B2o3 Hedefin 103-127 Kev Enerjili Protonlar İle Bombardımanı Sonucu Gerçekleşen 11b(p,α)2α Reaksiyonunun Kısmi Diferensiyel Tesir Kesitlerinin Ölçülmesi

Abstract

Bor elementinin çeşitli malzeme yüzeylerindeki dağılımının tespit edilmesi için NRA (Nuclear reaksiyon analizi), RBS (Rutherford geri saçılma spektrometresi), HERDA- Ağır Iyon Elastik Geri Saçılma Tespiti Analizi (Heavy ion recoil detection analysis) gibi teknikler kullanılmaktadır. Nükleer Reaksiyon Analizi (NRA) tamamlayıcı bir metot olarak Rutherford Geri-Saçılma Spektrometresi (RBS) ile birlikte kullanılır. Hafif elementlerdeki hassasiyeti NRA'nın RBS'ye göre üstünlüğüdür. NRA sayesinde özellikle hafif elementlerin nanometre (nm) düzeyinde derinlik analizi gerçekleştirilebilir. Bu çalışmada Bor elementinin çeşitli malzeme yüzeylerinde NRA metoduyla analizi için gerekli olan, 11B(p,α)8Be reaksiyonunun Kütle merkezi sistemine (CM) göre 103-127 keV enerji aralığındaki ve 135º'deki tesir kesitleri ölçülmüştür. Işınlamalar Çekmece Nükleer Araştırma ve Eğitim Merkezi (ÇNAEM)'de kurulu bulunan SAMES J-15 iyon hızlandırıcısında gerçekleştirilmiştir. SAMES J-15 hızlandırıcısı Van de Graff tipi bir yüksek gerilim kaynağına sahip olup maksimum 150 kV gerilim üretebilmektedir. İyon kaynağı quartz malzemeden imal edilen bir plazma tüpü, 100 MHz RF (Radyo Frekans) kaynağı ve içi yaklaşık ~9 atm. basınçta H2 gazı ile doldurulmuş ozmoregülatör tüpten oluşmuştur. Işınlamalar ~5µA demet akımında gerçekleştirilmiştir. Işınlama süresi 4250 s. olarak seçilmiştir. Her ışınlama sonucunda hedefin konumu değiştirilerek tüm ışınlamaların taze bir hedef ile gerçekleştirilmesi sağlanmıştır. Demet hattındaki vakumu sağlamak için bir adet mekanik pompa ve bir adet difüzyon pompası kullanılmış ve ışınlamalar sırasında demet hattındaki vakum değeri ~5*10-6 Torr olarak ölçülmüştür. Işınlanacak olan hedefler, ÇNAEM Nükleer Fizik Birimi hedef kaplama laboratuvarında PVD- Fiziksel Buhar Biriktirme (Physical Vapor Deposition) yöntemiyle ~10-4 Torr basınç ortamında hazırlanmıştır. Hedefler toz halinde Borik Asit (H3BO3) kullanılarak ısıl işlem yoluyla elde edilen B2O3'ün ince Alüminyum folyo üzerine kaplanmasıyla elde edilmiştir. Kaplama kalınlığı, kaplamadan önce ve sonra yapılan tartımlar sonucu 56 µg/cm2 olarak hesaplanmıştır. Reaksiyonda oluşan alfa parçacıklarının algılanması 300 mm2'lik yüzey engelli dedektör ile gerçekleştirilmiştir. Dedektör, demet doğrultusu ile 1350 açı yapacak şekilde yerleştirilmiştir. Hedeften saçılan protonların dedektöre ulaşmasını engellemek amacıyla ince 250 µg/cm2 mayler film kullanılmıştır. Protonların hedef içerisindeki ortalama enerjisini hesaplamak için SRIM (Stopping Range of Ions in Matter) kodundan faydalanılmıştır. Katı açı hesaplamalarında üç farklı yöntem kullanılmış ve her bir yöntem sonucu elde edilen katı açı değerleri kullanılarak 11B(p,α0)8Be ve 11B(p,α1)*8Be reaksiyon kanallarının tesir kesitleri hesaplanmıştır. Ayrıca Nükleer Reaksiyon Analizi çalışmalarında kullanılmak üzere Toplam Kapsamındaki Tesir Kesiti (Cross Section Under Convention) hesaplanmıştır. İlk katı açı hesaplama yönteminde kaynağın noktasal olduğu kabul edilerek, ikincisinde ise dairesel olduğu kabul edilerek katı açı hesaplanmıştır. Üçüncü yöntemde ise hedefin eliptik geometriye daha yakın olduğundan yola çıkılarak Monte Carlo yöntemini kullanan Sacalc Ellipsoid bilgisayar yazılımından faydalanılmıştır. Noktasal hedef yaklaşımından ortaya çıkan katı açı değeri kullanılarak hesaplanan tesir kesiti verileri literatürdeki değerlerle karşılaştırılmıştır.

The cross section data are used in a lot of areas. Nuclear Physics study, nuclear reactor design, nuclear fuel development, shilding material development, surface analysis, trace element analysis, radyoisotop production etc. are some of these areas. In Turkiye there was some studies at Cekmece Nuclear Research and Training Centre (CNAEM) about neutron reaction cross sections. But these studies are not continued because of different reasons. Reaction cross section measurements required some facilities like particle accelerators or research reactors designed for researh activities. To establish these facilities are very expensive. In Turkey because of lacking these facilities the nuclear and particle physicists at the universities maintain their studies by only modelling the experiment using some computer codes. It is used some IBA (Ion Beam Analysis) methods, such as NRA (Nuclear reaktion analysis), RBS (Rutherford backscattering spektromety), HERDA (Heavy Ion Elastic Recoil Detection Analysis), for the measurement of Boron element at the surface of some materials. Nuclear Reaction Analysis (NRA) is used as a complementary method of Rutherford Back-Scattering (RBS) analysis. NRA is more sensitive than RBS for light elements. By using this method the depth analysis of the light elements can be performed. Protons, deuterons and alpha particles are used mostly as projectile. To prevent the backscaterred particles from reaching the detector, usually thin foils are used in front of the detector. In this study partial differential cross-sections of the 11B(p,α)8Be reaction at the angle of 1350 for 103-127 keV CM (Center of Mass) energies have been measured. The irradiations have been made at the SAMES J-15 accecelarator at the Cekmece Nuclear Research and Training Centre (CNAEM). SAMES J-15 accelerator is a DC accelerator. High voltage unity of the accelerator produces 150 kV maximum. This unity consists of two parts which are high voltage generator and high voltage feed source. The high voltage feed source produces 30 kV exitation voltage and this exitation voltage is exited to up to 150 kV at the Van de Graff type high voltage generator. The stabilization of the high voltage unity is about ±1%. the High volgage generator cylinder is filled with ~15 atm. Hydrogen gas because of the chemical stability of this gas for the structeral materials. Ion source is composed of a quartz plasma tube, an osmoregulator tube and an 100 MHz RF source. The osmoregulator tube is filled with H2 gas of 99.9% purity at ~9 atm pressure. In the osmoregulator, H2 gas is ionized and atomic (H+) and moleculer (H2+) ions are produced. The ions produced are ~80% atomic (H+) and ~20% moleculer (H2+). The irradiations of the target have been performed at ~5±0.5 µA beam current and 4250 s. irradiation time. The 10% error in the beam current is the main reason of the ~12% error of the cross section data obtained in this study. All the irradiations have been made with fresh target. The value of the beam line pressure has been measured as ~5*10-6 Torr while irradiations were being performed. The targets have been prepared at CNAEM (Cekmece Nuclear Reaserch and Training Center) with PVD (Physical Vapor Deposition) method at ~10-4 Torr vacuum pressure. The targets have been prepared by depositon of B2O3 obtained from Boric Acid (H3BO3) powder on thin Aluminum foil substrate. Target thickness has been calculated as 56 µg/cm2 by measuring the mass of the substrate before and after the deposition. By using this thickness the calculated number of 11B is 7.748 ×〖10〗^17± 1.33*1016 atoms/cm2. The 6 % error in this value is the other reason for the 12% error in the cross section values. 300 mm2 surface barrier detector is used for the detection of alfa particles formed at the 11B(p,α)8Be reaction. The angle between the detector and the beam direction is chosen as 1350. To prevent the protons which was scattered from the target from arriving the detector, it is used 250 µg/cm2 thin mylar foil in front of the detector. The mylar foil used as the absorbant of the scattering protons caused some deformation in the low energy part of the alpha spectrum because the energy spectrum of α01, α02, α11 ve α12 alpha particles coming from the 11B(p,α0)8Be and 11B(p,α1)*8Be reaction channels starts from 0 keV and continues to ~5000 keV. The 250 µg/cm2 mylar foil used in this experiment caused the energy spectrum of alpha particles coming from the calibration source to shift approximately 200 keV to the left. The calibration source has alpha particles about ~5000 keV. If 5000 keV alpha particles loose ~200 keV energy, it is obvious that the alpha particles with the energy of 0-1000 keV will loose more energy than 200 keV. In this work the number of alpha particles, having 0-600 keV enegy and coming from the 11B(p,α0)8Be and 11B(p,α1)*8Be reaction channels, arrived to the detector and counted is very low because of the absorbtion of the mylar foil. This loss in the low energy part of the spectrum have prevented the accurate cross section measurement of the 11B(p,α1)*8Be reaction channel. SRIM (Stopping Range of Ions in Matter) computer program is used to calculate the average energy of the protons while passing through the target. It is found that the protons while passing through the target have lost approximately ±16-17 keV energy. Althoug this energy loss is not important for the prottons in the MeV energy scale, it is very high for 100-150 keV protons of J-15 accelerator. Three different approaches are used to calculate the detector solid angle. The three different solid angles obtained by these three approaches are used to calculate the partial differential cross sections of 11B(p,α0)8Be ve 11B(p,α1)*8Be reaction channels. Furthermore, for the 11B(p,α1)*8Be reaction, Cross Section Under Convention which can be utilised at Nuclear Reaction Analysis studies is calculated too. For the first, the second and the third solid agle approaches the source (the target at this study) is accepted as a point source, a disc source, and an elliptic source respectively. At the first and second approaches the solid angle is calculated manually. At the third approach the calculation is performed by Monte Carlo method using Sacalc Ellipsoid computer program. The calculated solid angels (Ωpoint, Ωdisk ve Ωelliptic) obtained from the point source, the disc source and the elliptic source approaches are used to calculate three different cross section sets. The cross section data set calculated from Ωpoint solid angle compared with some literature data. It is concluded that calculated cross section data set of 11B(p,α0)8Be reaction channel is agree with the literature data. On the other hand, the cross section data set of 11B(p,α1)*8Be reaction channel is some different than the literature data. Furthermore the cross section data sets obtained by three different approaches are very close to each other. The difference between them is less than 1%. It is concluded from this result that, at the situation in which the source dimensions are small enough compared to the detector diameter it is not problem what is the shape (disc, ellips, square, rectangle) of the source. The point source approach can be used independently from the source shape. This will not affect the results more than 1%. The cross section values of the 11B(p,α0)8Be and 11B(p,α1)*8Be reaction channels in the energy range of 103-127 keV are changing in a syncronized form. It is concluded from this result that the variation of the cross section values for the reaction channels 11B(p,α0)8Be and 11B(p,α1)*8Be are similar to each other.