As disussed in the introduction, tritium has two advantages over an x-ray tube: (1) As a radioactive source it is very stable. Since some typical tritium strengths are a fraction of a Curie, the fractional fluctuation level should be about per second. (2) Since commercial x-ray tubes incorporate windows their emitted beams contain very few low energy photons. Thus, these sources are inefficient for fluorescing low Z targets. On the other hand, an unsealed tritium source emits a rich population of low energy electrons. In essence, tritium beta particles shining upon a target act as an electron-impact tube in which the electron spectrum is broad band. These electrons are not only efficient for creating inner shell ionizations for low Z atoms, but create them very near the surface so there is very little self absorption of the emitted k-alpha x-rays. This increased efficiency becomes important for low Z materials since their fluorescent yield is increasingly small.
Tritium is a radioisotope of hydrogen whose nucleus contains one proton and two neutron. It has a half life of 12.32 years, and decays via . Tritium is the lowest energy beta emitter known with a total transition, or endpoint, energy of 18.6 keV. The beta energy spectrum is easily described by the Fermi theory as where E and p are the electron energy and momentum respectively. C is the Coulomb correction term given by where The mean decay energy is 5.685 keV, and a most probable decay energy of 3.8 keV.
Since our CCD calibration environment prohibits the use of tritium in a gaseous state (since the CCD requires a cold vacuum environment, and any membrane or window containing tritium gas would greatly absorb low energy electrons or x-rays) a solid form is needed. (Cryogenic tritium is possible but deemed too difficult to handle). This leaves a metal tritide as the leading candidate since typical source strengths required for adequate calibration purposes are on the order of 1 Ci. One Ci corresponds to tritium atoms, or about . If the metal to tritium ratio is 1.6 (theoretical maximum), then for titanium we need about 1.2 mg. For a surface area of about this corresponds to a thickness of about 2.7 microns. The only large manufacturer remaining in North America capable of making tritium sources this strong is Safety Light Corp, whose primary application is with the airline industry in tritium loaded signs for aircraft safety illumination. The geometry of each source is that of a disk with an outer diameter of 3/4'' and an inner hole of diameter 1/2''. This gives a surface area of and a tritium thickness of 1.8 microns. Each sample consists of a titanium tritide layer on one surface of a 0.010'' thick copper substrate. The nominal activity densities are for a total source strength of 1.25 Ci. The source is manufactured by first evaporating a thin layer of metal (e.g. titanium) onto a cleaned substrate. The layer's thickness determines the amount of of final tritium activity. Then the substrate and metal are heated in a vacuum to a high temperature (e.g. 920 C) and tritium gas is introduced, which absorbs into the hot metal in about 10 minutes. The temperature is slowly lowered in a helium atmosphere thereby locking the tritium into the metal. The essential physics is that hydrogen absorption and permeability are strongly temperature dependent.
A holder for the tritium samples has been designed and built as shown in Fig. 18. It is made from 1.25'' diameter iron rod. As shown, beta particles from tritium illuminate a disk-shaped target, whose emitted characteristic x-rays can shine through the central aperture and form a beam. Powerful neodymium magnets surround the long portion of the aperture, creating a magnetic ``gate'' that stops backscattered (and the few Compton scattered) electrons from striking the CCD. A magnet underneath the target helps to guide the incident betas and increase efficiency. The x-ray output chamber is baffled to help prevent secondary emission and x-ray scattering. The outside material is made from soft iron to help guide the magnetic fields. The unit is designed for replaceable thin targets with a 15 mm diameter. A calculation shows that the effective or average solid angle subtended by the central target area from the entire tritium surface is .
Targets ranging from beryllium to copper have been tested in the tritium cup. High Z targets are inefficient for K emission due to strong competition by L emission. Very low Z targets are also inefficient because of their fluorescent yield. The CCD calibration program uses targets of polyethylene, boric oxide, and lithium fluorine to produce carbon, oxygen, and fluorine x-rays, respectively, to complement the higher energy x-rays produced by HEXS. Figures 19 - 21 are log plots of the spectra from these three targets. The total exposure time for the oxygen and fluorine targets is about 40 hours, spread over 100 days. The carbon data exposure time is about 10 hours. These spectra are from the same CCD used in the HEXS presentation. Many features are observed in addition to the main peaks at 277, 525, and 677 eV. Most prominent is the bremsstrahlung continuum. The titanium K and K lines from Compton scattering off the tritium target are evident at 4511 eV and 4931 eV, as well as small contaminations at iron and copper K at 6400 and 8048 eV. The continuum reveals the silicon
Figure 18: Tritium Source.
Figure 19: Carbon spectrum from tritium source.
Figure 20: Oxygen spectrum from tritium source.
Figure 21: Fluorine spectrum from tritium source.
edge at 1840 eV and hints at the titanium edge (from the source). Carbon, oxygen and fluorine contamination is seen with all targets. The family of lines in the fluorine spectra around 3500 eV are unknown, perhaps a diffraction effect similar to that discussed with HEXS.
There are two theoretical approaches to understanding the spectra: The first and easiest uses existing empirical formulas for general x-ray emission from target materials due to an electron beam of known energy. This approach is simply an extension of model presented in section 2, except using an incident electron beam with an energy spread. Thus we model the beta spectrum from tritium as the sum of many electron beams with different energies and currents. Such an assumption can be difficult since the emergent beta spectrum is not known (although the nuclear emission spectra is well known, the effect and strength of self-absorption and self-scattering of the beta electrons in the titanium metal before they emerge as a beam is not well known). The second approach is more fundamental and uses models developed by the electron microbeam analysis efforts from the last 30 years. In essence, what are required are (1) the inner shell ionization cross section for an electron on the target atom, and (2) the electron slowing down formula.
The theoretical tritium spectra from a model using the first approach are overlaid in Figs. 19 - 21 as solid lines. Unlike the HEXS model spectra, the tritium model spectra are normalized to give a good overlap to the data, using factors of about 2. This is probable due to uncertainties in the source strength, and self absorption effects. The fit around the silicon edge is good, although the general shape of the high energy tail diverges above 10 keV. Also, the absolute value of K emission is different by factors of three to ten. This discrepancy may reflect limitations in the given x-ray emission model at low energy.
An indication of the tritium source stability is shown in Fig. 22, which plots the total detected fluorine K rate observed on the same CCD over a period of 70 days. The natural decay rate of tritium predict a 1.1% drop over 70 days, which is far less than the drop observed at days 60 and 70. The outgassing rate would be even less. It's possible that a thin layer of material is slowly building up on the tritium surface, or that the geometry was disturbed since both the sources and CCDs were removed and reinstalled during the calibration run. Nevertheless, the short term stability of the tritium sources is very good.
Figure 22: Tritium source stability.
Figure 23: Tritium source uniformity.
The detected fluorine K uniformity is presented in Fig. 23. Each contour neighboring lever differs by 2%. The circular uniformity is due to the close proximity of the tritium source to the CCD. With a 10 cm source-CCD distance, the variation of a point source across the CCD would be about 1%.
A difficulty in using a tritium source is the slow outgassing of tritium molecules from the titanium tritide. The migration of sufficient tritium atoms from the source to the CCD could appear as an enhanced background, thereby confusing delicate cosmic x-ray background measurements while in orbit. There are two possible mechanisms for outgassing. Both start with tritium which migrates from the bulk through microcracks in the protective oxide layer to sit on the oxide-vacuum interface. Then (1) tritium atoms can be attracted to surface defects where they combine to form molecules are thereby leave the oxide surface; or (2) a water molecule from the vacuum strikes the oxide surface near a tritium atom and an exchange reaction occurs between a water hydrogen and the tritium atom. To directly measure the outgassing rate the sources were inserted into a closed ionization chamber. The slow increase in ionization current was related to an outgassing rate. Typical measured values at room temperature were around ppd. This value is consistent with reported values of and . Attempts were made to reduce the outgassing rate by evaporating a thin barrier layer onto the source's surface which was transparent to electrons. On average, a reduction in the outgassing rate of 3-4 was observed. A reduction of 10 could be observed by cleaning the surface with methanol and keeping the source under vacuum. Experimental observations from the CCDs of enhanced background rates in the vacuum chambers indicated an increase of about a factor of two, with no subsequent contamination of the CCDs. Although high, the enhanced background rate was steady over 100 days.
Figure 24: Calibration Chamber.