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sec neutron generator for neutron radiography
NUCLEAR INSTRUMENTS
([97[) 6~3-6t7;
AND METHODS 92
©
NORTH-HOLLAND
PUBLISHING
CO.
USE OF A 1011 n/see NEUTRON GENERATOR FOR NEUTRON RADIOGRAPHY* A. R. SPOWART
United Kingdom Atomic Energy Authority, Dounreay Experimental Reactor Establishment, Thurso, Caithness, U.K. Although the experiment is still at an early stage, it is already clear that the unit can produce useful neutron radiographs of a variety of specimens with exposure times varying from 5-30 rain depending on exposure conditions. (Exposure times of 5 min for the direct technique, and 30 rain for transfer exposures.)
The tube output is being further increased and an enriched uranium booster is being installed to allow the collimation of the beam to be improved while keeping the specimen exposure times constant. The enrichment of the boosters is to be 75 % 2ssU.
I. Introduction Neutron radiography is a relatively new nondestructive testing technique, the first experimental work having been done in the period 1939-1945. Its main advantages are to offer increased penetration in certain materials (such as zirconium and niobium), very high sensitivity for the detection of certain elements (such as hydrogen, boron), and a complete discrimination against gamma ray fogging of the radiographic film. This latter advantage is only relevant to certain specialised applications of the technique, such as in the examination of irradiated nuclear fuels. For further details of basic techniques in neutron radiography the reader is referred to refs. 1-4. Most of the experimental work in neutron radiography has been carried out with neutron beams extracted from nuclear reactors. These beams have typical angular divergences of about 1°, and give thermal neutron fluxes of the order of 107n/cm 2 sec at
the specimen. Such beams can detect structural defects as small as 0.001" and give neutron radiographs of a quality comparable with X-ray radiographs. It is obvious that the necessity of using a nuclear reactor as a source of neutrons can be a serious hindrance to the application of this technique, with the result that a number of workers are actively studying the feasibility of using non-reactor neutron sources for neutron radiography. In 1968 we completed a design study 5) for a neutron radiography unit based on a 14 MeV sealed tube neutron generator with a projected output of 1011n/sec. The actual experimental work with the generator started in January 1970, and it is the purpose of this paper to present an interim report on progress with the equipment.
2. Description of equipment It is accepted that fast neutrons provide low radio-
Proofread by the Publisher.
NEUTRON
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TRAN$FER TECHNIQUE
=REc'r V,EW,.~. ~ T R A,.,SFOR,,,, .
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AI~DCONTROL PANELS. I l
Fig. 1. Mobile neutron radiography equipment layout
613 VII. M I S C E L L A N E O U S A P P L I C A T I O N S
614
A.R. SPOWART
Fig. 2. Mobile neutron radiography unit.
Fig. 3. Sealed tube P-type generator. 1. Target of tritium in erbium; 2. dummy Pirani; 3. Pirani; 4. suppressor electrode ( - 4 0 0 V); 5. extractor electrode ( + 115 kV); 6. ion source electrode ( + 120 kV); 7. cooled electron backstop (copper with enap. aluminium on front face); 8. solenoid focussing coil; 9. heat exchanger coils; 10. rf coils; 11. rf oscillator value; 12. steel case, inner polythene; 13. oil, 60 kV/2 m m strength.
A 1011
n/sec N E U T R O N G E N E R A T O R FOR N E U T R O N R A D I O G R A P H Y
615
Fig. 4. Comparison between a n¢utron (a) and a 150 kV X-ray radiograph (b). 1. Araldite seal; 2. steel spring; 3. cadmium tube in lead block; 4. dysprosium spacers; 5. silicon fluid in aluminium can; 6. steel plug; 7. rubber plug; 8. copper base for copper bellows.
graphic image contrast between different materials, so that the basic constituent of the unit is a moderating tank to moderate the approximately 14 MeV neutrons, produced by the (d,T) reaction in the sealed tube generator, down to thermal energies (approximately 0.025 eV). For reasons of geometric flexibility, a liquid moderator was chosen, namely transformer oil, which is basically a CH2 molecule. Transformer oil and water have virtually identical moderating powers due to their hydrogen content, but transformer oil was chosen because of its elimination of corrosion problems and its lower (n,7) capture cross section. Having selected the moderator, calculations showed that a 3" dia. tank filled to a depth of 3" with oil would give a reasonably "infinite" moderating system. The
neutron generator (14" diameter × 33" long), is mounted horizontally on the centre line of the tank, with the neutron producing target (which is at earth potential and at the end of the generator), on the centre line of the tank. The target is thus effectively surrounded by an 18" radius sphere of oil, since the inside of the generator can is itself filled with transformer oil apart from the volume occupied by the sealed tube and rf oscillator. Five ports are available in the tank for the insertion of neutron collimators, which are made either of aluminium with cadmium lining or of boral sheet (the boral sheet is about 0.25" thick and contains 250 mg/cm 2 of boron-uniformity distributed). All exit horizontally from the centre line of the tank; one in a VII. M I S C E L L A N E O U S A P P L I C A T I O N S
616
A.R. SPOWART
Fig. 5. Comparison between a neutron (a) and a 150 kV X-ray radiograph (b). 1. Brass casing on lead bullet; 2. explosive.
direction in-line with the generator, 2 inclined at 30 ° to the line of the generator, and 2 inclined at 90 ° to the line of the generator. Two basic designs of collimator are available; one in a plane parallel type, 3" dia. x 36" long, the other a divergent type with the same d/L ratio but giving a beam area of 24"×6" (d = beam collimator diameter, L = collimator length). The unit is shown schematically in fig. 1, and the actual arrangement is shown in fig. 2. The generator canister is shown in detail in fig. 3.
3. Experimental results Thermal neutron flux scanning was carried out using the 55Mn(n,y)56Mn reaction in a 0.25" dia. ×0.005" thick, the foils having a 2.6 h half life following neutron irradiation. A 2200 m/s cross section of 13.26 is
used to calculate the neutron flux from the measured gamma decay rate of the foil. With the tube operating at a power level of 5 x 10 l° n/sec, the peak thermal flux in the pure oil system was measured to be 6.8 × 10 v n/cm 2 sec using the standard manganese foil activation technique, this value being achieved at the target shield plate. This fell off to 6 × 105 n/cm 2 sec at the tank wall. By placing a block of natural uranium about 3" thick × 6" long x 8" high in front of the generator target, the peak thermal flux increased to 108 n/cm 2 sec at 1" from the uranium, falling to 5 × 105 n/cm 2 sec at the tank wall. The boost is achieved by fast fission in the 23sU, and by thermal neutron fission in the 0.7%
of 235U. The neutron flux at the specimen using the above
A l011
n/sec N E U T R O N
G E N E R A T O R FOR N E U T R O N R A D I O G R A P H Y
collimator design was measured to be 105 n/cm 2 sec with art intrinsic gamma ray contamination of approximately 6 R/h of about 1 MeV average energy. Collimation with a d/L ratio of 1 : 12 has been shown to give good image resolution for thin specimens, but the resolution falls off rapidly with increasing specimen thickness, and results to date indicate that the maximum tolerable specimen thickness with this degree of collimation is about 1" (see later radiographs of 1" thick specimen). Neutron exposure times of approximately 30 min are required to give film densities of approximately 1.5 using a 0.005" thick dysprosium converter screen and a medium speed non-screen X-ray film. Exposure times of approximately 5 min give similar densities using glass scintillator detectors and screen-type X-ray films with no problems of gamma fogging.
617
4. Typical radiographs Figs. 4(a) and 4(b) compare a thermal neutron radiograph and an X-radiograph of 1¼" dia. composite test piece. The neutron radiograph was produced with the dysprosium transfer technique (details as above). Figs. 5(a) and 5(b) compare a thermal neutron radiograph and an X-radiograph of some live ammunition. The neutron radiograph was again produced with the dysprosium transfer technique under identical exposure conditions to fig. 4(a). References x) H. Berger, Neutron radiography (Elsevier Publ. Co., Amsterdam, 1965). 2) j. Thewlis, AERE Report M/TN37 (1956). 3) H. V. Watts, Report ARF-1164-27 (1962). 4) j. p. Barton, Phys. Med. Biol. 9, no. 1 (1964). 2) A. R. Spowart, Nucl. Eng. 13, no. 144 (May 1968).
VII. M I S C E L L A N E O U S A P P L I C A T I O N S
([97[) 6~3-6t7;
AND METHODS 92
©
NORTH-HOLLAND
PUBLISHING
CO.
USE OF A 1011 n/see NEUTRON GENERATOR FOR NEUTRON RADIOGRAPHY* A. R. SPOWART
United Kingdom Atomic Energy Authority, Dounreay Experimental Reactor Establishment, Thurso, Caithness, U.K. Although the experiment is still at an early stage, it is already clear that the unit can produce useful neutron radiographs of a variety of specimens with exposure times varying from 5-30 rain depending on exposure conditions. (Exposure times of 5 min for the direct technique, and 30 rain for transfer exposures.)
The tube output is being further increased and an enriched uranium booster is being installed to allow the collimation of the beam to be improved while keeping the specimen exposure times constant. The enrichment of the boosters is to be 75 % 2ssU.
I. Introduction Neutron radiography is a relatively new nondestructive testing technique, the first experimental work having been done in the period 1939-1945. Its main advantages are to offer increased penetration in certain materials (such as zirconium and niobium), very high sensitivity for the detection of certain elements (such as hydrogen, boron), and a complete discrimination against gamma ray fogging of the radiographic film. This latter advantage is only relevant to certain specialised applications of the technique, such as in the examination of irradiated nuclear fuels. For further details of basic techniques in neutron radiography the reader is referred to refs. 1-4. Most of the experimental work in neutron radiography has been carried out with neutron beams extracted from nuclear reactors. These beams have typical angular divergences of about 1°, and give thermal neutron fluxes of the order of 107n/cm 2 sec at
the specimen. Such beams can detect structural defects as small as 0.001" and give neutron radiographs of a quality comparable with X-ray radiographs. It is obvious that the necessity of using a nuclear reactor as a source of neutrons can be a serious hindrance to the application of this technique, with the result that a number of workers are actively studying the feasibility of using non-reactor neutron sources for neutron radiography. In 1968 we completed a design study 5) for a neutron radiography unit based on a 14 MeV sealed tube neutron generator with a projected output of 1011n/sec. The actual experimental work with the generator started in January 1970, and it is the purpose of this paper to present an interim report on progress with the equipment.
2. Description of equipment It is accepted that fast neutrons provide low radio-
Proofread by the Publisher.
NEUTRON
~
I
•
i
7-
.
I
~
l
.
X--
-t l
J /
I
.
"
I
.
I-".' F....T. COOt.ANt
.
.
.
INI'r~FIE~ "~'~ ~
.,EO,Ro,
.
l ma
---(
~.__
_ k'
LE,,
COLLIMATORS
"~'=~ _ ~¢'~ - - - - I r~
.
.
.
i/~--=-~
"7 -/UZ:SS . . ~
BoosT ,
: --
" [I FI
~
~=_ " ~ , ~
1
' /~S** MIRROR
CADMIUM FILTER.
~
(O)METAL FOCL FO~
TRAN$FER TECHNIQUE
=REc'r V,EW,.~. ~ T R A,.,SFOR,,,, .
.
.
.
.
/
AI~DCONTROL PANELS. I l
Fig. 1. Mobile neutron radiography equipment layout
613 VII. M I S C E L L A N E O U S A P P L I C A T I O N S
614
A.R. SPOWART
Fig. 2. Mobile neutron radiography unit.
Fig. 3. Sealed tube P-type generator. 1. Target of tritium in erbium; 2. dummy Pirani; 3. Pirani; 4. suppressor electrode ( - 4 0 0 V); 5. extractor electrode ( + 115 kV); 6. ion source electrode ( + 120 kV); 7. cooled electron backstop (copper with enap. aluminium on front face); 8. solenoid focussing coil; 9. heat exchanger coils; 10. rf coils; 11. rf oscillator value; 12. steel case, inner polythene; 13. oil, 60 kV/2 m m strength.
A 1011
n/sec N E U T R O N G E N E R A T O R FOR N E U T R O N R A D I O G R A P H Y
615
Fig. 4. Comparison between a n¢utron (a) and a 150 kV X-ray radiograph (b). 1. Araldite seal; 2. steel spring; 3. cadmium tube in lead block; 4. dysprosium spacers; 5. silicon fluid in aluminium can; 6. steel plug; 7. rubber plug; 8. copper base for copper bellows.
graphic image contrast between different materials, so that the basic constituent of the unit is a moderating tank to moderate the approximately 14 MeV neutrons, produced by the (d,T) reaction in the sealed tube generator, down to thermal energies (approximately 0.025 eV). For reasons of geometric flexibility, a liquid moderator was chosen, namely transformer oil, which is basically a CH2 molecule. Transformer oil and water have virtually identical moderating powers due to their hydrogen content, but transformer oil was chosen because of its elimination of corrosion problems and its lower (n,7) capture cross section. Having selected the moderator, calculations showed that a 3" dia. tank filled to a depth of 3" with oil would give a reasonably "infinite" moderating system. The
neutron generator (14" diameter × 33" long), is mounted horizontally on the centre line of the tank, with the neutron producing target (which is at earth potential and at the end of the generator), on the centre line of the tank. The target is thus effectively surrounded by an 18" radius sphere of oil, since the inside of the generator can is itself filled with transformer oil apart from the volume occupied by the sealed tube and rf oscillator. Five ports are available in the tank for the insertion of neutron collimators, which are made either of aluminium with cadmium lining or of boral sheet (the boral sheet is about 0.25" thick and contains 250 mg/cm 2 of boron-uniformity distributed). All exit horizontally from the centre line of the tank; one in a VII. M I S C E L L A N E O U S A P P L I C A T I O N S
616
A.R. SPOWART
Fig. 5. Comparison between a neutron (a) and a 150 kV X-ray radiograph (b). 1. Brass casing on lead bullet; 2. explosive.
direction in-line with the generator, 2 inclined at 30 ° to the line of the generator, and 2 inclined at 90 ° to the line of the generator. Two basic designs of collimator are available; one in a plane parallel type, 3" dia. x 36" long, the other a divergent type with the same d/L ratio but giving a beam area of 24"×6" (d = beam collimator diameter, L = collimator length). The unit is shown schematically in fig. 1, and the actual arrangement is shown in fig. 2. The generator canister is shown in detail in fig. 3.
3. Experimental results Thermal neutron flux scanning was carried out using the 55Mn(n,y)56Mn reaction in a 0.25" dia. ×0.005" thick, the foils having a 2.6 h half life following neutron irradiation. A 2200 m/s cross section of 13.26 is
used to calculate the neutron flux from the measured gamma decay rate of the foil. With the tube operating at a power level of 5 x 10 l° n/sec, the peak thermal flux in the pure oil system was measured to be 6.8 × 10 v n/cm 2 sec using the standard manganese foil activation technique, this value being achieved at the target shield plate. This fell off to 6 × 105 n/cm 2 sec at the tank wall. By placing a block of natural uranium about 3" thick × 6" long x 8" high in front of the generator target, the peak thermal flux increased to 108 n/cm 2 sec at 1" from the uranium, falling to 5 × 105 n/cm 2 sec at the tank wall. The boost is achieved by fast fission in the 23sU, and by thermal neutron fission in the 0.7%
of 235U. The neutron flux at the specimen using the above
A l011
n/sec N E U T R O N
G E N E R A T O R FOR N E U T R O N R A D I O G R A P H Y
collimator design was measured to be 105 n/cm 2 sec with art intrinsic gamma ray contamination of approximately 6 R/h of about 1 MeV average energy. Collimation with a d/L ratio of 1 : 12 has been shown to give good image resolution for thin specimens, but the resolution falls off rapidly with increasing specimen thickness, and results to date indicate that the maximum tolerable specimen thickness with this degree of collimation is about 1" (see later radiographs of 1" thick specimen). Neutron exposure times of approximately 30 min are required to give film densities of approximately 1.5 using a 0.005" thick dysprosium converter screen and a medium speed non-screen X-ray film. Exposure times of approximately 5 min give similar densities using glass scintillator detectors and screen-type X-ray films with no problems of gamma fogging.
617
4. Typical radiographs Figs. 4(a) and 4(b) compare a thermal neutron radiograph and an X-radiograph of 1¼" dia. composite test piece. The neutron radiograph was produced with the dysprosium transfer technique (details as above). Figs. 5(a) and 5(b) compare a thermal neutron radiograph and an X-radiograph of some live ammunition. The neutron radiograph was again produced with the dysprosium transfer technique under identical exposure conditions to fig. 4(a). References x) H. Berger, Neutron radiography (Elsevier Publ. Co., Amsterdam, 1965). 2) j. Thewlis, AERE Report M/TN37 (1956). 3) H. V. Watts, Report ARF-1164-27 (1962). 4) j. p. Barton, Phys. Med. Biol. 9, no. 1 (1964). 2) A. R. Spowart, Nucl. Eng. 13, no. 144 (May 1968).
VII. M I S C E L L A N E O U S A P P L I C A T I O N S