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

E_α(dE_α) [keV]

I_α(dI_α) [%]

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.

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.

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.

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.

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

For E_α=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.

 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%.

For E_α=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.