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Author:Holm, Philip
Title:Neutron detection with high-energy photons using NaI portal monitor
Neutrondetektering med NaI gammaport genom högenergiska fotoner
Publication type:Master's thesis
Publication year:2012
Pages:xiv + 50 s. + liitt. 31      Language:   eng
Department/School:Teknillisen fysiikan laitos
Main subject:Ydin- ja energiatekniikka   (Tfy-56)
Supervisor:Salomaa, Rainer
Instructor:Toivonen, Harri
Electronic version URL: http://urn.fi/URN:NBN:fi:aalto-201209213143
OEVS:
Electronic archive copy is available via Aalto Thesis Database.
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Location:P1 Ark Aalto  236   | Archive
Keywords:neutron detection
gamma spectrometry
NaI
security technology
neutrondetektor
gammaspektrometri
NaI
säkerhetsteknologi
Abstract (eng): New neutron detection methods are developed worldwide due to the shortage of the isotope 3He.
In this thesis, an indirect detection method, that has not received much attention, is described.
The method utilizes high-energy photons emitted in neutron reactions such as neutron capture reactions and inelastic scattering.

One of the advantages of this method is that the detector can be used as a gamma spectrometer simultaneously.
Experimental studies were performed with a 4"x4"x16" NaI portal monitor and 252Cf and AmBe neutron sources.

The measurement results show that the most important reaction is the neutron capture reaction by iodine in the detector.
The detection efficiency can be improved significantly by covering three sides of the detector with polyethylene and polyvinylchloride.
The plastics serve as a moderator and converter.
A theoretical survey of possible converter materials restricted the materials to chlorine and iron.
The polyvinylchloride performed better than the iron.
The moderator and converter also lower the background radiation, which further improves the limits of detection.
The effect of the converter is insignificant if the thickness of the moderator and converter layer is under 4 cm.
With thicker layers than 10 cm, the efficiency does not improve significantly.

The NaI detector, boosted with the plastics, was compared with a He portal monitor.
The efficiencies and minimum detectable activities were of the same order of magnitude.
In fact these values were better for the NaI detector in most cases.
The NaI detector preserved its detection ability when neutron shields were used around the sources.
The NaI detector is thus a very promising alternative from a security point of view.
Abstract (swe): Nya metoder för neutronmätning utvecklas världen runt på grund av brist på isotopen 3He.
I detta diplomarbete beskrivs en indirekt detekteringsmetod som hittills fått mycket lite uppmärksamhet.
Metoden utnyttjar högenergiska fotoner som emitteras i neutronreaktioner, så som neutronkapningar och inelastisk spridning.
Denna metod har bland annat fördelen att detektorn simultant kan användas som en vanlig gammaspektrometer.
Experimentella studier utfördes med en 4"x4"x16" NaI detektor avsedd för portmonitorbruk och 252Cf och AmBe neutronkällor.

Mätresultaten visar att den viktigaste reaktionen är neutronkapning av jod i detektorn.
Detektorns effektivitet kan förbättras avsevärt genom att täcka tre av detektorns sidor med polyeten och polyvinylklorid.

Plasten fungerar som moderator och konverter.
En teoretisk studie över möjliga konvertermaterial begränsade konvertermaterialen till klor och järn.
Polyvinylkloriden konstaterades vara en effektivare konverter än järn.
Moderatorn och konvertern sänker också backgrundsstrålningen, vilket förbättrar alarmgränsvärdet ytterligare.
Konvertern har obetydlig verkan i upp till 4 cm tjocka moderator- och konverterlager.
Effektiviteten stiger inte avsevärt med tjockare lager än 10 cm.

NaI detektorn (förbättrad med plasten) jämfördes med en He-portmonitor.
Effektiviteten och de minsta möjliga detekterbara aktiviteterna var av samma storleksordning.
De facto var dessa värden bättre för NaI detektorn i de flesta fall.
NaI detektorn bibehöll sin detekteringseffektivitet bättre då neutronkällorna skyddades med neutronabsorbatorer.
NaI detektorn är således ett mycket lovande alternativ speciellt för säkerhetsapplikationer.
ED:2012-05-14
INSSI record number: 44521
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