transfers to shift register SR-1 an average count of When the incoming pulse rate incre8lles near a neutron source the load and reset pulses for binary counter A increase in frequency momentarily caus ing a number mailer than 256 to be transferred to shift register SR-l The count CA is shifted out of ohift register SR-1 lnto shift register SR-2 and into a serial adder In the 3-fold circuit used in the suitcaae three such sequential time intervals are summed and the sum is presented to the compare circuit which compares the current sum to the operator-selected trip level If the time between incoming pulses has been compressed enough the trip level is reached and the alarm relay is activated A push-to-test switch end OK condition light are provided for the Internal low-voltage batteries Fig 4 The control panel withdrawn from the suit- ca11e 4 I • E Alarm An audible signal is generated when the trip level is reached if the audible alarm is not inhibited The signal duration is determined by the position of the system reset switch · F System Overflow A light-emitting diode is activated if the selected M causes the capacity of the clock pulse binary counter D to he exceeded G Panel Meter and Rate Divide Fig 5 The suitcase control panel C System Reset A three-position toggle switch provides the following alerm-enable functions UP - The alarm la NlBet at ihe end of each sampling interval MIDDLE - The alarm is inhibited DOWN - The alarm is continuous once activated until reset It is reset by placing the switch in the MIDDLE position and returning it to the DOWN position The time Interval number from the serial adder is converted from digital to analog to provide a meter deflection for operator convenience To reduce unnecessarily large meter deflections the serial adder output can be divided by using the rotary rate divide switch H B-fold Interface A keyboard ill provided fot inputing appropriate values for the prescale multiplier M and the trip level TL The M and TL values are determined for the speed of the vehicle cerrying the suitcase background in the area tolerable false alarm rate and distance to the structures suspected of containing the device D Remote Output Recorder An internal digltel-to-analog converter produce• an analog output of the 3-fold time interval for use with a strip-chart recorder • I Alarm A relay is elooed momentarily when the trip level is Nlached in the logic circuit The resulting signal maY be used as an event merk on a strip-c art record or it can be scaled for alarm rate determinations OPERATIONS The neutron detector suitcase is deployed as shown in Fig 4 The control panel is extracted Crom the center section and taken to the operator's seat With the suitcaee powered the M and TL values are put in These values are determined from the expressions ' M•'f•S - 3 and TL• 1i r 11 meaeurementa of CA for a nonfluctuating input count rate i e it represents the adder output that corresponds 'to an input detector count rate of tJ The number from the serial adder is reduced by the facto x 68 where B is ths average background count rate S is the minimum source signal above background expected for alarming usually taken from Polsaon tablee consletent with an acceptable talse alarm rate and 'C ill the time over which the source signal is expected to be detectable above the background In terms of the vehicle speed v and the distance to suspect objects t the time 'C is eitpressed as 1f T T to give an Input detector count rate that correeponde to the sum of the nonfluctuating background 11 and the source signal S The mobile eeareh can start when appropriate M and TL values have been registered REFERENCES l Group R-2 Quarterly Progrees Report OctoberDecember 1975 unpubllllhed LASL data The numbet 768 in the formula for TL ill the number that normally comes from the serial adder shown In Fig 8 It reptesenta the sum of three 6 2 See for eumple R D Evans 7lul Atomic NucleWJ McGraw-HIii 1955 p 794