What is measured? M. C. Moxon Jan. 2011

Size: px

Start display at page:

Download "What is measured? M. C. Moxon Jan. 2011"

Verity Evans
5 years ago
Views:

1 What is measured? M. C. Moxon Jan. 2011

2 1 Observed data 2 Things infrequently mentioned in reports Dead time Resolution Background Sample thickness variations Sample temperature

3 What is observed? Counts per timing channel t x x C t B e t e R e Y e T e C t t x t t t t t t e e δ α δ δ φ τ + = = +Δ = Δ = = = 1, 2 / 2 / 0

4 Dead time Dead time due to the detector pulse. The longest dead time must be introduced as close electronically to the out put of the detector as possible. Subsequent dead times must be shorter if not there are much bigger problems in calculating corrections.

5 Time spectrum Amplitude Bias level Signal Intrinsic dead time Intrinsic output Fixed dead time 10 channels output Time

6 Pulse height spectrum for a Li-glass oise and background Reaction pulses "observed" 16 Relative count rate Pulse height

7 Observed dead time 6.0E E+05 Exponential fit to time>4 o dead time Intrinsic dead time Fixed dea d time 4.0E+05 Count rate 3.0E E E E Time

8 Count loss correction 1.0E-01 Equation 1/1-X Equation 1/[1-x/ x^2] 1.0E-02 - difference ratio 1.0E E E Observed count rate times fixed dead time

9 Fixed dead time 1.0E-01 Ratio equ. 1 Ratio 8.0E E E-02 Spectrum count rate = 4 per start Background count rate =0.5 per start Dead time 2.5 time channels Ratio = corrected counts/true counts-1 Equ 1 = 1/1-x Equ 2 = 1/1-x/ x X= Cn over the dead time Ratio equ.2 Count loss correction equ.1 Tmie gate 2.0E E E Time

10 Resolution function in REFIT for Geel 1.0E E-01 1 to 5 ev 1 to 2 kev 100 to 200 kev 1.0E-02 Intensity 1.0E E E Effective distance cm

11 Effect of resolution Geel 1.00E-02 Measured data Fitted curve including background adjustment Capture cross-section * a/b 1.00E-03 Capture yield 1.00E E E Energy ev

12 U-238 Capture Harwell 1.0E E+03 Measured data "Fitted curve" Capture X-section Total X-section 1.0E+02 Cross-section b 1.0E E E E Energy ev

13 Background Geel 1000 Data for Hf-178 Fitted Background Counts Time Micro-secs

14 Background Geel 1.E+04 1.E+03 ormalised counts 1.E+02 1.E mm77k In 15 mm 77K out 15 mm 77K in Background. 15mm 77k out Background. 1.E Energy ev

15 Mean densities relative to published values Geel Schillebeeckx Weighted mean / ISIS Kocklemann Weight mean / Harwell Moxon Si9.593 mm xtal / Graphite from to gm/cc Published value varies from 1.80 to 2.10 Fe 25.6 mm /

16 Variations in density Decrease in the capture area due to a log-normal distribution in the thickness for n-sigma-zero= "Decrease in resonance area" Resonance area resonace peak Spread in thickness about the mean

17 Published data Total cross-section from a single sample Reaction cross-section, corrected for self screening and may be multiple scattering. Transmission. Reaction yield. Resonance energy eutron width Other partial widths uclear radius Spin?

18 What is published! Transmission Reaction = S O O O O S S S K K t B F C t B F C T R ST R ST ST ST ST R R R R K K t B F C t B F C Y ε ε = 1 e MS e e e e Y T R R e n R T + = σ σ ε σ e n e T σ

19 Transmission Geel Pb Measured data Fitted curve 0.6 Transmission Energy ev

20 Capture Geel Pb E E-02 Measured data Fitted curve, including background adjustment Capture cross-section * a/b 1.0E-03 Capture yield 1.0E E E E Energy ev

21 As an evaluator I would like to see more information published about the measurements. What was the sample temperature? How was the dead time measured? Background formalism and how its magnitude was determine. Details of the normalisation in reaction measurements. Some details about the neutron time of flight resolutions and its time dependence. Some of these request could be fulfilled by publishing the actual observed counts!

23 It is VERY difficult to measure the capture cross-section of a fissile isotope, especially using time of flight techniques with gamma-ray detectors!

24 Why? 1 The prompt gamma-ray energy emitted from a fission event is ~12 MeV and from a capture event ~7 MeV 2There are can be up to ~12 prompt gamma-rays emitted from a fission event where as for a capture event it is generally less than 6.

25 Detectors ~4 Π γ-ray detector measuring the total γ-ray energy from an event plus a fission chamber in anti-coincidence Many γ detectors covering ~4 Π to determining the number of γ-rays per event in the sample and the total energy γ-ray energy from the reaction. Using a GeLi γ-ray detector to record the low energy γ- ray transitions following a neutron capture event. This may be dependent on the spin of the resonance formed in the compound nucleus U-236.

26 Ba-F detector ORL Gwinn Multi-detector data 1.0E E+02 Cross-sectionb 1.0E+01 Measured fission, o gammas >6 Fitted fission Measured capture +fission o gammas >1,<7 Fitted fission +capture 1.0E Energy ev

27 Difference 2 to 6 γ-rays and 7 to 12 γ-rays 1.0E+03 Difference <7 and >6 Fit Observed capture cross-section 1.0E E E Energy ev

28 A fit to Gwin et al multi BaF detector to the fission and fission + capture data Temperature coefficients for all isotopes in sample 1, 2, 3, 4 gas model Sample temperature = E-02 +/ E-04 ev ** 14 ** E+02 +/ E+00 K FLIGHT PATH LEGTH 1,2,3,4 = / E-02 METRES ** 15 ** IITIAL DELAY 1,2,3,4 = / E-01 MICROSECODS ** 9 ** EFFICIECY FOR CAPTURE ISOTOPE 2 SAMPLE 1 = / ** 10 ** ORMALISATIO OF SAMPLE 1 = A+B*T**C A = E+00 +/ E-03 ** 5 ** BACKGROUD FOR SAMPLE 1 = A+B*T**C A = E-04 +/ E-05 ** 1 ** RESOLUTIO DATA FOR SAMPLE 1,2,3,4 EFFECTIVE MEA FREE PATH OF EUTROS I THE MODERATOR AT 1 ev = E-03 +/ E-04 METRES ** 16 ** EFFICIECY FOR CAPTURE ISOTOPE 2 SAMPLE 2 = / ** 11 ** ORMALISATIO OF SAMPLE 2 = A+B*T**C A = E+00 +/ E-03 ** 6 ** BACKGROUD FOR SAMPLE 2 = A+B*T**C A = E-04 +/ E-05 ** 2 ** EFFICIECY FOR CAPTURE ISOTOPE 2 SAMPLE 3 = / ** 12 ** ORMALISATIO OF SAMPLE 3 = A+B*T**C A = E+01 +/ E-02 ** 7 ** BACKGROUD FOR SAMPLE 3 = A+B*T**C A = E-05 +/ E-06 ** 3 ** EFFICIECY FOR CAPTURE ISOTOPE 2 SAMPLE 4 = / ** 13 ** ORMALISATIO OF SAMPLE 4 = A+B*T**C A = E+01 +/ E-02 ** 8 ** BACKGROUD FOR SAMPLE 4 = A+B*T**C A = E-05 +/ E-06 ** 4 **

29 COVARIACE MATRIX Temp. all Cap. eff S or. S Bac. S Zero delay all Flight len. all MFP all Cap. eff. S or. S Bac. S Cap. eff S or. S Bac. S Cap. eff. S or. S Bac. S

30 Pb E E E E E E E-01 ORL trans data Trans fit Geel Cap data Cap. fit 0.0E

31 Pb ORL Transmission Fit Geel capture *5000 Fit*5000 Yield/transmission Energy ev

Nuclear Reactions A Z. Radioactivity, Spontaneous Decay: Nuclear Reaction, Induced Process: x + X Y + y + Q Q > 0. Exothermic Endothermic

Nuclear Reactions A Z. Radioactivity, Spontaneous Decay: Nuclear Reaction, Induced Process: x + X Y + y + Q Q > 0. Exothermic Endothermic Radioactivity, Spontaneous Decay: Nuclear Reactions A Z 4 P D+ He + Q A 4 Z 2 Q > 0 Nuclear Reaction, Induced Process: x + X Y + y + Q Q = ( m + m m m ) c 2 x X Y y Q > 0 Q < 0 Exothermic Endothermic 2