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Neutron collimators with plates of self-contracting foils
NUCLEAR INSTRUMENTS AND METHODS IO8 (I973) I O 7 - I I I ; © NORTH-HOLLAND PUBLISHING CO.
N E U T R O N C O L L I M A T O R S W I T H P L A T E S OF S E L F - C O N T R A C T I N G F O I L S H. MEISTER and B. WECKERMANN Physics Division, Euratom C.C.R., Ispra, Italy
Received 7 November 1972 Airplane model paper and thermoplastic foils have been used two identical paper collimators of 10' collimation angle are for the construction of neutron collimators. A methodical reported. instruction and the results of a performance test carried out with 1. Introduction Collimators used in thermal neutron spectrometry usually consist of a number of equidistant and parallel metal plates, covered with neutron absorbers such as cadmium or boron (Soller collimators). To obtain optimum performance these plates should be very thin and very straight. Depending on the dimensions of the collimator and the degree of collimation desired, it often becomes necessary to construct quite sophisticated plate stretching devices which make the collimators bulky and expensive. The collimator construction can then be simplified by replacing the metal plates with a material which contracts upon special treatment. In section 2 of this report we give methodical instruction in how to build neutron collimators of airplane model paper. Another method based on the use of plastic foils which shrink at temperatures around 100°C is outlined briefly. The performance data of a 10 min paper collimator tested in a reactor beam are presented in section 3.
with a series of holes for the collimator bolts to pass through. The width of the sheet is about that of the final collimator plate plus double the width of one distance plate. One glues this sheet onto two distance plates which are kept at a clear width corresponding to that of the collimator. The paper extending equally far beyond both distance plates is folded around and glued onto their reverse sides. This sheet of paper with the distance plates on it is then suspended between two rows of bolts which are fixed in one of the collimator support plates. They are short dummy bolts which exactly take the place of the
2. Collimator fabrication The composition of a paper collimator resembles that of an ordinary Soller collimator without any plate stretching devices. The central part of the collimator consists of the collimator plates with distance plates on top and bottom in between them, pressed together by means of two rows of bolts and a plane and rigid support plate on either side of this package (see fig. 1). The novelty of the paper collimator derives from the materials used for the plates and their respective fabrication procedure. We will describe this procedure for one collimator plate, leaving out all mechanical details. The final assembly of the collimator is straightforward once the collimator plates are available. The basic material of the collimator plate is the kind of tissue paper which is commonly used to cover the frames of airplane models. One cuts a sheet of the length of the distance plates which are metal stripes
Fig. 1. Paper collimator with clear cross section of 10 x 25 cm2. 107
]08
H. M E I S T E R A N D B. W E C K E R M A N N
real collimator bolts, but have a slightly reduced diamater at their free ends which pass through the holes of the distance plates. The paper which at this stage is quite flabby, is painted with the proper lacquert). The paper stretches as it dries. It is then painted again, this time with a paint containing boron z) to make it absorb neutrons. Finally, the wavy parts on either free edge of the paper (see fig. 2) must be cut away. This cut follows the stress line in the paper which for this purpose can be determined sufficiently well by eye. Collimator plates fabricated in this way come out all alike without much attention being paid to tolerances during the fabrication process. The final thickness of the plates is between 0.12 and 0.15 ram. They still retain a certain elasticity, which makes it possible to stretch them by the difference between the diameters of the dummy bolts and the real collimator bolts during final assembly of the collimator. This difference has to be chosen in accordance with the clear width of the collimator. In our case, for example, the clear width is 250 m m and the difference of bolt diameters is 0.3 ram. The tension of the collimator plates in the final arrangement is not influenced by air humidity or changes in the room temperature. Another construction method we tried out in a preliminary way after the paper collimators were completed and which seems promising and less time consuming, is that of using a type of plastic foil which shrinks at about 100°C3). The bolts which are fixed in one of the support plates have sharpened edges at their free ends and inside threads for the screws which hold the second support plate. The beginning of a long plastic foil which is as wide as the collimator is long, is stretched across the two rows of bolts. Pressing on top with a pair of distance plates, one cuts holes through the plastic foil which is then folded around one distance plate and stretched on top of both again across the two rows of bolts. This wrapping-up procedure is
repeated till the package has the necessary dimensions, and then the second support plate is mounted. The beginning and the end of the foil are fastened on the outside of the collimator. The whole unit is put into an oven and heated up to 100°C. When cooled down again, it is immersed in the boron paint. With an appropriate cutting device (see fig. 3) one must finally cut away the wavy end parts of the collimator plates as described above.
3. Collimator performance The experimental arrangement for testing the collimator is sketched in fig. 4. Using only collimator 1 together with the BF3-counters, one first measures the angular dependence of the reactor beam intensity and determines the beam axis. Subsequently, collimator 2 is set up so that its axis coincides with the beam axis. The distance between the two identical collimators is about 2 m. Their plates are vertical. While rocking collimator I around its vertical symmetry axis, one records the intensity transmitted through both collimators as a function of the rocking angle 7. To avoid dead-time errors in neutron counting, the beam emerging from the 2 cm dia. beam channel is attenuated by a Cddiaphragm with horizontal slit. The slit width is 1 . 0 m m for 4.2x10-3~
coLlimat or sheet
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D/STANCE PLATE ,*"-
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DUMMY B O L T S T R E S S
LINE
i
~---- --,----;- --g- --g-- -g-Fig. 2. Collimator plate u n d e r tension with wavy e n d parts.
~
'"'\support prate
'\spacers Fig. 3. Schematic sketch o f collimator fabrication procedure using a thermo-plastic foil.
NEUTRON COLLIMATORS
BEAM
109
INTENSITY
oC
• I,," IE
--n
C:_-
SHIELDING
Cd - DIA PHRA GM Cd-SHUTTER
COLL. I
TRA NS M~ 5SION
/ COUN;
COLL. 2
-eft. RA TE
IRPRRPI
B F3
COUNTERS
Fig. 4. Collimator test arrangement. In order to obtain a reference standard for the collimator p e r f o r m a n c e we have calculated the collim a t o r rocking curve for two identical collimators of collimation angle F and transmission T (see fig. 4) with absolutely nontransparent and plane plates. Assuming that the angular dependence of the p r i m a r y neutron intensity in the angular range ]el ~< F can be a p p r o x i m a t e d by I = Io(1 - c e Z ) , where ~ is the angle between the direction of neutron incidence and the b e a m axis, this calculation gives: [I~+2(y)]-_rr = I o ( T / r ) 2 [ 4 C 3 / 3 - 4 c r ~ / 1 5 +
+ ];(2/'2 -- 2cF'*/3) + y 2 ( F - c F 3 ) + + ~3(1/6 - 5eF2/6) - ~,4cC/3 - ~ c / 2 0 ] ,
(1)
for the angular range - 2 F ~ < 7 ~ - F , and [11 +z(7)]°-r = I o ( T / F ) 2 [2F3/3 - cF5/15 - 72F _ y3(1/2 _ cF2/6) + y4cF/3 + y5 3c/20],
(2)
for the angular range - F ~ y ~ 0. (The indices refer to the collimators placed in the beam.) 11 + 2 (7) is of course symmetrical to ? = 0 and is zero for 2 F ~< ]7[.
110
H. M E I S T E R A N D B. W E C K E R M A N N
2,0"
I 0 "J
-t,o
1,8
o,8
/
L4 t,2
o,6
t,o . tO-J
o,4
o,a o,6
.o,2
0,4 O,2 0
.;,w.,~, .=,~,==
- 12
-8
.
"l
-4
I
0
,
4
n=,l " . " " =,
8
"-
(T"
lO-3FO
12
Fig. 5. Collimator rocking curve; - - theoretical curve for two identical collimators taking into account the angular dependence of the primary beam intensity, x x experimental results (wings x 500).
With F = 2.94 x 10 -3 and c = 1.8 x l04 which are the parameters describing the collimation angle of our collimators and the actual angular dependence of the primary neutron intensity, and after normalization to I1 +2(0)= 1, one obtains the theoretical rocking curve depicted by the solid line in fig. 5. Comparing the experimental points with the theoretical rocking curve, one finds satisfactory agreement with the exception of the extreme wings. An intensity of some 10 .4 is observed where the theoretical curve has already reached zero. This happens precisely in the ,/-range where transmitted neutrons have to pass through one collimator plate only. At still larger values of y the intensity of transmitted neutrons diminishes. Since the effective absorber thickness is to a first approximation constant over the ),-range in question, we conclude that the average absorber thickness is certainly adequate, but that there are small inhomogeneities in the collimator plates. This conjecture is confirmed if one holds one of the collimator plates, which are black due to the boron paint, against a bright lamp: light shines through many minute holes in the paper.
Another essential quantity involved in the performance of the collimator is the transmission T. It can be deduced from the maximum intensity of neutrons transmitted through collimator 1 and 2 and the intensity after removal of collimator l: 11+2 = Io T 2 ( 2 F / 3 -- cF3/15),
(3)
12 = I o T ( F -- cF3/6),
(4)
and consequently: T = (11 + 2/lz) (1 - c F 2 / 6 ) / ( 2 / 3 -
cr2/l 5).
(5)
The experimentally determined value of I~+2/12 is 0.573 +_0.004 which leads to T = 0.850+0.006. The value T = 0.85 corresponds to an effective increase of the plate thickness by wavyness of 0.1 mm which seems quite moderate in view of the plate dimensions of about 465 x 250 mm 2.
NEUTRON COLLIMATORS
4. Conclusion
The use of materials such as airplane model paper or plastic foils which shrink upon special treatment, has been proved to offer considerable advantages for the fabrication of neutron collimators because of (a) simple and therefore lightweight construction, (b) high transmission values due to small plate thickness and little wavyness and (c) collimation features which closely approximate the theoretical ones. With regard to the latter point, plastic foils give even better performance than paper, which has a less homogeneous tissue texture.
111
For the collimator fabrication, the authors are greatly indebted to the technical services of the C.C.R., Ispra, and especially to Mr H. Geist, Head of the Construction Office, who developed the plastic foil method.
References 1) The airplane model paper and the stretching lacquer have been bought in a toy shop. 2) Supplier of the boron paint is: Bollig und Kemper, Lackfabrik, 5 Kbln 30, Postfach 320 370, Federal Republic of Germany. 3) Thermo-plastic foils on polyethylene basis as used for industrial packing.
N E U T R O N C O L L I M A T O R S W I T H P L A T E S OF S E L F - C O N T R A C T I N G F O I L S H. MEISTER and B. WECKERMANN Physics Division, Euratom C.C.R., Ispra, Italy
Received 7 November 1972 Airplane model paper and thermoplastic foils have been used two identical paper collimators of 10' collimation angle are for the construction of neutron collimators. A methodical reported. instruction and the results of a performance test carried out with 1. Introduction Collimators used in thermal neutron spectrometry usually consist of a number of equidistant and parallel metal plates, covered with neutron absorbers such as cadmium or boron (Soller collimators). To obtain optimum performance these plates should be very thin and very straight. Depending on the dimensions of the collimator and the degree of collimation desired, it often becomes necessary to construct quite sophisticated plate stretching devices which make the collimators bulky and expensive. The collimator construction can then be simplified by replacing the metal plates with a material which contracts upon special treatment. In section 2 of this report we give methodical instruction in how to build neutron collimators of airplane model paper. Another method based on the use of plastic foils which shrink at temperatures around 100°C is outlined briefly. The performance data of a 10 min paper collimator tested in a reactor beam are presented in section 3.
with a series of holes for the collimator bolts to pass through. The width of the sheet is about that of the final collimator plate plus double the width of one distance plate. One glues this sheet onto two distance plates which are kept at a clear width corresponding to that of the collimator. The paper extending equally far beyond both distance plates is folded around and glued onto their reverse sides. This sheet of paper with the distance plates on it is then suspended between two rows of bolts which are fixed in one of the collimator support plates. They are short dummy bolts which exactly take the place of the
2. Collimator fabrication The composition of a paper collimator resembles that of an ordinary Soller collimator without any plate stretching devices. The central part of the collimator consists of the collimator plates with distance plates on top and bottom in between them, pressed together by means of two rows of bolts and a plane and rigid support plate on either side of this package (see fig. 1). The novelty of the paper collimator derives from the materials used for the plates and their respective fabrication procedure. We will describe this procedure for one collimator plate, leaving out all mechanical details. The final assembly of the collimator is straightforward once the collimator plates are available. The basic material of the collimator plate is the kind of tissue paper which is commonly used to cover the frames of airplane models. One cuts a sheet of the length of the distance plates which are metal stripes
Fig. 1. Paper collimator with clear cross section of 10 x 25 cm2. 107
]08
H. M E I S T E R A N D B. W E C K E R M A N N
real collimator bolts, but have a slightly reduced diamater at their free ends which pass through the holes of the distance plates. The paper which at this stage is quite flabby, is painted with the proper lacquert). The paper stretches as it dries. It is then painted again, this time with a paint containing boron z) to make it absorb neutrons. Finally, the wavy parts on either free edge of the paper (see fig. 2) must be cut away. This cut follows the stress line in the paper which for this purpose can be determined sufficiently well by eye. Collimator plates fabricated in this way come out all alike without much attention being paid to tolerances during the fabrication process. The final thickness of the plates is between 0.12 and 0.15 ram. They still retain a certain elasticity, which makes it possible to stretch them by the difference between the diameters of the dummy bolts and the real collimator bolts during final assembly of the collimator. This difference has to be chosen in accordance with the clear width of the collimator. In our case, for example, the clear width is 250 m m and the difference of bolt diameters is 0.3 ram. The tension of the collimator plates in the final arrangement is not influenced by air humidity or changes in the room temperature. Another construction method we tried out in a preliminary way after the paper collimators were completed and which seems promising and less time consuming, is that of using a type of plastic foil which shrinks at about 100°C3). The bolts which are fixed in one of the support plates have sharpened edges at their free ends and inside threads for the screws which hold the second support plate. The beginning of a long plastic foil which is as wide as the collimator is long, is stretched across the two rows of bolts. Pressing on top with a pair of distance plates, one cuts holes through the plastic foil which is then folded around one distance plate and stretched on top of both again across the two rows of bolts. This wrapping-up procedure is
repeated till the package has the necessary dimensions, and then the second support plate is mounted. The beginning and the end of the foil are fastened on the outside of the collimator. The whole unit is put into an oven and heated up to 100°C. When cooled down again, it is immersed in the boron paint. With an appropriate cutting device (see fig. 3) one must finally cut away the wavy end parts of the collimator plates as described above.
3. Collimator performance The experimental arrangement for testing the collimator is sketched in fig. 4. Using only collimator 1 together with the BF3-counters, one first measures the angular dependence of the reactor beam intensity and determines the beam axis. Subsequently, collimator 2 is set up so that its axis coincides with the beam axis. The distance between the two identical collimators is about 2 m. Their plates are vertical. While rocking collimator I around its vertical symmetry axis, one records the intensity transmitted through both collimators as a function of the rocking angle 7. To avoid dead-time errors in neutron counting, the beam emerging from the 2 cm dia. beam channel is attenuated by a Cddiaphragm with horizontal slit. The slit width is 1 . 0 m m for 4.2x10-3~
coLlimat or sheet
t
/ }cutting device
/
/
D/STANCE PLATE ,*"-
@ .
.
.
® .
.
.
.
.
• .
.
.
.
® .
.
.
.
',
® .
.
.
.
.
.
~
® '.
.
.
.
.
DUMMY B O L T S T R E S S
LINE
i
~---- --,----;- --g- --g-- -g-Fig. 2. Collimator plate u n d e r tension with wavy e n d parts.
~
'"'\support prate
'\spacers Fig. 3. Schematic sketch o f collimator fabrication procedure using a thermo-plastic foil.
NEUTRON COLLIMATORS
BEAM
109
INTENSITY
oC
• I,," IE
--n
C:_-
SHIELDING
Cd - DIA PHRA GM Cd-SHUTTER
COLL. I
TRA NS M~ 5SION
/ COUN;
COLL. 2
-eft. RA TE
IRPRRPI
B F3
COUNTERS
Fig. 4. Collimator test arrangement. In order to obtain a reference standard for the collimator p e r f o r m a n c e we have calculated the collim a t o r rocking curve for two identical collimators of collimation angle F and transmission T (see fig. 4) with absolutely nontransparent and plane plates. Assuming that the angular dependence of the p r i m a r y neutron intensity in the angular range ]el ~< F can be a p p r o x i m a t e d by I = Io(1 - c e Z ) , where ~ is the angle between the direction of neutron incidence and the b e a m axis, this calculation gives: [I~+2(y)]-_rr = I o ( T / r ) 2 [ 4 C 3 / 3 - 4 c r ~ / 1 5 +
+ ];(2/'2 -- 2cF'*/3) + y 2 ( F - c F 3 ) + + ~3(1/6 - 5eF2/6) - ~,4cC/3 - ~ c / 2 0 ] ,
(1)
for the angular range - 2 F ~ < 7 ~ - F , and [11 +z(7)]°-r = I o ( T / F ) 2 [2F3/3 - cF5/15 - 72F _ y3(1/2 _ cF2/6) + y4cF/3 + y5 3c/20],
(2)
for the angular range - F ~ y ~ 0. (The indices refer to the collimators placed in the beam.) 11 + 2 (7) is of course symmetrical to ? = 0 and is zero for 2 F ~< ]7[.
110
H. M E I S T E R A N D B. W E C K E R M A N N
2,0"
I 0 "J
-t,o
1,8
o,8
/
L4 t,2
o,6
t,o . tO-J
o,4
o,a o,6
.o,2
0,4 O,2 0
.;,w.,~, .=,~,==
- 12
-8
.
"l
-4
I
0
,
4
n=,l " . " " =,
8
"-
(T"
lO-3FO
12
Fig. 5. Collimator rocking curve; - - theoretical curve for two identical collimators taking into account the angular dependence of the primary beam intensity, x x experimental results (wings x 500).
With F = 2.94 x 10 -3 and c = 1.8 x l04 which are the parameters describing the collimation angle of our collimators and the actual angular dependence of the primary neutron intensity, and after normalization to I1 +2(0)= 1, one obtains the theoretical rocking curve depicted by the solid line in fig. 5. Comparing the experimental points with the theoretical rocking curve, one finds satisfactory agreement with the exception of the extreme wings. An intensity of some 10 .4 is observed where the theoretical curve has already reached zero. This happens precisely in the ,/-range where transmitted neutrons have to pass through one collimator plate only. At still larger values of y the intensity of transmitted neutrons diminishes. Since the effective absorber thickness is to a first approximation constant over the ),-range in question, we conclude that the average absorber thickness is certainly adequate, but that there are small inhomogeneities in the collimator plates. This conjecture is confirmed if one holds one of the collimator plates, which are black due to the boron paint, against a bright lamp: light shines through many minute holes in the paper.
Another essential quantity involved in the performance of the collimator is the transmission T. It can be deduced from the maximum intensity of neutrons transmitted through collimator 1 and 2 and the intensity after removal of collimator l: 11+2 = Io T 2 ( 2 F / 3 -- cF3/15),
(3)
12 = I o T ( F -- cF3/6),
(4)
and consequently: T = (11 + 2/lz) (1 - c F 2 / 6 ) / ( 2 / 3 -
cr2/l 5).
(5)
The experimentally determined value of I~+2/12 is 0.573 +_0.004 which leads to T = 0.850+0.006. The value T = 0.85 corresponds to an effective increase of the plate thickness by wavyness of 0.1 mm which seems quite moderate in view of the plate dimensions of about 465 x 250 mm 2.
NEUTRON COLLIMATORS
4. Conclusion
The use of materials such as airplane model paper or plastic foils which shrink upon special treatment, has been proved to offer considerable advantages for the fabrication of neutron collimators because of (a) simple and therefore lightweight construction, (b) high transmission values due to small plate thickness and little wavyness and (c) collimation features which closely approximate the theoretical ones. With regard to the latter point, plastic foils give even better performance than paper, which has a less homogeneous tissue texture.
111
For the collimator fabrication, the authors are greatly indebted to the technical services of the C.C.R., Ispra, and especially to Mr H. Geist, Head of the Construction Office, who developed the plastic foil method.
References 1) The airplane model paper and the stretching lacquer have been bought in a toy shop. 2) Supplier of the boron paint is: Bollig und Kemper, Lackfabrik, 5 Kbln 30, Postfach 320 370, Federal Republic of Germany. 3) Thermo-plastic foils on polyethylene basis as used for industrial packing.