Characteristic α particles of a Triple-Alpha Source

Note that the intensities don't quite add to 100%. There are other weak α-decay branches of 239Pu and 241Am. I am interested to hear about your setup if you are observing those.

Isotope

Eα(dEα) [keV]

Iα(dIα) [%]

239Pu

5105.50(80)

11.94(07)

239Pu

5144.30(80)

17.11(14)

239Pu

5156.59(14)

70.77(14)

241Am

5388.**(??)

01.66(02)

241Am

5442.80(13)

13.10(30)

241Am

5485.56(12)

84.80(50)

244Cm

5762.64(03)

23.10(10)

244Cm

5804.77(05)

76.90(10)

A quick note: I will refer to the smaller angle SSB detector as 33° until the issue has been clarified further. This is what the default DRAGON root histograms suggest and the data show less discrepancy with this value than 30°

Fit to 239Pu peaks in 33° SSB detector

Here is a fit to the 239Pu peaks. The fitting function for the lineshapes is purely Gaussian; the non-Gaussian shape is due to the multiple peaks folded together.

fit to 239Pu peaks from Triple-Alpha Source

Here are the dirty details of the fit.

 chisq=  134.720817
 ndof= 94
 sigma=  15.144588
 (+  0.51899807/-  0.34596321)
 area=  3501.474
 (+  106.579302/-  106.553011)
 cent 1=  2275.14099
 (+  0.529522968/-  0.530706628)

The only centroid fit was the 5156.59-keV α. The others were fixed relative to it. The relative values would change for a large DC offset. I made one pass at fitting with the relative positions of the centroids free to vary, but the results of the fit were consistent with a negligible offset.

Hence channel 2275.14(53) corresponds to 5156.59(14) keV.

Fit to 241Am peaks in 33° SSB detector

Here is a fit to the 241Am peaks. Once again, the fitting function for the lineshapes is purely Gaussian; the non-Gaussian shape is due to the multiple peaks folded together.

fit to 241Am peaks from Triple-Alpha Source

Here are the dirty details of the fit.

 chisq=  125.715751
 ndof= 98
 sigma=  15.2013118
 (+  0.696409543/-  0.316609559)
 area=  3525.58594
 (+  110.615054/-  110.307372)
 cent 1=  2421.36719
 (+  0.557337565/-  0.559052171)

The only centroid fit was the 5485.56-keV α. The others were fixed relative to it. Hence channel 2421.37(56) corresponds to 5485.56(12) keV.

Fit to 244Cm peaks in 33° SSB detector

Here is a fit to the 244Cm peaks. Once again, the fitting function for the lineshapes is purely Gaussian; the non-Gaussian shape is due to the multiple peaks folded together.

fit to 244Cm peaks from Triple-Alpha Source

Here are the dirty details of the fit.

 chisq=  90.0543143
 ndof= 86
 sigma=  14.4737855
 (+  0.643290072/-  0.506232657)
 area=  2316.39538
 (+  90.2146347/-  90.0722384)
 cent 1=  2562.90692
 (+  0.65639956/-  0.666038005)

The only centroid fit was the 5804.77-keV α. Hence channel 2562.91(67) corresponds to 5804.77(05) keV.

Best-fit calibration of 33° SSB detector

In all three cases, dE/E is significantly smaller than dC/C. Here is an expression for χ2 using a linear calibration.

ugly equation

Solving this equation for the minimum χ2, the result is E(ch)=2.252178992•ch+32.45505990.

The fit gives χ2=0.06 for one degree of freedom; the probabilitiy of a value equal to or greater than this is approximately 81% for a χ2 distribution with one degree of freedom.

Calibrated 33° SSB detector spectrum

Calibrated SSB spectrum

For Eα=6.546 MeV, one expects 3He and 4He at 33° with 4.5 and 4.6 MeV, respectively. Alphas scattered off either oxygen or nitrogen at 33° would have about 6 MeV.

Fit to 239Pu peaks in 57° SSB detector

The gain was too high to see the Am or Cm peaks in the other SSB detector. But the higher gain gave better resolution. I fit the three centroids separately with the intensities fixed, to get a calibration from this one set of peaks.

another fit!

 chisq=  80.4493227
 ndof= 67
 sigma=  7.03696406
 (+  0.553938074/-  0.415515258)
 area=  2234.375
 (+  111.5625/-  114.84375)
 cent 1=  3765.5625
 (+  0.44375/-  0.545366482)
 cent 2=  0.9966875
 (+  0.000710182397/-  0.000562861408)
 cent 3=  0.99025
 (+  0.000485450824/-  0.000382847653)

Using the larger of the error bars in each case, the three calibration points follow.

E[keV]        Channel
5156.69(14)   3765.56(0.55)
5144.30(80)   3753.09(2.67)
5105.50(80)   3728.85(1.81)

Calibrated spectrum for 57° SSB detector

The best fit for a linear calibration to the data is E(ch)=1.383472502•ch-52.70906983 with χ2=1.6. The probability of a χ2 greater than or equal to 1.6 for one degree of freedom is approximately 21%.

calibrated spectrum for 57deg SSB detector

For Eα=6.546 MeV, one expects 3He and 4He at 57° with 1.9 and 1.94 MeV. Also, 4He should elastically scatter off 16O with 5.2 MeV. 16O and protons scattered by the alphas at 57° should both have about 1.24 MeV. Could the alphas scattered by oxygen just be off the scale?

The next step is to simulate the beam scattering off various partial pressures of 4He, 3He and 16O (H2O). Maybe not today.