- Email: [email protected]
A method for the preparation of 13C targets by the cracking procedure
438
Nuclear Instruments and Methods in Physics Research 219 (19~4) 438 439 North-Holland, Amsterdam
L e t t e r to the Editor
A METHOD FOR TI-IE P R E P A R A T I O N E. R A M S T R O M
O F 13C T A R G E T S
BY TIIE CRACKING
PROCEDURE
a n d B. T R O S T E L L
The Studsoik Science Research Laboratory, S-611 82 NykOping, Sweden Received 7 October 1983
A cracking chamber is described which has been designed for the preparation of carbon targets enriched in ranging between 15 and 150 # g / c m 2.
Carbon targets highly enriched in 13C are needed for the investigation of different nuclear reactions such as 13C(ot, n ) l r O and 13C(p, n)13N, which are of interest for instance in astrophysics. For the preparation of such targets the method of cracking methyl iodide enriched in 13C has been found useful [1]. In the following a cracking chamber developed for this type of target preparation will be described. Fig. 1 shows the cracking apparatus. A foil of a suitable backing material, for instance tantalum, is clamped between two electrodes as shown in the figure. The chamber is evacuated by pumping through a liquid nitrogen trap in order to prevent contamination from the natural carbon in the oil of the pump. When the pressure in the chamber is about 1 Pa, t3C-enriched methyl iodide is let in to a pressure of about 0.01 MPa and the tantalum foil is heated to bright red with an ac current. The heat induces cracking of the surrounding gas and a carbon layer is built up on the foil. The thickness of the carbon layer is of course depending on the time of cracking. As an example the thickness obtained using a 0.1 m m thick tantalum foil and a current of 58 A is given as a function of the time of heating in fig. 2. In order to get a large area of homogeneous temperature in the foil during the heating, it was found necessary to reduce the cooling effect of the current leading holders by decreasing the area of the foil near the holders. Thus foils with the dimensions given in fig. 3 were used. Methyl iodide enriched in 13C was obtained in bulbs made of glass *. Such a glass bulb is connected to the vacuum system by glueing it into a pipe, smashing a part of the bulb inside the p i p e and then connecting the pipe to the system. During this procedure as well as during the pumping of the system the bulb is cooled in liquid nitrogen in order to prevent C H 3 I from evaporat* From CEA, France. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
il
13C
of thickness
k
Fig. 1. "Cracking chamber": A, brass cylinder; B, tantalum backing; C, plates of stainless steel; D, screw M4; E, bush of stainless steel; F, electrical feedthrough; G, current connections; H, insulation; I, O-ring; J, insulation of steatite; K, O-ring; L, glass-plate; M, LN 2 trap; N, vacuum gauge tube, O, thimble; P, CH31 enriched in 13C; Q, glass bulb; R, pressure gauge; S, cooling trap; T, valve to the vacuum gauge tube; U, admission valve for air; V, admission valve for CH31 i X, valve to the pump; Y, valve for the admission of CH31 from the cooling trap; Z, valve used for evacuating CH3I from the chamber via the cooling trap.
E. Ramstr6m, B. Trostell / Preparation of 13C targets
U
439
100
X Z
Q
L 1
i 2
Fig. 3. Dimensions of the tantalum foils used in the target preparation.
TIME (MIN)
Fig. 2. The thickness of the carbon layer obtained using a 0.1 mm thick tantalum foil with the dimensions given in fig. 3 and a current of 58 A as a function of the time of heating.
ing. When the cracking procedure is finished the remaining C H 3 I is evacuated from the chamber via a cooling trap, where the gas is transferred to liquid. This
liquid of C H 3 I can be reused in further preparations of targets.
Reference [1] C.N. Davids, Nucl. Phys. A l l 0 (1968) 619.
Nuclear Instruments and Methods in Physics Research 219 (19~4) 438 439 North-Holland, Amsterdam
L e t t e r to the Editor
A METHOD FOR TI-IE P R E P A R A T I O N E. R A M S T R O M
O F 13C T A R G E T S
BY TIIE CRACKING
PROCEDURE
a n d B. T R O S T E L L
The Studsoik Science Research Laboratory, S-611 82 NykOping, Sweden Received 7 October 1983
A cracking chamber is described which has been designed for the preparation of carbon targets enriched in ranging between 15 and 150 # g / c m 2.
Carbon targets highly enriched in 13C are needed for the investigation of different nuclear reactions such as 13C(ot, n ) l r O and 13C(p, n)13N, which are of interest for instance in astrophysics. For the preparation of such targets the method of cracking methyl iodide enriched in 13C has been found useful [1]. In the following a cracking chamber developed for this type of target preparation will be described. Fig. 1 shows the cracking apparatus. A foil of a suitable backing material, for instance tantalum, is clamped between two electrodes as shown in the figure. The chamber is evacuated by pumping through a liquid nitrogen trap in order to prevent contamination from the natural carbon in the oil of the pump. When the pressure in the chamber is about 1 Pa, t3C-enriched methyl iodide is let in to a pressure of about 0.01 MPa and the tantalum foil is heated to bright red with an ac current. The heat induces cracking of the surrounding gas and a carbon layer is built up on the foil. The thickness of the carbon layer is of course depending on the time of cracking. As an example the thickness obtained using a 0.1 m m thick tantalum foil and a current of 58 A is given as a function of the time of heating in fig. 2. In order to get a large area of homogeneous temperature in the foil during the heating, it was found necessary to reduce the cooling effect of the current leading holders by decreasing the area of the foil near the holders. Thus foils with the dimensions given in fig. 3 were used. Methyl iodide enriched in 13C was obtained in bulbs made of glass *. Such a glass bulb is connected to the vacuum system by glueing it into a pipe, smashing a part of the bulb inside the p i p e and then connecting the pipe to the system. During this procedure as well as during the pumping of the system the bulb is cooled in liquid nitrogen in order to prevent C H 3 I from evaporat* From CEA, France. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
il
13C
of thickness
k
Fig. 1. "Cracking chamber": A, brass cylinder; B, tantalum backing; C, plates of stainless steel; D, screw M4; E, bush of stainless steel; F, electrical feedthrough; G, current connections; H, insulation; I, O-ring; J, insulation of steatite; K, O-ring; L, glass-plate; M, LN 2 trap; N, vacuum gauge tube, O, thimble; P, CH31 enriched in 13C; Q, glass bulb; R, pressure gauge; S, cooling trap; T, valve to the vacuum gauge tube; U, admission valve for air; V, admission valve for CH31 i X, valve to the pump; Y, valve for the admission of CH31 from the cooling trap; Z, valve used for evacuating CH3I from the chamber via the cooling trap.
E. Ramstr6m, B. Trostell / Preparation of 13C targets
U
439
100
X Z
Q
L 1
i 2
Fig. 3. Dimensions of the tantalum foils used in the target preparation.
TIME (MIN)
Fig. 2. The thickness of the carbon layer obtained using a 0.1 mm thick tantalum foil with the dimensions given in fig. 3 and a current of 58 A as a function of the time of heating.
ing. When the cracking procedure is finished the remaining C H 3 I is evacuated from the chamber via a cooling trap, where the gas is transferred to liquid. This
liquid of C H 3 I can be reused in further preparations of targets.
Reference [1] C.N. Davids, Nucl. Phys. A l l 0 (1968) 619.