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Bir polarize döteron hedefin polarizasyon miktarının ölçümleri

Measurement of polarisation of the deuteron polarised target at spin muon collaboration

  1. Tez No: 39753
  2. Yazar: İSKENDER A. REYHANCAN
  3. Danışmanlar: PROF.DR. H. HÜSEYİN GÜVEN
  4. Tez Türü: Yüksek Lisans
  5. Konular: Fizik ve Fizik Mühendisliği, Physics and Physics Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1994
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 43

Özet

ÖZET Avrupa Nükleer Araştırma Merkezi (CERN)'de Spin Muon Collab oration (SMC) deneyinde nükleonlarm spine bağlı yapı fonksiyonları (g% ve g") ölçülmektedir. Bunun için enerjisi 200 GeV'ye kadar olan polarize lep- tonların (m+)i polarize edilmiş nükleonlardan (proton ve nötron İçin döteron) derin inelastik saçılmasına bakılmaktadır. Bu spine bağlı yapı fonksiyonlarını hesaplayabilmek için deneyin kalbi sayılan hedefin polarizasyon miktarını iyi bir doğrulukla bilmek gerek lidir. Bu nedenle polarizasyonu ölçmek için Nükleer Manyetik Rezonans (NMR) tekniği kullanılmaktadır. Bunun için Radyofrekans (RF), Alçakfre- kans (LF) ve sayısal (dijital) bölümlerinden oluşan NMR devresi kullanıldı. Bu devreden elde edilen NMR spektrumlarını kullanarak Isısal Denge (TE) veya Asimetri metotlarıyla hedefin polarizasyonu ölçülebilir. Deneyin çalışması esnasında diğer adı Alan metodu olan TE metotu kullanılmaktadır. Bu metotda hedefin polarizasyonu ile TE(NMR) spektru- munun altındaki alan arasında bir doğru orantı vardır. Buradaki orantı sabiti ise TE kalibrasyonu yapılarak bulunmaktadır. Diğer bir metot olan asimetri metotunda ise polarizasyon, NMR devresinden elde edilen spektrumun bir fonksiyona fit edilmesi ile spektrumun tepeleri arasındaki oran (R) bulun maktadır. Bu iki metotla bulunan polarizasyon değerlerinin birbirlerine çok iyi bir uyum içinde oldukları görülmektedir. iv

Özet (Çeviri)

SUMMARY MEASUREMENT OF POLARISATION OF THE DEUTERON POLARISED TARGET AT SPIN MUON COLLABORATION The NA47 experiment at CERN of Spin Muon Collaboration (SMC) is aimed at measuring the deep inelastic scattering of polarised muons from polarised nucleons to provide information on the spin dependent structure function of the proton and the neutron. Recently 100 GeV/c muons have been scattered from a deuterated butanol polarised target to provide infor mation on the spin structure function of the neutron. SMC experiment setup consists of polarised target, which is the heart of the experiment, the spectrometer and polarimeter. The spectrometer has been adapted to the high flux of 4 xlOr muons per pulse of 2.6 s repeated every 14.4 s. The scattered muon, whose momentum is analysed by the For ward Spectrometer Magnet (FSM), is identified by a set of trigger hodoscopes after having penetrated an iron absorber wall (Fig. 1 a). Besides there is an array of streamer and drift tubes, altogether consisting of 44 planes. Another 8 planes of proportional chambers exist in this region. Additional chambers upstream of the absorber increase the redundancy also in this region. The polarimeter, which is located downstream of the main spec trometer, (Fig. 1 b) determines the beam polarisation by two independent methods. A combination of both methods will result in a beam polarisation measurement of about 5 % overall accuracy. The polarised target which is the heart of the experimental setup consists of the 3He - 4He dilution refrigerator, superconducting solenoid providing a longitudinal magnetic field of 2.5 Tesla, two seperate microwave systems for Dynamic Nuclear Polarisation (DNP) and the Nuclear Magnetic Resonance (NMR) measuring the polarisation of polarised target. During Dynamic Nuclear Polarisation (DNP), the temperature in the target region is about 0.5°K. Under these conditions free electron spins are completely aligned, whereas protons and deuterons are essentially unpo- larised. DNP transfers the electron polarisation to the protons or deuterons by means of microwave radiation close to the electron spin resonance line of 70 GHz. The slightly different frequencies needed to generate parallel and antiparallel longitudinal polarisation are provided by two seperate microwave systems. This allows to build up opposite polarisations in the two target sec tions simultaneously. So the polarised target consists of an upstream and aa) H1V/H H2 FSM W2 W45 pvl Wl P45 PV2İ H3V/H ST57 DT67 Target P1P2P3 POB POCPOD POE POA H4H H5 H6B9 ÜU * ft Ata t tt *t tt SI HI' S2H3'S3 H4' S4.30 -20.10 10 20 Distance from FSM-Cenne (m) b) SVH BVU PPC6 PPC4 Fe HMU OTW I Vacuum Pipe Target I Mayiftuod Target BVD MNP26 ppqi iizzhcd PBC7 I HMS Q34Q35 Pb J PBCİ I PBC2 PBC4 PBa psa PBCS PPC! PPC5. ppa VLG HLO Pb LO 30 40 50 I Chamber i Ve» İHodncope 60 70 80 DUaacc from FSM-Cenne (m) Figure l.a) The SMC Spectrometer b)The SMC beam Polarimeter VIdownstream target cell. Once the maximum polarisation is reached and the microwave power is switched off, the target is cooled down to 50 m°K, where the nuclear spin-lattice relaxation becomes extremely slow and the target po larisation is effectively frozen, which is named as Frozen Spin Mode. For the measurement of g? and g\, the nucleons in both target sections are polarised longitudinally and in opposite sense with respect to each other. In this experiment, beads of butanol and deuterated butanol as pro ton and neutron targets respectively are used. To generate the paramagnetic centers needed for DNP, the butanol is doped with a metallo-organic sub stance EHBA - Crv. The beads are contained in the two 40 cm long target sections of 5 cm diameter, which are seperated by a 20 cm long gap. The NMR system has measured the polarisation of the deuteron and hence, neutron polarisation. This large target is split into halves (upstream and downstream) with one half polarised in the opposite direction to the other. Four coils in each target half measure the deuteron polarisation. Two other coil can monitor the polarisation of the residual protons in the butanol material. The NMR system consists of three basic sections; a Radiofrequency (RF) section, a Low Frequency (LF) section and a digital section. The RF section is subdivided into four parts; the NMR pick-up coils inside the cryo- stat, the A/2 rigid cables, the Q meters and the digital frequency synthesizer. The target polarisation around each coil is measured by a Q me ter and a series LCR resonant circuit. The NMR coil is the inductor and imbedded in the target material. The resonant frequency of the circuit is adjusted for the deuteron NMR frequency at the external magnetic field (2.5 T.)which is used. The coil is connected to the Q meter by a cable of length A/2, which contributes little to the impedance of the circuit at the resonant frequency, so that the circuit can be tuned to resonance frequency by means of a capacitor external to the cryostat. The coils are made of 60 cm long, 2 mm diameter Cu-Ni tubes with a 1 mm Teflon (Dupont PTFE) coating. Each coil has three windings and an inductance of 450 nH. The coils are all in line with 4 cm spacing and oriented in such a way to create a magnetic field perpendicular to the main field, needed to process the nuclear spins and perpendicular to its neighbour coils to reduce the manual inductance. The LF section is subdivided into two parts; the offset-cards and the receving amplifiers. The Q meter output has a DC level of about -3 V. This DC level must be subtracted before further amplification is possible and this is done at the start of a sweep averaging period. viiThe digital section consists of the A/D converter and digital proces sor part. It converts the amplified signal to a binary number at each step with high precision and accumulates an average over a few thousand sweeps. The Q meter measures the magnetic susceptibility of the butanol beads as a function of frequency. The coil L, A/2 cable and capacitor C form a tuned circuit. The voltage at the input of the RF amplifier is proportional to the Q of the circuit. This voltage signal is amplified and applied to a double balanced mixer to measure the absorptive part of the magnetic susceptibility of the butanol beads. The polarised nuclei have a magnetic susceptibility which modifies the Q of the circuit (hence Q meter). The change in Q is measured by a change in the voltage across the coil, when the tuned circuit is modulated around the resonant frequency. The integral of this response is proportional to target polarisation. This integral is normalised by measuring the inte grated signal at thermal equilibruim (TE) with no microwaves applied and under the same experimental conditions. The deuteron TE signal is very small and buried in noise. Signal averaging is used to average out the statistical noise and improve the signal to noise ratio. For this to work there should be no systematic effects which change the TE signal by the signal averager. To calculate the integrated signal from the measured input, we pro ceed in three steps. The NMR signal plus Q curve is measured in 400 points over the frequency sweep from lo.l to 16.6 MHz and averaged for a certain number of sweeps, usually 200 sweeps for NMR signal and 10000 sweeps for TE signal (Fig. 2). After that, the Q curve alone is measured by shifting the magnetic field to a 5 % lower value, which shifts the NMR signal outside the frequency sweep. The first step is to subtract the Q curve alone from the signal plus Q curve, which results in a NMR signal with usually only a small residual Q curve, which is removed in the second step by subtracting a parabolic fit to the wings of the signal. For the fit, we took 70 points in each wing, which is a compromise between fit accuracy and cutting too much into the signal. The third step is to determine the integrated signal, which we have defined as the straight sum all the points divided by the total number of points. This is named as TE method. The second type of calculation of the deuteron polarisation is Asym metry Method. In this method, the deuteron polarisation is measured from the asymmetry of the deuteron NMR spectrum (Fig. 3). To determine Ra tio parameter (R), we have used to the amplitudes of three extrema in the NMR spectrum in the case of a Boltzmann distribution of spins. Then the polarisation is calculated from Vlllp= R2-1 R2 + R+l' Finally the agreement between TE and Asymmetry Methods was found very well. IX> UJ o 0.7 0.6 0.5 0.4 0.3 - 0.7 - 0.1 - 0 I- I. I_.l J- L..I I I I I I I I.. I I I ' I.. I J I I I J I I I I. I I. I J. I. I. I I I I I.1 0 50 100 150 700 750 300 350 400 KANAL SAYISI Figure 2. The TE Spectrum> v O 500 f= 400 300 200 00 0 50 100 150 700 L..I..1..I...I..U..I..1 - 1 i t l I - 1 _ t [ I 250 300 350 400 KANAL SAYISI Figure 3. The NMR Spectrum xı

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